CHAPTER 1

GENERAL INTRODUCTION

1.1 Co-operative breedilg in birds.

Co-operative breeding in birds is usually defined

as the regular involvement of individuals, other than the

parents, in the care and feeding of offspring (Brown 1978).

The presence of 'helpers' (Skutch 1935) has been of

considerable interest to behavioural ecologists and a number

of reviews and theoretical discussions have been published

in recent decades (eg. Brown 1975, Wilson 1975, Brown 1978,

Koenig and Pitelka 1981, Emlen and Vehrencamp 1983 among

others). Much of the interest in co-operative breeding has

been due to the apparently altruistic behaviour exhibited

by helpers toward the offspring of others and, in order to

explain the evolution of suer behaviour, there has been much

use made of the concept of ir.clusive fitness (Hamilton 1964)

and the theory of kin-selection (Maynard Smith 1964).

Co-operative breeding has been documented in many

species of bird, with at least thirty-two families and sub-

families represented in the Passeriformes alone (Brown 1978,

Fry 1977, Emlen 1978, 1984). In Australia, co-operative

breeding is widespread with one survey (Dow 1980) listing

sixty-five species in twenty families. Among the better

studied species in Australia are the babblers Pomatostomus

sp. (Brown and Baida 1977, 3rown et al. 1978, 1982, 1983),

Noisy Miner Manorina melana:7ephala (Dow 1970,

1977, 1978a, 1978b, 1979a, 1979b), fairy-wrens Malurus

cyaneus and M. splendens (Rowley 1965, 1981) and the

White-winged Chough Corcorax melanorhamphus (Rowley 1978).

Many species however, have not been studied in detail and

the occurrence and type of co-operative breeding in these

species is almost unknown.

Most co-operative breeding species live in small

(2 - 20 birds), relatively stable social groups in permanent,

all-purpose territories, are monogamous, and insectivorous in

their diet (Ricklefs 1975, Grimes 1976, Brown 1978). In a

recent survey of Australian species, Ford et al. (unpubl.

ms.) found that most inhabiter eucalypt woodlands and forests

rather than rainforests or deserts, were insectivorous, and

were generally ground-based foragers. Such species included

the babblers, White-winged Chough, and the fairy-wrens.

A variety of terms have been used in descriptions

of co-operative social systems and in the present study I

have followed the terminology of Emlen (1984). Co-operative

breeding is defined as any situation in which more than one

adult of either sex (ie. the minimum requirement for sexual

reproduction) participate in the raising of offspring.

Within this general definitic , n I further separate 'helper-at-

the-nest' systems and 'commural l systems.

Helper-at-the-nest systems comprise a single,

monogamous breeding pair together with one or more non--

breeding helpers. Helpers are often the grown offspring

of the pair that have failed to disperse from their natal

group. Although immatures from previous nests within the

2

same breeding season may also help at later nests, this is

not considered to be co-operative breeding. Some well-

studied species which are typEcal helpers-at-the-nest species

include the Florida Scrub Jay Aphelocomacoerulescens

(Woolfenden 1975, 1978, Woolfnden and Fitzpatrick 1984),

Stripe-backed Wren Campylorhynchus nuchalis (Rabenold

and Christensen 1979, Rabenold 1984, Austad and Rabenold

1985), and the fairy-wrens (Rowley 1965, 1981).

In communal breeding species however, there may

be more than one breeding pair in each breeding g roup and

helpers may also be breeders. Communal breeding species

include the Acorn Woodpecker Melanerpes formicivorous

(MacRoberts and MacRoberts 1976, Stacey 1979, Koenig 1981)

and the Groove-billed Ani Crotophagasulcirostris

(Vehrencamp 1978). Communal breeding is often treated

separately in discussions of co-operative breeding and will

not be treated in detail in this study. The evolution of

communal breeding is addressed specifically by Emlen (eg.

Emlen 1982b, 1984, Emlen and Vehrencamp 1983, 1985), and

Vehrencamp (eg. Vehrencamp 1983).

1.2 The adaptive significance of co-operative breeding.

In order to understand better the adaptive

significance of co-operative breeding it is necessary to ask

three general questions about the individuals involved;

(1) Why delay dispersal? (2) Why help?, and (3) Why do

breeders tolerate helpers?

Q.1 : Why do some birds delay their dispersal from

the natal group?

In species such as the Superb Fairy-wren, delayed

dispersal is a necessary precursor to co-operative breeding.

Since Superb Fairy-wrens, as with many other co-operative

breeding species (Woolfenden and Fitzpatrick 1978, 1984),

disperse directly into breeding positions, delayed dispersal

also implies delayed breeding. Groups of non-breeding

birds or 'floaters', for example, which live in marginal

areas or wander throughout the habitat, are not generally

found in co-operative breeding species (Koenig and Pitelka

1981). The question of delayed dispersal therefore raises

two further points; (i) what are the advantages of residing

in the natal group as opposed to other types of social group,

or alone, and (ii) why do some birds delay breeding?

However, it is difficult to test the first of these points

in a species such as the SupErb Fairy-wren because single

birds are rarely recorded, particularly during the breeding

season. Instead, I have con s idered two potential advantages

of association with the natal group; (i) the opportunity

to help raise additional siblings, and (ii) the opportunity

for greater access to breeding positions (eg. through

inheritance of the breeding position). Both of these

points are important to the social organisation of fairy-

wrens and are considered in later chapters.

There must also be a consideration of the likely

success of any dispersal movement, ie. what is the probability

that dispersal will lead to breeding? Emlen (1982a)

identified four factors which should influence the success

of dispersing individuals; (i) the risks associated with

dispersal itself, (ii) the probability of establishment

on a suitable territory or patch of habitat, (iii) the

probability of finding a mate, and (iv) the probability

of successful reproduction onc.?. established.

(i) Dispersal should always involve some

significant risk, most importantly from predation or

starvation. It is also possible that this risk is propor-

tional to the time taken and the distance moved during

dispersal, and may be greater for immature or inexperienced

birds. In addition, many co-operative breeding species are

restricted to specific habitat types or have specific

requirements (Selander 1964, Brown 1974, 1978, Brown and

Saida 1977, Craig 1979, 1980a, 1980b, Stacey 1979, Trail

1980, Koenig 1981, Koenig and Pitelka 1981, Zack and Ligon

1985a). These specific requirements may require extensive

searching by dispersing birds, further increasing the risks

associated with dispersal.

(ii) For species with specific habitat require-

ments, there may not be sufficient suitable habitat to

accommodate all of the potential breeders (Selander 1964,

Brown 1969, 1974, Koenig and Pitelka 1981). In addition

to the abundance of suitable habitat, spatial and temporal

variability in habitat quality may also influence the

5

probability of successful establishment (Brown and Balda 1977,

Trail 1980, Nias 1984, Zack ald Ligon 1985b).

(iii) The success of a dispersing bird in becoming

established as a breeder will also depend on the probability

of locating a sexual partner. Skewed sex-ratios are a

feature of many co-operative breeding species and a lack of

mates may be an important constraint on reproductive options

(Dow 1978a, 1978b, Fry 1972, Emlen 1978, 1984, Reyer 1980,

Rowley 1981). The male-based sex-ratio found in Superb

Fairy-wrens at Gungahlin (Rowley 1965) for example, has

been widely discussed as a constraint on the reproductive

options of potential breeding males and a factor promoting

delayed dispersal (Brown 197E, Emlen 1978, 1984, Emlen and

Vehrencamp 1983, 1985, Wittenberger 1984).

(iv) Even if dispersing birds are able to obtain

suitable habitat and a mate, poor environmental conditions

may reduce the probable success of any reproductive efforts.

This type of 'ecological constraint' has been discussed by

Emlen (eg. Emlen 1981, 1982a, Emlen and Vehrencamp 1983, 1985)

with particular reference to co-operative species (eg. the

White-fronted Bee-eater Mera:Ds bullockoides ) living in

variable environments.

In summary, the probability of successful dispersal

and establishment by individual birds may be reduced by a

number of factors or 'ecological constraints' (Emlen 1982a).

If this probability is sufficiently low, then individuals

may benefit from delayed dispersal and association with

close relatives. The factors which reduce the dispersal

and reproductive opportunities for potential breeders have

been discussed in much of the literature concerning co-

operative breeding (eg. Selander 1964, Brown 1974, 1978,

Emlen 1982a, Emlen and Vehrencamp 1983, 1985 among others).

Q.2 : Why do non-dispersing birds help to raise

the offspring of others?

