IBIS I 34; 128-1.3 3

Regulation of the period of postfledging dependence in the Spanish Imperial Eagle Aquila adalberti

MIGUEL FERRER Estacion Biologica de Donana CSIC, Avda. Maria Luisa, Pabellon del Peru, E-41013 Sevilla, Spain

From r986 to 1988, 30 Spanish Imperial Eagle Aquilaadalhertinestlings, equipped with radio transmitters, were studied in Donana National Park, southwestern Spain, to evaluate factors that determine when the young become independent. The results support the view that the dependence period is composed of two different periods, each representing almost half the total, and so the variations produced in one affect the overall period. The first part of the dependence period begins with leaving the nest and ends with the first soaring flight. The duration of this period is related to the physical condition of the young. The second period extends from the first soaring flight to the date of independence. Its duration is strongly related to hatching date. The young are not forced to become independent because of parental aggression. Aggressive behaviour allows the parent to evaluate the flight capacity of the young.

For some time after a chick leaves the nest, it depends on its parents, especially for food. For adults, the optimum length of the dependence period is that which maximizes their net lifetime reproductive success. For the offspring, the optimum length is that which maximizes their probability of surviving to reproductive age, even if it means a lower reproductive success for the parents. The interests of parent and offspring thus may come into conflict (Trivers 1974).

Birds of prey show considerable variation in the length of the dependence period and also in some of the factors which determine its end (Sherrod 1983, Johnson 1986, Konrad & Gilmer 1986, Alonso et al. r987, Bustamante & Hiraldo 1989, r990). According to some authors, the adults control the length of the dependence period by reducing food and by aggressive behaviour towards the young (Alonso et al. 198 7) independently of the capacity of chicks to fend for themselves. This was thought to occur in the Spanish Imperial Eagle Ayuila adalberti (Alonso et al. ry87) , suggesting that the chicks’ maturation of flight does not affect the Iength of the dependence period. It has been proposed for other species that the dependence period is determined by the young, whose interest in adults disappears when they are able to fend for themselves (Brown 1966), or when they are obliged to migrate (Bustamante & Hiraldo 1990).

The aim of the present study was to identify factors related to the length of the dependence period in the Spanish Imperial Eagle. and to evaluate the roles of adults and young in the independence process.

METHODS

The study was carried out in the Doiiana National Park, southwest Spain, in 1986, 1987 and 1988. The population of Spanish Imperial Eagles a t Donana consisted of I 5 pairs

which produced 42 nestlings during the three years. Of these, j o were equipped with solar radiotransmitters (HSPB J 4003 x A from Wildlife Materials Inc.), fixed on the backs of the nestlings by a harness (Kenward 198 7). The set weight of transmitter and harness ranged between 28 g and 37 g (approximately r % of the bird’s weight). Nestlings were also marked with PVC coded rings: 8 nestlings were marked in 1986. 8 in 1987, and 14 in 1988. Three other nestlings which were not equipped with transmitters, but whose siblings were marked, could be studied during the depen- dence period. In total 5 chicks from broods of one young, 16 from broods of two and I 2 from broods of three were studied. The sex ratio was I 5 males and 18 females. The transmitters were attached when nestlings were between 50 and 70 days old.

At the same time a 2-111111~ blood sample was extracted from the radial vein of the wing. I used the level of urea in the blood to evaluate nutritional condition. As is known for many bird species, including Spanish Imperial Eagles, when tissue protein sources are mobilized by starvation. provoking an increase in nitrogenous excretory components in the blood, the urea value increases (Ferrer et ul. I 987, Garcia- Rodriguez et al. I 987a. Ferrer I99oa, b). Blood samples were extracted between I roo h and I 500 h to minimize circadian variations of the blood parameters (Garcia-Rodriguez et al. 1987b. Ferrer 199oa). Analyses were carried out with a Hitachi 705 multichannel automatic analyser.

The age of the chicks was known in every case by previous checks of nests. Observations were started when the young left the nest, defined as the first time that a young bird was observed flying, or seen on a perch inaccessible from the nest, and were concluded the first time that the young did not sleep in the parental territory.

During this period, each young was located by triangula- tion at least once every 2 days. All the nests were observed for

128

I Y Y 2 POSTFLEDGING DEPENDENCE I N SPANISH Ih4PERIAL EAGLES 129

4-8 complete days, during which time as much visual contact was kept of the young as possible; all their activities, as well as the adults’, were recorded on a tape recorder. Observations were carried out using 10 x 40 binoculars and a 25-40 x magnification telescope from a variable distance depending on the nests, but always greater than 300 m. Observations were timed with a stopwatch and triangulation was repeated every 10 minutes if visual contact with the young was lost.