Given that some birds delay their dispersal from

the natal group, there may be a number of potential benefits

LO be gained from helping behaviour. Emlen and Vehrencamp

(1983, 1985) for example, suggest that helpers; (i) may gain

valuable experience in parental duties, (ii) may be able

to disperse along with the young that they raise, (iii) may

be able to raise their own, future helpers, and (iv) may be

able to enhance their inclusive fitness by the production

of additional relatives. Sone of the benefits of remaining

within the natal group and helping may be immediate, such

as increased survival, or they may be delayed, such as being

in a position to inherit the natal territory (Emlen 1984,

Wiley and Rabenold 1984). In addition, benefits may be in-

direct (ie. affecting the fitness of genealogical relatives,

Brown 1980). There has been much debate on the relative

importance of direct versus indirect benefits but the

available data suggest that both types of benefits may be

gained from helping behaviour (Vehrencamp 1979). Helpers

may, for example, gain experience at parental duties at the

same time as raising additional relatives.

7

However, in order for helpers to gain indirect

benefits from helping behaviour, it must first be established

that helpers increase the reproductive output of breeders.

Although a correlation between the presence, or number, of

helpers and reproductive output may indicate a positive

effect of helpers, it does not establish a causal link.

Lack (1968) for example, recognised that at least two other

factors, parental age and territory quality, may influence

both the probability that helpers are present, and the

reproductive success of the breeders. A number of studies

for example, have demonstrated a relationship between group

size (and hence the presence or number of helpers in the

group), territory quality, and reproductive output by

breeders (Zahavi 1974, Brown 1978, Gaston 1978b, Koenig

and Pitelka 1981, Lewis 1981, Zack and Ligon 1985b). The

only experimental evidence of the effect of helpers on group

reproductive success (independently of habitat effects) has

come from a study of the Gre y -crowned Babbler Pomatostomus--_-_-_-_-__

temporalis. By manipulating the number of helpers present

in groups of babblers, Brown et al.(1982) were able to show

that groups with fewer helpers produced fewer fledglings than

did groups with more helpers, although the mechanism by

which helpers increased group reproductive success was not

clear. It had earlier been shown, for example, that helpers

did not increase the rate at which food was delivered to

nestlings (Brown et al. 1978) and it was later shown that

nestling growth was not influenced by the presence of helpers

(Dow and Gill 1984). It was possible, however, that

8

breeding females expended less energy on parental care when

helpers were present and were therefore able to make more

nesting attempts (Brown and Brown 1981).

An alternative view is that helping behaviour is

simply mis-directed parental care and not, in itself, an

adaptive trait (Price et al. 1983, Jamieson 1986). Helpers

may perform parental duties simply because they are

stimulated by begging nestlings. Selection against helping

behaviour might therefore work against the later performance

of normal parental behaviour and be counter to the fitness

of the individual (Brown and Brown 1984). Such a hypothesis

might be refuted however, if it could be shown that helpers

vary the amount of aid given in accordance with the degree

of potential benefit to be obtained. Helpers may, for example,

selectively aid closer relatives (Clarke 1984) or adjust the

amount of aid given according to their dominance position

within the group (Carlisle and Zahavi 1986).

Q.3 : Why should breeders tolerate the presence

of grown offspring in the natal group?

Although there are a number of potential costs

and benefits to group living (Alexander 1974, Wilson 1975,

Hoogland and Sherman 1976, BErtram 1978), a number of these

relate more specifically to co-operative breeding. In

particular, there are severa] important benefits which

might be gained by allowing g rown offspring to remain

within the natal group if they participate in parental

duties. Helpers might increase the survival of offspring

by feeding, or reduce the losses associated with brood

parasitism or predation by mobbing. Even if no immediate

enhancement of reproductive output results from the presence

of helpers, assistance given During the nesting period may

enhance the survival or future reproductive output of

breeders (eg. Woolfenden and Fitzpatrick 1984). Gaston

(1978a) suggested that assistance given by helpers may be

a form of 'payment' to the breeders. Therefore, any cost

to the breeders, such as increased competition for food,

might be compensated for by helpers during the raising of

nestlings.

In a number of co-operative breeding species,

helpers may eventually inherit the breeding position within

the natal group (Brown 1974, Woolfenden and Fitzpatrick 1978,

Brown and Brown 1981, Rowley 1965, 1981). Co-operative

breeding might therefore be considered as a strategy for

retaining good quality territories in the possession of

genetic relatives, and increasing the probability that

offspring gain a breeding position (Brown 1974).

1.3 The Superb Fairy-wren.

The Superb Fairy-wren is a small (ca. 10g)

insectivorous passerine common in much of south-eastern

Australia (Fig. 1.1). During the 1950s and 1960s Ian Rowley

made a detailed and long-term study of the species at

Gungahlin, near Canberra A.C.T. Rowley's work was one of

the first to use individuall y recognizable (colour-banded)

subjects and his work on the Superb (Rowley 1965) and later

- 10 -

the Splendid (Rowley 1981) Fairy-wrens have become landmarks

in Australian ornithology.

Rowley (1965) found that breeding groups of

Fairy-wrens comprised a single monogamous breeding pair

together with any adult, non-breeding helpers that were

present. Immature birds from previous nests during the

breeding season also accompanied the breeding pair. Adult

helpers were almost always males, and the offspring of at

least one of the breeding pair. Helpers were usually one-

year old birds that had not dispersed from their natal

group although some older helpers were also present.

Helpers and immatures participated in a number of

the parental duties, including the feeding of nestlings and

dependent fledglings. Helpers also assumed much of the

responsibility for the tendir:g of dependent fledglings while

the breeding female began re-nesting. Breeding males also

fed nestlings and tended fledglings but only the breeding

female built the nest, laid and incubated the eggs, and

brooded the nestlings.

During the breeding season (approximately September

to January) each breeding group occupied a territory

containing dense understory vegetation interspersed with

clear, open ground. In the non-breeding season groups often

wandered outside their breeding territories into nearby

'neutral areas', sometimes joining other fairy-wrens in

temporary feeding aggregations, more permanent 'super-groups',

or in mixed-species feeding-flocks.

Eggs were laid from September to January with a

peak of egg-laying in October. Up to three successful

clutches (of 3 or 4 eggs each: were laid within the one

breeding season and replacement clutches were laid for

unsuccessful nesting attempts. The eggs were laid on

successive days, sometimes with a one-day gap between the

third and fourth egg. Incubation lasted about 14 days and

nestlings fledged 12 or 13 days after hatching. Fledglings

remained partially dependent on other birds for food up to

30 days after leaving the nest.

Rowley found that pairs with helpers produced more

fledglings on average (1.06 independent fledglings per

pair) than did pairs without adult helpers (0.75 independent

fledglings per pair) on each nesting attempt. Rowley

therefore concluded that helpers made a significant

contribution to the reproductive output of the breeders

that they helped and it was suggested that helpers might

enhance their fitness to a greater extent by helping, than

they could by breeding. However, there was little evidence

of a direct causal link between helping behaviour and

increased reproductive output for breeders. Rowley did

suggest however, that the brEeding female, freed from some

of the responsibilities of parental care, was able to direct

her efforts into producing extra clutches.

As with many of his contemporaries in the 1960s,

Rowley viewed social organisation as an adaptation for the

benefit of the species, rather than the individual. With the

synthesis of social behaviour and population genetics

however (eg. Hamilton 1964, Maynard Smith 1964,

Brown 1975, Wilson 1975), social organisations were seen

more as systems brought about by selection acting on

individuals, or groups of closely related individuals,

rather than the species. Several authors (eg. Brown 1975,

Emlen 1978) have since made a re-assessment of Rowley's

data in light of modern theories. In particular, Brown

(1975) used Hamilton's inequality (Hamilton 1964) in order

to determine which strategy, breeding or helping, would

maximise the fitness of an individual. Brown (1975) concluded

that the benefits derived by helpers in aiding close relatives

(ie. through the operation of kin-selection) were insufficient

to explain why some birds chose to help, rather than to breed

and that helping behaviour would have been favoured only if

birds were in some way prevented from reproducing. The most

obvious cause of this reproductive constraint on individuals

was that helpers (mostly males) were unable to find a sexual

partner. Rowley found that females tended to disperse more

frequently, and further, than did the more sedentary males.

As a consequence, it was suggested that females suffered

a higher mortality rate than males. Some males would there-

fore be unable to obtain a mate and, unable to reproduce

independently, would benefit most from remaining in the natal

group. Males that remained as helpers might improve their

fitness through better survival, helping to raise additional

siblings, and by gaining experience. Helpers were therefore

seen as birds which had been prevented from breeding by a

- 13 -

lack of sexual partners, and helping behaviour as a means

of enhancing their inclusive fitness while remaining in the

natal group.

1.4 Objectives of this study.

In general, empirical studies and long-term

observations of co-operative breeding species still lag

behind the development of theory. Further developments

may come most easily with comparative studies of related

species (eg. Zack and Ligon 1985a), studies of different

populations of the one species (eg. Reyer 1980, 1984), and

through experimental manipulation (eg. Brown et al. 1982).

In this study I have attempted to answer the

three questions posed in Section 1.2 which address the

adaptive significance of co-operative breeding in Superb

Fairy-wrens. A colour-banded population was monitored over

a four year period and particular attention was paid to the

demography of the population, the behaviour of helpers at

the nest, and factors which influenced the reproductive

success of breeders. The following hypotheses were

considered throughout the study; (i) that delayed dispersal

and breeding is only favoured when the probability of

successful dispersal and establishment is sufficiently

low, and (ii) that helping behaviour is an adaptive trait

which benefits both the helper and the breeder.

Fig. 1.1 The location of Armidale in relation to the

distribution of the Superb Fairy-wren Malurus cyaneus

(Blakers et al. 1984).

tf' A rmidale

CHAPTER 2

GENERAL METHODS AND STUDY SITE

The study was conducted from September 1982 to

February 1986 at Eastwood State Forest (Fig. 2.1), 10km

south-east of Armidale, New South Wales (30° 35' S, 151P

44' E). Situated on the New England Tablelands, the

altitude of the study-site varied from about 975m to 1036m

above sea-level.

Table 2.1 shows the monthly rain

Armidale during the study period. Figures are taken from a

local newspaper 'The ArmidalE Express' 2nd January, 1986.

Total annual rainfall averaged about 800mm with most rainfall

being recorded in late sprin g and summer (October to

February). However, the firE;t year of the study coincided

with an extreme drought in the area and only 462.8mm were

recorded from January to December in that year. Approximately

average rainfall was recorded for the remainder of the study.

Summers in Armidale are warm with January temperatures

ranging from 140 C (mean daily minimum) to 280 C (mean daily

maximum). Winters are cold by Australian standards with

July temperatures ranging from 0 - 14° C, light snow was

recorded (mostly in July) each year.

Eastwood S.F. is a remnant patch (about 200ha) of

open eucalypt woodland dominated by stringybarks (mostly

Eucalyptus caliginosa), with some gums (E. blakelyi and

E. viminalis) and boxes (E. :nellidora) also present.

fall recorded at

- 16 -

There was little understorey vegetation present with only

5% cover at the 1 - 5m level being recorded by Ford et al.

(1985). Grasses, woodpiles, fallen trees, patches of eucalypt

regeneration, acacias (mostly Acacia filicifolia) and black-

berry brambles (Rubus vulgaris comprised the bulk of the

understorey vegetation or thicket present. The study-site

was surrounded by partially cleared grazing lands.

A description of the bird community at Eastwood

is given by Ford and Bell (1982) and Ford et al. (1985).

Fairy-wrens were captured with mist-nets, or taken

from the nest, and banded with numbered aluminium bands

supplied by C.S.I.P.O. and, later in the study period, by

the Australian N.P.W.S. Bird and Bat Banding Scheme.

Throughout the thesis individuals are referred to by the

number on their bands (eg. 92733, 30824 etc.) Individual

combinations of plastic colour-bands were also given to

birds which could be accurately sexed on the basis of

plumage (see Rowley 1965) enabling visual identification

of individuals.

Censuses were made of each group of fairy-wrens

once or twice monthly in the non-breeding season (March

to August) and weekly in the breeding season (September to

February). During the censu3 of each group its position

was recorded relative to a 50m by 50m grid and then mapped.

Individual group members were identified and the number of

new birds or immatures in the group counted.

Further details of specific methods are given in the

relevant chapters.

- 1 7 -

m

Table

2.1

Mon

thly

rain

fall (in

mm

) re

cord

ed at

Arm

idale

, N

.S.W

. (fr

om

"Th

e A

rmid

ale

Expre

ss"

2 Jan., 1986 .--

Yea

r Ja

n

Feb M

ar

April M

ay J

un

e

Ju

ly

Au

gS

ept

Oct

Nov

Dec

An

nu

al Tota

l

1982

82.4

55.2

71.2

16.6

7.6

14.8

11.8

0.6

40.2

92.2

9.8

60.4

462.S

1983

66.0

55.8

35.8

137.0

137.4

32.4

37.8

24.4

79.6 111.6

55.4 138.0

914.2

1984

198.6 117.2

45.0

63.4

15.4

28.4

108.0

37.6

46.4

75.2 117.4

40.8

893.4

1985

41.4 100.4

41.0

83.6

23.2

42.6

57.4

70.4

56.4

95.8

26.6 118.0

756.8

1986

21.0 39.4

--

--

--

--

--

-

Ave

rage

81.9

73.6

48.3

75.2

45.9

29.6

53.8

33.3

55.7

93.7

52.3

89.3

747.8

Fig. 2.1 Eastwood State Forest, near Armidale N.S.W.

Contour intervals at 15m, broken lines indicate drainage

channels, open circles indicate dams.

CHAPTER 3

POPULATION BIOLOGY AND SOCIAL ORGANISATION

3.1 Introduction

Early studies of animal societies relied on

correlations between aspects of the environment and the

type of social organisation adopted by different species

(eg. Crook 1965, Lack 1968, Emlen and Oring 1977).

Although these correlations provided many insights into

the ecological factors which shaped social systems, they

were insufficient in themselves to explain how certain

social systems had evolved. In the study of co-operative

breeding, for example, simple models based on environmental

influences have proved inadequate. Although some authors

(eg. Selander 1964, Brown 1974, Woolfenden 1975, Ricklefs

1975) stressed the importance of stable and predictable

environments, others (eg. Rowley 1965, 1968, Fry 1972) saw

co-operative breeding as an adaptation to harsh and

unpredictable environments.

Recent studies have concentrated on the demography

of the species concerned in attempts to explain the adaptive

significance of co-operative breeding (Brown 1978, Emlen

1982a). In the field, long-term studies have attempted to

quantify the factors which influence the reproductive

options of individuals (eg. Rowley 1981, Woolfenden and

Fitzpatrick 1984). Some experimental testing of hypo-

theses has begun (eg. Brown et al. 1982) and comparisons are

- 20 -

being made between co-operative and non-co-operative

breeders (eg. Zack and Ligon 1985a, 1985b).

In this chapter, the major aim has been to

quantitatively describe the demography of fairy-wrens at

Eastwood. The basic social organisation of the fairy-wrens

is also described and much of the data collected and

analysed form a basis for later chapters. This chapter

also relates directly to the first of the three questions

posed in Section 1.9, ie. Why do some birds delay their

dispersal from the natal group? An attempt is therefore

made to describe how and when individuals disperse,

obtain breeding positions within the population, and which

factors may influence the probable success of dispersing

birds.

3.2 Methods

The study-site, banding and identification of

individuals was described in Chapter 2.

At least once each month between January 1983

and December 1985 all groups of fairy-wrens were located

and individuals identified. The presence of immatures

and unbanded birds was recorced and missing group members

noted. Immature birds were deemed to be adult in the

September following their flEdging. Group composition

was then determined from the census date nearest to the

beginning of each month. Some data on group composition

were also collected in 1982.

- 21 -

3.3 Results

Although the study area was searched regularly,

fairy-wrens were rarely recordedoutside of a few areas

during the breeding season. These areas typically

contained patches of brambles, sapling re-growth, woodpiles

and fallen trees (see Chapter 4.)

The total number of fairy-wrens recorded in

monthly samples ranged from 13 to 56 birds and a mean

monthly estimate of 36.2 birds (±10.8) was calculated.

There were generally fewer birds recorded during the winter

months (Table 3.1) but this was probably due to the wider

ranging movements of groups at this Lime (see Chapter 4).

The total number of fairy-wrens present in each breeding

season was tabulated according to age, sex, and social

status (Table 3.2). On occasions, censuses recorded groups

without breeding males or females and so the number of

breeding adults present did not always match the number of

groups present (eg. in November 1982). The age structure

of the population (Fig. 3.1) changed as immatures were added

to the population, and as the number of immatures declined

due to mortality and dispersal. In September, all immatures

present were re-classified as adult birds.

Group 0 showed a nuTiber of features typical of

the changes that occurred in group composition (Fig. 3.2).

Group 0 consisted initially of a breeding pair (30840 M and

30839 F) and two female helpers (308:38 F and 30846 F).

After the disappearance of the breeding male, the breeding

- 22 -

position was filled by a male (44875 M) who arrived,

together with a subordinate male (44877 M) from an unknown

origin. The subordinate male remained as a helper until

December 1983 after which it was not seen again. An unbanded

female (later banded as 44844 F) arrived in the group and

also acted as a helper but the origin of this female was not

known. The two unrelated helpers (44877 M and 44844 F)

left the group in October, and made several unsuccessful

attempts at nesting together before returning to the group

in January 1984. 44844 F left again but returned in February

to fill the vacancy created by the disappearance of the

previous breeding female. In June and July 1984, both

breeding birds in Group 0 disappeared and were replaced by

a breeding pair from a nearby group (Group P). Shortly

after the arrival of this pair (30842 M, 56499 F) a resident

immature male (44824 M) left to form a new group with an

immigrant (unbanded) female. It is possible that the

immature male had been usurped in his position by the older

immigrant male, and may otherwise have inherited the

territory.

Another group history, that of Group M, is also

illustrated in Fig. 3.2. Group M consisted initially of a

breeding pair (30884 M, 18874 F) and a single male helper

(18872 M). The male helper inherited the breeding position

(and was joined by an immigrant female from Group L) after

the disappearance of the breeding pair in July 1983. When

the breeding male (18872 M) also disappeared in April 1984,

the breeding position was inherited by a young male (44870 M).

- 23 -

The pairing of this male with his mother was the only

recorded instance of close inbreeding during this study.

The relationship between breeders and helpers was

known positively for only 11 helpers (9 males and 2 females).

Most helpers were the offspring of at least one member

of the breeding pair - the only exception being a male

helping at the nest of an older male sibling (Table 3.4).

Helpers were most frequent in the 1982/83 breeding

season when from 55.6% to 83.3% (from monthly census records)

of all groups had helpers with male and female helpers

being about equally common. Fewer helpers were present in

the 1983/84 breeding season when from 0.0% to 28.6% of

groups had helpers, but again there were about equal

numbers of male and female helpers present. In both the

1984/85 and 1985/86 breeding seasons helpers were of

intermediate frequency relative to other years when from

22.2% to 62.5% of groups had helpers, of which virtually

all were males. The only female helper present in the

1984/85 or 1985/86 breeding season left shortly after the

start of the breeding season.

Group composition was further tabulated during

the first nesting attempt of each pair. A total of 31.4%

of pairs, in all breeding seasons, had helpers present at

their first nest of the season but there was rarely more

than one helper present in any group (Table 3.5).

If it is assumed that the sex-ratio at fledging is

1.00, then the number of first-year helpers present in each

breeding season can be expressed as a percentage of the total

- 24 -

number of potential helpers (ie. fledglings from the previous

breeding season) present in the population. If it is also

assumed that the mortality rate of immatures, over the non-

breeding season, was similar in each study-year, then this

figure represents the proportion of offspring that remained

as helpers (Table 3.5).

Fairy-wrens of known origin (ie. those banded as

nestlings or immatures) could be aged accurately but many

birds could only be assigned minimum ages based on the

number of years they were seen in the study-site. In total,

31 breeding males and 24 breeding females could be aged at

the beginning of each breeding season (Table 3.3).

Although females were more likely to breed in their first

year than males (X 2 = 7.5, p < 0.01), this result is likely

to be biased since most newly breeding females were of

unknown origin and therefore could not be aged. Eleven

male helpers could be aged at the first nest of each

breeding season. The majority of male helpers (10 out of

11) were one-year old and the other was two-years old.

However, of the 7 male helpers that could not be aged

accurately, 3 were known to he at least two-years old and

1 was at least three-years old. Therefore, in total, 17 male

helpers were at least one-year old, 4 were at least two-

years old, and 1 was at least three-years old.

Although none of the 9 female helpers could be

aged accurately, 3 were present as helpers for two years in

succession, making them at least two-years old.

- 25 -

Dispersal could only be recorded when known

individuals were again located in another area after an

absence from their former group. In Table 3.6 dispersal

events are tabulated separately for both sexes and for

breeding birds and helpers. Few examples of dispersal by

breeding males were recorded and in both cases the two

males (31167 M and 30842 M) moved to fill a breeding

vacancy in a nearby group just prior to the start of the

breeding season. In contrast to the relative sedentariness

of the males, however, 11 departures by breeding females

were recorded. These movements occurred throughout the

year but predominantly in July. Five of the females that

dispersed (18830 F, 56498 F, 56009 F, 18874 F, 30849 F)

moved after the absence and presumed death of their mate,

five females (44844 F, 44900 F on three separate occasions,

and 30867 F) left their mates;, and one (56499 F) moved

together with her mate, to breed in a nearby unoccupied

territory.

Dispersal by helpers was most frequent just prior

to, or early in, the breeding season (Table 3.6). Of the ten

movements recorded for male helpers, nine occurred when they

left their previous group to become breeders - forming new

groups in unoccupied patches of habitat. Most of the

females that joined these males were unbanded and had

presumably come from outside of the study area. Only once

was a male helper (30837 M) seen to leave one group and

join another as a helper.

-

Seven dispersal movements were recorded for

female helpers of which two (78221 F, 56648 F) formed new

groups within the study-area with males that had also

previously been helpers, two (30881 F, 30858 F) left the

study area completely. One female (30858 F) had also

moved earlier, along with a breeding female, to join a

nearby unpaired male.

The distances moved by birds in dispersal

movements were calculated in terms of territory diameters

(approximately 100m). A movement to an adjacent territory

was therefore counted as a movement of 1 territory diameter.

Male fairy-wrens rarely moved more than 1 territory diameter

in distance, but many females. moved 3 or more territory

diameters when dispersing (Table 3.7).

The rate of dispersal by helpers to areas outside

the study area could not easily be determined because of the

likelihood of mortality and the size of the area to be

surveyed. However, an estimate of the sex ratio of

dispersers and the timing of dispersal movements can be

made if it is assumed that bards arriving in the study-

site, after dispersal from outside areas, are similar in

these respects to those leaving the study-site. During

the three years a total of eight male arrivals in the study-

site were recorded, seven of these just prior to the start

of the breeding season (Table 3.6). Four of the males

either replaced absent breeding males, or formed new

breeding groups of their own. Two of the male arrivals

disappeared after a short period, one arrived as a helper

- 27 -

with a dominant male, and one immature male arrived, with

a small group of others in MErch, before forming a new

group in October.

In contrast to the number of male arrivals, a

total of 27 females arrived in the study-site during the

three-year period. Most femE:les arrived in the population

just after, or just before each breeding season. In

contrast, adult male arrivals were only recorded in the

months just prior to the breeding season (Table 3.6).

Fourteen of the female arrivals replaced absent

females in breeding groups, usually within a week after

their disappearance, or formed new breeding groups with

unpaired males. Thirteen females arrived as subordinates

and remained so until the breeding season. Many of the male

and female arrivals, particularly those arriving during

the non-breeding season, entered the study-site as pre-

formed groups. These groups then attached themselves to

existing groups until the beginning of the breeding season.

During the formation of breeding groups in July and August

some of these birds remained to replace a missing breeder

while others were not observed again.

A total of 120 nestlings were banded in the 1982/83,

1983/84, and 1984/85 breeding seasons of which 103 (85.8%)

fledged successfully. Of these 103 fledglings, only 16

(15 males and 1 female) were still present in the study-site

in the following September. Six of the males were in

breeding positions, three after inheriting the breeding

position in their natal group and three after dispersal.

- 28 -

Nine males were still in their natal group as helpers as

was the single female still present. However not all

nestlings were banded and these figures may be an under-

estimate.

For males which were still present as helpers

in their natal group, breeding positions also arose during

the breeding season either through the disappearance of

the breeding male within the group, or through the

occurrence of vacancies in nearby groups. Of all the

helpers recorded, half (11 out of 22) of the males

remained as helpers throughout the breeding season.

Of those that found breeding positions, seven dispersed

and one inherited within the group. Three of the male

helpers disappeared, presumably dying or dispersing to an

area outside of the study-site. However, few females (4

out of 15) remained as helpers throughout the breeding

season and most either left their natal group (6) or

disappeared (5).

Annual mortality rates were calculated for known

birds according to sex and status (Table 3.8). Most of

the mortality recorded for breeding males, breeding females

and helper males occurred during the months of May, June

and July (Fig. 3.3). Female helpers were not included in

the analysis because of the possibility of dispersal to

areas not covered by censuses. The annual mortality of

breeding males (55.6%) and breeding females (56.0%) did not

differ significantly, nor did the mortality of breeding

or helping (50.0%) males (al± Chi-square tests n.s.).

Although mortality rates for immatures were

difficult to calculate because some would have dispersed

a minimum survivorship rate for immatures was calculated for

the period from March (when the last nestlings of the

breeding season had fledged) to the following August (after

which immatures were classified as adults). In August 1983

only two out of 11 (18.2%) immatures produced in the 1982/83

breeding season were still present in the study-site. In

August 1984 a total of 10 out of 22 (45.5%), and in August

1985 a total of 6 out of 11 154.5%), immatures were still

present in the study-site. The average survivorship (40.9%)

was significantly lower than the 65.8% achieved by adults

over the same period (X 2 = 4.1, p < 0.05).

3.4 Discussion

The Superb Fairy-wren is a typical 'helper-at-

the-nest' species in which a single monogamous pair may be

accompanied during the breeding season by non-breeding

helpers, usually their offspring from previous generations.

The construction of group histories and genealogies

enabled the relationships between helpers and breeding birds

to be determined in a number of cases. All helpers of

known origin, except one, were the offspring of at least

one member of the breeding pair, and often the offspring

of them both. The only exception was the case of a male

helper (18837 M) attending the nest of his older brother

(92733 M). All male helpers were therefore related to the

breeding male at least. However, the two female helpers

- 30 -

for which genealogies could be constructed were both the

offspring of the breeding female of the group, but unrelated

to the male. Although the data are few for female helpers,

there is the suggestion of a correlation between the sex

of the helper and the sex of the breeding bird to which

it is related. There are several reasons why this may be

advantageous for both the breeders and helpers. For male

breeders, at least, their inheritors as breeding birds are

almost certain to be closely related (ie. their sons), thus

ensuring some greater chance of successful breeding for

their offspring and thereby improving their own fitness.

However, this argument cannot be applied to female helpers

as they did not inherit breecing positions within their

natal group. Similarly, female helpers cannot be seen as

being potential mates for male breeders because of their

pre-breeding dispersal (cf. Rowley 1981). In addition, no

evidence was collected which might suggest that super numerary

females breed while acting aE: helpers. Only a single nest

was ever discovered in a territory at any one time and female

helpers were never observed incubating eggs or brooding

nestlings. Furthermore, the number of eggs present in any

nest never exceeded four (except when a fifth egg was found

to be a cuckoo's egg); making it unlikely that more than one

female laid in any one nest. Rowley (in Emlen and Vehrencamp

1983) found that in M. splerLdens, supernumerary females may

also build nests and lay eggs within the same territory as

the dominant breeding female. These females may have been

mated either with the dominant breeding male or one of the

male helpers if these were present.

- 31. -

The degree of relatedness between Superb Fairy-wren

helpers and the offspring that they helped to raise could

only be determined for those birds of known origin. When

helpers are the offspring of both parents, for example, they

have a minimum relatedness coefficient of 0.5 and when they

are the offspring (or sibling) of only one of the parents

their coefficient of relatedness is 0.25 (eg. Emlen 1978).

The average relatedness of helpers to offspring at Eastwood

was therefore calculated to be 0.34. Rowley calculated a

similar coefficient (0.36) for M. splendens but no figure

is available for his study of M. cyaneus at Gungahlin.

Woolfenden and Fitzpatrick (1984) compared the

average relatedness (r ) of Florida Scrub Jay helpers with a

figure calculated by the folThwing equation :

p r . S x T

Where r between full sibs is 0.5, S is the annual

survivorship of adults and T is the age of the helpers.

This equation gives an average relatedness between helpers

and offspring which would be predicted if helpers remained

within their natal groups, regardless of the degree of

relatedness between them and their beneficiaries. Using this

reasoning, Woolfenden and Fitzpatrick (ibid. pp 86-89) showed

that the average relatedness between Florida Scrub Jay helpers

was almost exactly the same as predicted, suggesting that the

helpers remained regardless of their degree of relatedness

to the breeders or their offspring.

Using the method of Woolfenden and Fitzpatrick

(1984), the predicted degree of relatedness among superb

Fairy-wrens (r p = 0.22) was found to be considerably lower

than the actual figure obtained for helpers in the population

(r a = 0.34). This may indicate that helpers were unlikely to

remain in their natal group unless they were closely related

to the offspring of the breeding pair. As pointed out earlier,

helpers were always themselves the offspring (or sibling)

of at least one of the breeding pair (ie. r a was at least

0.25).

Immature: and former helpers acquired breeding

positions either by dispersal_ into unoccupied patches of

habitat or into established (jroups which had lost one

member of the breeding pair. In addition, males also

inherited the breeding position within their natal group

following the death, or disappearance, of the breeding male.

Of the males banded as nestlings, and still present on the

study-site as adults, 60% started their first year of adult-

hood as helpers while 20% inherited their breeding positions

and 20% dispersed and bred outside of the group. During the

breeding season 50% of the male helpers remained as helpers

while less than 5% inherited and about 32% dispersed.

In summary, of the males that survived to their first

breeding season, about 30% remained as helpers throughout

the breeding season while about 23% inherited a breeding

position and 39% dispersed to breed outside of the group.

Given that the remaining percentage of males died before

breeding, then each male has about a 62% chance of breeding

- 3:i -

in his first year. The data for females are quite different

with only a single banded female still present on the study-

site in her first year as a helper in her natal group. Some

females, however, were already helpers at the beginning of

the study and, of these, 26% remained as helpers throughout

the season. The number of female helpers varied between the

breeding seasons, being notably absent in the 1984/85 and

1985/86 breeding seasons.

One of the major d*fferences between male and

female fairy-wrens was their tendency to disperse. Male

fairy-wrens, for example, rarely moved after they had

acquired a breeding position and any movement was almost

invariably to an adjacent unoccupied territory. If the

breeding female disappeared, then the male remained on the

territory and was later joined by a new female. Most of the

newly breeding females arrived within a week of a breeding

vacancy occurring and most were unbanded, indicating that

they probably originated outside the study-site. In contrast,

many females left their terr:_tories at some point in their

life as a breeding adult, us ually to occupy a breeding

position in another group. Cases of 'divorce' in which a

female left her mate to breed with another were not uncommon,

in contrast to other co-operative breeding species where it

is virtually unknown (Rowley 1983).

An estimate of the sex-ratio of dispersing birds

was made on the basis of those birds entering the population.

Females were found to be 3.4 times more likely to enter the

population from areas outside the study-site and this figure

- 34 -

is similar to the estimate of 3.5 female to male arrivals

made on the basis of figures reported in Rowley's study at

Gungahlin. It is likely that dispersal entails some risk.

Birds moving through unfamiliar areas, mostly on their own

or with unrelated birds, miglt be expected to experience a

greater risk of mortality from predation or starvation, than

non-dispersers or those that move with their natal group.

Females often arrived alone just prior to the

breeding season, usually filling a breeding vacancy in one

of the local groups. The speed with which many of these

vacancies were filled suggests that females may continually

monitor breeding vacancies in the population. However, it

was not clear if females were 'floating' in the population

or if they made occasional forays from their natal groups.

In the non-breeding season, dispersive forays may also occur

as a group wanders over its :_arger non-breeding season

home range (see Chapter 4). In other species (eg. Florida

Scrub Jays, Woolfenden and Fitzpatrick 1984; White-browed

Sparrow Weavers Plocepasser mahali, Lewis 1982) it has

been argued that group wanderings may provide opportunities

for birds to monitor breeding vacancies in the population,

while remaining in contact with their natal group.

In two of the breeding seasons (1984/85 and 1985/86)

there was a skew in the sex-ratio of the population with

males being about 1.7 times more abundant than females

at the start of the season. Unfortunately, I was not able

to determine the sex of many immatures prior to their

dispersal as problems were ercountered in trying to locate

- 35 -

groups with immatures in the winter months, and in

identification. Many immatures tentatively identified as

'females' later turned out to be males and it was also

difficult to separate adult: females from immatures on the

basis of plumage. At Gungahlin, however, Rowley (1965)

found similar numbers of male and female immatures. The

skewed sex-ratio might be better explained as a result of

greater female dispersal. IL is suggested that females

suffer higher mortality because of their greater tendency

to disperse. It is also likely that since the Eastwood

and Gungahlin populations were surrounded by (mostly)

unsuitable habitat, then each study-site might be considered

as an 'island'. Since the number of females arriving on

the 'island' might not equal the number leaving, as the

birds disperse outwards from a source, then this may

further reduce the number of females in the population.

Although mortality rates at Eastwood were higher than those

at Gungahlin (data analysed in Rowley 1981), there were

no significant differences in the mortality rate of male

and female breeders. This would also suggest that the

origin of the skewed sex-ratio is in the greater mortality

of females during dispersal, or the imbalance in the number

of female emigrants and immigrants due to the 'island'

effect.

Some of the demographic attributes of the fairy-

wren population seem likely Lo severely curtail the

reproductive opportunities available to potential breeders.

Included in these attributes are the continuous residence of

- 36 -

adults on breeding territories, the 'surplus' production of

offspring relative to the occurrence of breeding vacancies,

and a skewed sex-ratio (Emlen 1984, Emlen and Vehrencamp

1983, Woolfenden and Fitzpatrick 1984, Wiley and Rabenold

1984, among others). The frequency with which breeding

opportunities arise in the population will therefore

depend on (i) the current level of resource monopolization

by breeding birds (particularly if territories and/or

mates are in short supply), (ii) breeder mortality, and

(iii) competitor natality and survivorship (Brown 1969,

1974, Ricklefs 1975).

In the following model (adapted from Woolfenden

and Fitzpatrick 1984) some of the factors which may

promote delayed dispersal among potential breeders are

considered in more detail.

Individuals are expected to follow a delayed

dispersal strategy ( h ) when the expected lifetime

fitness (Wh ) gained from suct. a strategy is greater than

that expected from an early ddispersal strategy (W 0 ), ie.

when :

Wh > W o

and when :

ph Rh > L 0 . Do . Ro

where lifetime fitness ( W ) = Li .Di .R and Li = the

probability of survival to dispersal age i, Di = the

probability of successful establishment as a breeder at

age i, and R = the total number of offspring expected from

the first year of breeding onwards.

37 -

The equation can bE . expanded by the inclusion of

the term ( ! ) which is a meEsure of the additional fitness

gained through helping behaviour; ie. through the

production of extra relative: (Vehrencamp 1979, Brown

1980), thus ;

L . . (R h -I k ) > Lo.D0.R0.

The ratio between the survivE.1 of late versus early dispersers

(ie.L.=Lh/Lo) can then be included in the equation thus ;

L 1 (Dh.Rh + Fz ) Do.R0-

The ratio between the probabilities of successful late

versus early dispersal (I) can also be defined (ie. I =

D h/D o) and the ratio between the lifetime reproductive

success of early versus late dispersers (ie. R = R0/Rh).

If both of these terms are included in the equation, then

it can be expressed in the following form ;

(1 - L i . I) D 0 < L i . f2/R.

The cost of delayed dispersal is therefore reduced by the

inclusion of I in the equation. If the term (1 - Li.I)

is negative (ie. I is large), then delayed dispersal is

favoured under all permutations of L i , K, D 0 and R because

the right-hand side is always greater than, or equal to,

zero. The equation in this form produces a very interesting

result; delayed dispersal can be favoured even in the

absence of any indirect benefits from helping behaviour

(ie. k = 0). It is even possible for k to be negative,

if I is sufficiently large, and delayed dispersal to still

- 38 -

be favoured. The model in the form expressed above predicts

that delayed dispersal should be favoured over earlier

dispersal and breeding when ;

(i) I is greater than 1.0 ; ie. when delayed

dispersal results in a greater probability of success at

acquiring a breeding position. Among Superb Fairy-wrens

this condition is most likely to be met when the probability

of early success is low (due to a lack of breeding vacancies

or mates in the population) or when success can be enhanced

by remaining for a period in the natal group (as in

situations where inheritance can occur).

(ii) Li > 1.0; ie. when the survivorship of

late dispersers to the age of first breeding (eg. the first

year) is greater than the survivorship of earlier dispersers

to the same age. In other words, delayed dispersal will

be favoured if the survivorship of individuals in their

natal group is greater than their survivorship outside of

the group. This should most obviously be the case for

immatures that spend their first winter with their natal

group. For older birds, however, and particularly for

males, this advantage might diminish in relationship to the

cost of non-breeding.

(iii) When (z > 0; ie. when non-dispersers

are able to enhance their indirect fitness through the

production of extra relatives. This condition would only

be met at Eastwood when helpers were able to enhance the

reproductive success of the breeders and this point is

considered in Chapter 6.

- 39 -

In summary, there are several reasons why non-

dispersal by potential breeders may be favoured. In this

Chapter some aspects of the population biology of fairy-

wrens at Eastwood have been discussed. In particular I have

suggested that the probability that individuals can acquire

a breeding position within the population is an important

influence on the costs and benefits of delayed dispersal.

If potential breeders are unable to obtain suitable

breeding positions, then they may benefit from remaining

within the natal group until conditions improve. Two of

the most important benefits to delayed dispersal are

(i) potentially better survivorship, and (ii) the

opportunity to inherit a breeding position within the

natal group itself. In addition there are several factors

which may lower the probability of successful establishment

by potential breeders including (iii) a lack of breeding

vacancies within the population because of a 'surplus'

production of offspring in previous seasons, and (iv) a

lack of sexual partners (in this case females) due to

differential mortality of the sexes at some stage of their

life-history.

Table

3.1

Mon

thly

cen

su

s r

ecord

s o

f fa

iry-w

ren

s a

t E

astw

ood S

.F.

Year

Cate

gory

Jan

Feb

Mar

Apri

lM

ayJu

ne

Ju

lyA

ug

Sep

tO

ctN

ovD

ec

1982

Nu

mber

of;

Adu

lts

--

--

--

--

4442

5554

Imm

atu

res

--

--

--

--

40

02

Un

know

ns

--

--

--

--

10

20

Tota

l Popu

lati

on

--

--

--

--

4542

5756

Nu

mber

of

Gro

ups

--

--

--

--

1214

1816

1983

Nu

mber

of;

Adu

lts

4940

4044

3916

2629

3334

3230

Imm

atu

res

02

138

101

50

00

1721

Un

kn

own

s0

00

01

00

10

00

0

Tota

l popu

lati

on

4942

5350

5020

3030

3334

4951

Nu

mber

of

Gro

ups

1512

1213

125

911

1415

1514

Table

3.1

(Con

t'd)

Year

Cate

gory

Jan

Feb

Mar

Apri

lM

ayJu

ne

Ju

lyA

ug

Sep

tO

ctN

ovD

ec

1984

Nu

mber

of;

Adu

lts

2823

1723

2023

1417

2524

2229

Imm

atu

res

2824

2228

2326

1311

06

1811

Un

know

ns

00

00

01

10

00

00

Tota

l popu

lati

on

5647

3951

4351

2828

2530

4030

Nu

mber

of gro

ups

1412

911

1013

98

89

99

1985

Nu

mber

of;

Adu

lts

1913

1315

177

2227

2228

2525

Imm

atu

res

1915

11 IIn

n .-2n

0 u..)

00

312

Un

know

ns

20

22

22

20

00

00

Tota

l popu

lati

on

4028

2626

2718

3330

2228

2837

Nu

mber

of gro

ups

96

56

73

77

911

1111

Table

3.2

Com

posit

ion

of

bre

edin

g

gro

ups in

each

mon

th

Bre

edin

g s

eason

Cate

gory

Sep

tO

ctN

ovD

ecJan

Feb

1982/8

3

Nu

mber

of;

Bre

edin

g m

ale

s12

1416

1615

12

Bre

edin

g f

em

ale

s11

1417

1514

12

Help

er

male

s10

713

1313

11

Help

er

fem

ale

s11

79

107

5

Tota

l adu

lts

4442

4954

5940

Tota

l im

matu

res

uu

02

02

Adu

lt s

ex-r

ati

ol

1.0

1.0

0.9

0.9

0.8

0.7

Nu

mber

of

gro

ups w

ith

A CI

1 1

1 f

hP 1

p P

■ r q

7R

10

1111

ln

Tota

l n

um

ber

of

gro

ups

1214

1816

1512

1.

Nu

mber

of

fem

ale

s p

er

male

.

Table

3.2

(Con

t'd)

Bre

edin

g S

eason

Cate

gory

Sep

tO

ctN

ovD

ecJan

Feb

1983/8

4

Nu

mbe

rof;

Bre

edin

g m

ale

s14

1515

1414

11

Bre

edin

g f

em

ale

s14

1515

1414

12H

r.,1

17,r

,,,,a

lnc

-)n .,

n

Help

er

fem

ale

s3

31

10

0

Tota

l adu

lts

3334

3230

2823

Tota

l im

matu

r es

n Vn V

17-)

14.

1-)

0G

U')A 4..

.t

Adu

lt s

ex-r

ati

ol

1.1

1.1

1.0

1.0

1.0

1.1

Nu

mber

of

gro

ups w

ith

adu

lt h

elp

ers

41

11

00

Tota

l n

um

ber

of

gro

ups

1415

1514

1412

1.

Nu

mber

of

fem

ale

s p

er

male

.

Table

3.2

(C

on

t'd)

Bre

edin

g s

eason

Cate

gory

Sep

tO

ctN

ovD

ecJa

nFeb

1984/85

Nu

mber

of;

Bre

edin

g m

ale

s8

99

99

6

Bre

edin

g f

em

ale

s8

99

77

5

Help

er

male

s8

64

33

2

Help

er

fem

ale

s1

00

00

0

Tota

l adu

lts

2524

2219

1913

Tc,L

al im

atu

r es

n L,r

1 0

■,.

1 1

10_..

1

Adu

lt s

ex-r

ati

ol

0.6

0.6

0.7

0.6

0.6

0.6

Nu

mber

of

gro

ups w

ith

adu

lt h

elp

ers

55

32

22

Tota

l n

um

ber

of

gro

ups

89

99

96

1.

Nu

mber

of

fem

ale

s p

er

male

.

Tab

le 3

.2

(Cont

Id)

Bre

ed

in

g s

easo

n

Cate

gory

Sep

tO

ctN

ovD

ecJa

n

1985/8

6N

um

ber

of

;

Bre

ed

ing m

ale

s9

13

1111

11

Bre

edin

g f

em

ale

s8

1211

1111

Help

er

male

s5

33

33

He

1p

er f

em

ale

s0

00

00

i 1.t. cn

Tota

l adu

lts

rri

- 1

J. V L

. a _L

a L.

22

0

28

0

25

3

2525

Ad

ult

sex

-r a

t io

l0

. 60

. 80

.80

.80.8

Num

ber

of

gro

up

s w

ith

help

ers

53

33

3

To

tal

nu

mb

er

of

gro

up

s9

13

1111

11

Feb

1.

Num

ber

of

fem

ale

s p

er

male

.

Table 3 .3 Ales of known individuals at the f irst nest

of all breeding seasons.

Age in years Breed Lngmales

N

Breedingfemales

N

Helpermales

N

He 1perfemale s 1

N

(%) ( % ) (%) ( %)

1 8 16 10 0

(25.3) (66.7) (90.9) (00.0)

2 9 3 1 0

(29. ) (12.5) (09.1) (00.0)

3 + 14 5 0 0

45.2) (20.8) (00.0) (00.0)

Total ofknown age 31 24 11 0

Total ofunknown age 20 27 7 9

1. No female helpers cou ld be accurate ly aged .

- 47 -

Table 3 .4 Summary of the relationships between adult

helpers and breeders .

Re lat ions h ip ofhelper to breeder

Male Female Coefficient

He 1per Helper ofRelatedness(r )

Offspring of ;

both breeders 4 0 0.50

male breeder only 4 0 0.25

female breeder only 0 2 0.25

neither breeder 0 0 0.00

Sibling Of ;*male breeder 1 0 0. 50

female breeder 0 0 0.50

Unknown 11 7

Total 20 9 0.34

* half sibling (r = 0.25 )

- 48 -

Table

3.5

Th

e c

om

posit

ion

of

bre

edin

g

gro

ups a

t th

e

firs

t n

est

of

each

bre

edin

g s

eason

.

Bre

edin

g s

eason

Nu

mber

of

gro

ups

Nu

mber

of

Help

ers

01

23+

Mea

n n

um

ber

Sex-r

ati

o o

fper

gro

up

adu

lt h

elp

ers

Rete

nti

on

of

offspri

ng a

sh

elp

ers

1

Male

s F

emale

s

11982/8

3

15

73

32

1.1

3

9F:1

0M

75-1

00%

75-1

00%

,... co

1983/8

4

15

13

1 1

00.2

0

2F:1

M

23%

11%

11984/8

5

95

31

00.5

6

OF:

4M

15%

0%

1985/8

6

12

93

0 0

0.2

5

OF:

3M

18%

0%

Tota

l51

35

95

20.5

3

11F:1

8M

1.

Esti

mate

d %

of

all p

revio

us s

eason

's o

ffsprin

g r

em

ain

ing a

s h

elp

ers a

t th

e s

tart

of

each

bre

edin

g s

eason

(ie

. S

epte

mber)

.

Table

3 .

6

Th

e t

imin

g o

f dis

pers

al by f a

iry-w

ren

s a

ccord

in

g t

o s

ex a

nd s

tatu

s.

Cate

gory

of

movem

en

tJ

FM

AM

JJ

AS

0N

DTota

l

Bir

ds leavin

g g

rou

ps

Bre

ed in

g m

ale

s0

00

00

01

10

00

02

Bre

edin

g f

em

ale

s1

10

10

14

21

00

011

He

1per

male

s0

00

00

10

34

11

010

Help

er

fem

ale

s0

00

00

02

21

10

17

Tota

l1

10

10

27

86

21

130

Bir

ds a

rriv

ing :

Male

s0

01

00

00

43

00

08

Fem

ale

s3

04

50

14

41

50

027

Un

kn

own

s1

02

01

12

00

00

07

Tota

l4

07

51

27

84

50

043

Table 3 . 7 The distance moved by dispersing fairy-wrens

as measured in territory diameters (1 territorydiameter approximately = 100m) .

Sex of disperser

1

Dispersal Distance(territory diameters)

2 3 4 5 or more

Males

Females

7

2

2

0

0

3

1

0

0

4

Table 3.8 Mortality of known individuals over three years

'January 1983 to January 1986)

Category Number of birds present AnnualMortality

Jan. Jan.next

Breeding males 3( 16 55.6%

Breeding females 11 56.0%

Male helpers 14 7 50.0%

Total 34 54.7%

F ig . 3.1 Change in population age structure from

September 1984 to January 1986.

adults

> 1

yr

old

1st

- y

ear

adults

i mm

atur

es

SN

JM

M

JS

NJ

MO

NTH

Fig. 3.2 The history of two groups (0 and M) during

the study period. Each individual is identified by the

C.S.I.R.O. band number . De at h , or disappearance , of

individuals indicated by 3ar re d line -I ) .

Arrows indicate movements between groups or change of

status .

GRO

UP 0

Cat

egor

y19

82

1983

ON

D J F M

A M

J J

A

1984

1985

S0 N

DJ

F M

A M

J J A SO

ND

J FM

A M

J J A SO

N

D

Bre

edin

g30

840

mal

e

Bre

ed in

g 3

-fe

male

0839

Help

er

mal

e

Help

er

30838

fem

ale

30846

I m

mat

ures

-4 4

4875

-1 1

4-4

844--

----

-j56499-

h

448

77 -

----

-

2 2

1 1

44

844

3 2

4 3

1 14

30842 - -------------------- --- ---

56611-----

4 4

8 6

6

4 4

6

ub

GRO

UP

fem

ale

-I

44

82

4

GRO

UP

GRO

UP

J

2 ub

mal

es

GRO

UP

P

1.

see

text

for

det

ails

4487

0-1

1887

2

-1 3

0989

_I

I

4 3

Bree

ding

3088

4m

aleBr

eedin

gfe

mal

e 18

874

Helpe

r18

872

male

Helpe

rfe

mal

eI m

matur

esOt

hers

3098

9 F

GROU

PL

4 4

4 2

2 3

3 3

5649

3 —

1

u bF I

i

Ub

u bfe

male

fem

ale

GROU

P M

1983

1984

Cat

egor

y J

FM

A

MJ

JA

SO

ND

J F

M

AM

JJ

AS

ON

D

Fig . 3.3 Distr ibut ion of mortality recorded throughout

the year . Data inc lude s breeding males , breed ing females ,

and male helpers. Data from January 1983 to December 1986.

c_

MONTHLY MORTALITY (70 MISSING)

O tv cn Oo 0 Iv -P. 0) oo 0

1

O

CHAPTER 4

HABITAT AND SOCIAL ORGANISATION

4.1 Introduction

The type of habitat occupied by species is one

focus of attention in the study of co-operative breeding.

The habitat saturation model, for example, predicts that

delayed dispersal is only favoured when suitable breeding

habitat is fully occupied by breeding birds.

A number of studies have attempted to measure the

influence of habitat variables, particularly vegetation

structure, and its influence on social organisation. Group

size in co-operative breeding species may be influenced

by vegetation structure (Brown and Balda 1977, Gaston

1978b, Vehrencamp 1978, Craig 1979, Brown et al. 1983,

Zack and Ligon 1985a, 1985b) and the quality of breeding

territories may have an impertant influence on group

reproductive success and the reproductive options for young

birds (eg. Lewis 1981, Zack and Ligon 1985a, 1985b) as well

as adult survival.

An important consequence of any correlation

between group size and some measure of territory quality

(such as food abundance or t:he availability of nest-sites)

is the additional influence of territory quality on

reproductive success. It may therefore be difficult to

separate the effects of territory quality and the presence,

or number, of helpers on reproductive success (Lack 1968,

- 56

Zahavi 1974, Brown 1978, Brown and Brown 1981, Lewis 1981,

Woolfenden and Fitzpatrick 1984, Zack and Ligon 1985a).

In order to understand the evolution of co-operative

breeding systems more fully, it is therefore necessary to

determine the proximate factors that influence social

variables such as group size.

In the present study an attempt was made to

determine how certain habitat variables may influence

the social organisation of fairy-wrens in the Eastwood

population. A number of habitat variables were measured

and tested for correlation with group size. Territory

size was measured and differences in territorial behaviour

in the breeding and non-breeding season are described.

The following questions were considered :

(1) What are the characteristics of territories occupied

by fairy-wrens at Eastwood?

(2) How is suitable breeding habitat distributed through

the study area?

(3) Does the distribution or abundance of suitable habitat

influence the dispersion, demography or social

organisation of the population?

(4) Does habitat influence the reproductive opportunities

of potential breeders?

4.2 Methods

During periodic censuses of group composition,

and at other times when fairy-wrens were encountered, the

location of each group was recorded relative to a 50m x 50m

grid established throughout the study-site. Only one such

record was made for any one group within the same day.

During the occasional times when individual fairy-wrens

were separated from other group members (eg. during

incubation by the female), the location of the breeding

male was used as the group record.

Territorial boundaries and the area covered

by each territory (TAREA) were determined after twenty or

more records of group locat_ons had been made. Territorial

disputes were infrequently observed and no systematic

attempt was made to use thi3 behaviour to determine

territorial boundaries.

The composition and number of fairy-wrens present

in different groups varied within, and between, seasons and

years (Chapter 3). Group size (GSIZE) was therefore

defined as the maximum number of adult fairy-wrens present

on a territory after breeding groups had been formed.

In the 1982/83 an3 1983/84 breeding seasons

a number of habitat variables were measured in each

territory. Variables were chosen on the basis of being

potential determinants of the suitability of territories

for occupation and reproduction. Of particular interest

were features of the habitat used for nest--sites (brambles,

- 58 -

woodpiles and trees), and foraging sites (mostly small trees,

acacias, and open ground, see Chapter 5). No attempt was

made to determine the relative abundance of food (eg.

arthropods) in different terfitories.

The number of trees in twenty 20m quadrats within

each territory was recorded in three separate categories :

NTREE01 = the number of trees (other than Acacia spp) 0 to 5m

in height; NTREE02 = the number of trees (other than

Acacia spp) over 5m in height and; NACACIA = the number of

acacia trees. The presence of acacias (89% of which were

recorded as being less than 5m in height) was recorded

separately since they seemed to be preferred as foraging

sites over eucalypts (H.A. Ford pers comm., also Chapter 5).

A line-intercept method (Lucas and Seber 1977) was

used to estimate the total area covered by woodpiles, fallen

trees and dead branches within each territory (AWOOD) and

the proportion of the territory covered (PWOOD). The total

area of blackberry brambles (BAREA) was measured for each

territory, and the proportion of the territory covered by

brambles (PBAREA) was calculated. A percentage canopy

cover (CCOVER) was estimated visually from forty sample-

points within each territory.

In the 1984/85 and 1985/86 breeding season

measurements were made of group size (GSIZE), territory area

(TAREA) and bramble area pe. ) f territory (BAREA) only.

Habitat variables were tested for correlation with

GSIZE by determining the relevant Pearson correlation

coefficient. Percentile and proportionate variables were

transformed when appropriate using the arcsin transformation

(Sokal and Rolf 1969).

- 59 -

4.3 Results

Fairy-wren territories at Eastwood S.F. were found

in areas of disturbed vegetation usually along forest roads,

on the edge of the forest ad 4 oining cleared land, and

along temporary water courses (Fig. 4.1). These areas

appeared to contain larger areas of dense sapling regrowth,

fallen trees, woodpiles and brambles than the less disturbed

areas of the forest. The distribution of brambles in

particular, coincided with the distribution of fairy-wren

territories (Figs. 4.2, 4.3, 4.4, 4.5).

Table 4.1 presents the values obtained for

measurements of group size, territory area, and the density

of birds in occupied areas. There were no obvious relation-

ships between these measures in comparisons between breeding

seasons.

A summary of the habitat variables measured in

each of the breeding seasons is presented in Table 4.2.

Territory area was inversely related to group size in the

1982/83 breeding season (r = -0.47, p < 0.05), but no

significant correlations bet4een group size and territory

area were found in the other breeding seasons.

An inverse correlation was found between

territory area and the area of brambles per territory

in the 1982/83 breeding season, but not in other seasons.

There were positive correlations between territory area

and several of the habitat variables (NACACTA, AWOOD,

CCOVER) measured in the 1982/83 breeding season (Table 4.3).

- 60 -

A number of changes in the position and size of

particular territories in different years were noted and

can be seen in a comparison of figs. 4.2, 4.3, 4.4 and 4.5.

These changes are summarised as follows :

(1) The extinction of territories after the death or

disappearance of the occupants (groups A, L, in Fig.

4.2; groups Q, C2, Cl, B, V in Fig. 4.3; group J,

in Fig. 4.4).

(2) The creation (or re-creation) of new territories

when breeding pairs occupied a formerly vacant patch

of habitat (groups Q, X, in Fig. 4.3; groups V, W, B,

Q in Fig. 4.5).

(3) The extension of territorial boundaries to encompass

vacated patches of habitat (groups F, H, V, R, T in

Fig. 4.3; group 0 in Fig. 4.4).

(4) The splitting of a single territory into two smaller

territories (groups H1, H2 in Fig. 4.2; groups Cl, C2

in Fig. 4.3; groups Ml, M2 in Fig. 4.5). The

circumstances surrounding the split of territory H

into two separate territories in the 1982/83 breeding

season (Fig. 4.2) were unknown. However, in both

other cases (group C in 1983/84 breeding season,

group M in 1985/86 breeding season) the split occurred

when a male helper and a newly arrived female took

over part of the territory of the group for their

exclusive use. In both cases the areas used by the

newly formed pair were on the periphery of the former

single territory and did not contain substantial areas

- 61 -

of brambles. Neither of the newly formed pairs

produced independent fledglings and in one case (group

C2) the male returned to his former group at the

conclusion of the breeding season, after the

disappearance of his maze.

Territorial behaviour differed in the non-breeding

season (March to August) from the pattern found in the

breeding season (September to February). Territorial

boundaries were less well defended in the non-breeding

season as fairy-wrens foraged over a larger area, often

leaving their breeding territories for much of the day and

only returning in the evening to roost. It was common for

several groups of fairy-wrens to forage together in

temporary feeding aggregations, or in more permanent

'super-groups' (Rowley 1965) and mixed species flocks

were frequently observed.

Each of the habitat variables measured in the

1982/83 breeding season was tested for correlation with

GSIZE (Table 4.4). The significant habitat variables

(BAREA, and PBAREA) were both measures of the amount of

blackberry brambles present within a territory.

In the four breeding seasons, nine groups of

fairy-wrens were found on territories without significant

(ie. greater than lm in area) patches of brambles. All of

these groups were simple pairs without helpers.

Although several of the habitat variables

measured features of the habitat that seemed typical of

fairy-wren territories (eg. dense sapling growth, the

presence of acacias and woodpiles) none of these other

variables was correlated with GSIZE. A backward - stepping,

multiple-regression analysis (Nie et al. 1975) using all

habitat variables again showed the importance of brambles

as a determinant of group size (GSIZE = 2.55 + 1.06 PBAREA,

F(1,17

) = 19.30, P < 0.001).

In the 1984/85 breeding season a positive, but

non-significant correlation tetween GSIZE and BAREA (r = 0.60 ,

0.05 < p < 0.10) was also found but in the 1983/84 and

1985/86 breeding seasons the number of groups with helpers

was too few to make compariscns.

In order to determine why territories with a

large area of brambles present generally held larger groups,

a test of the survivorship of fairy-wrens was made.

Territories were divided intc, groups containing small (0 to

10m2 ), medium (10 to 100m2 ) End large (over 100m2 ) areas

of brambles. The percentage of fairy-wrens surviving from

one breeding season to the next was then calculated for each

of the three categories (Table 4.5). Although a trend

towards higher survivorship was seen with increasing

bramble size, the differences; between categories were not

significant (X2 = 3.9, P > 0.05). However survivorship

is only one of the possible determinants of group size and

others such as reproductive success of the breeders and

the frequency of dispersal of immatures a