During observations, variables related to the flight were recorded (pattern, frequency and length) and to the interac- tions with adults (frequency and distance of food deliveries, aggressive interactions, presence of an adult, presence of two adults, flights directed to adults and flights directed to young). Aggressive behaviour by adults to independent immature birds which came into the area and similar behaviour towards other birds of prey, was also recorded. Observations from complete days with a visual contact period of at least 6oo min were considered as valid. In total, for all the young the valid observation time was 953 hours and 42 minutes. For calculation of correlation coefficients brood means were used.

RESULTS

The average age at independence was 127 days (range I IO-

r47 days; n = a g ) . The average length of the dependence period was 5 I days (range 35-67 days; n= 22).

Flight development

The duration of all flight variables measured increased significantly during the dependence period (Table I). The appearance of the first soaring flight, which occurred on average 2 5 days after leaving the nest (range I 5-36 days: n = 22) is related significantly to the number ofsiblings (Table 2). being earlier in larger broods, and is not affected either by the hatching order or by sex (Table 2). The appearance of the first soaring flight was not influenced by the hatching date (ri5 =0.259, P = 0 . 3 $2). but there was a significant positive

Table 2. Efects of different factors on thr length (in dnys) of the period from lraving the nest to the first soaring flight in the Spanish lmprrial Eaglr

Average

Year 1986 24.7 I987 26.3 1988 24.0

Brood size I 32.8 2 24.3 3 20.5

Hatching order T 22.8 2 25.6

I 18.3 2 20.6 3 21.5

Sex Male 24.3 Female 25.3

n F d.f. P

6 6 2.47 2, 21 0.783

10

5

8 II 11.712 2, 21 <O.OOI

2 0.916 T, 9 0.37 3

3 3 0.937 2. 5 0.45 I

2

I1

1 5 0 . 1 59 1 , 22 0.698

relation with the urea level in the blood extracted at the end of the nesting period (r,,=o.564, P=0.035, Fig. I).

Relations with adults

The time that either adult spent in the neighbourhood of the young decreased significantly as the young matured but although the time that both adults were present also decreased, it was not significant (Table 3 ) . The number of flights of adults to the young did not increase significantly with age, but the number of flights of young to adults was greater when soaring flights first began (Table 3 ) . Parental feeding frequency decreased with age (Table 4). Distance of

Table I . Chnnges inflight variablrs with age Observation Total flight Longest Wing soaring of young Spanish Imperial Eagles. Figures are

Age (days) time (rnin) (seclday) flight flapping flight weighted means, in seconds per day, for all __ p u n g birds.

75-85 6032 -58 I 9 .3 8 0

86-9 5 7982 1731 1415 178 I553 Y h - I O j 9235 2732 1827 226 2506

106-11 5 1 o , 5 10 5189 2761 212 4977 I 16-1 2 5 9449 7028 .3533 141 6887 126-135 8368 7238 3930 249 6990 136-1 50 5646 14565 8360 I45 14420

4.263 3.922 2.787 4.219 6. 42 6, 41 6, 41 6, 42

P - 0.001 0.00 3 0.022 0.002

- 1: d.f. -

1 3 0 MIGUEL FERRER I B I S I34

h - 281

m

3 ?!

10 1'5 20 2'5 30 35 40

Age at the first soaring flight

Figure I. Brood means of urea levels (rngidl) in the blood of Spanish Imperial Eagles at the end of the nestling period in relation to the bird's age (in days after fledging) a t its fkst soaring flight (rI3 =o. 564, P=0.035).

food deliveries and the frequency of aggressive behaviour by the adults towards young increased significantly with age (Table 4).

In interactions of adults with independent immature birds and other birds of prey, the adults approached the bird from above at an angle of about 45Oin 100% of the cases ( n = I 53) and struck or tried to strike the back of the bird. By comparison, in interactions with their own young, the adult approach was from above at a 45O angle on only 12.5% (n=48) of occasions, striking the flexor apex of the wing. In the remaining 87.5%. the adult approach was from below at an angle of about 140'. with the effect of grasping s e young in its claws in a hanging attitude rather than striking it. Such parent-offspring interactions occurred only when the young was practising a soaring flight.

If, for each young, the period between the first soaring flight and the date of independence is divided into two periods of equal length, the number of aggressive approaches by the parent per day was the same at the beginning of soaring as it was later (sign-test n= 12 pairs, z=o, P= I).

Hunting behaviour of the young

On only two occasions were young seen to take prey

Table 3. Changes with age in the weighted One adult Both adults Flights of young Flights of adults mean time (minutes) spent per day parent

present present towards adults towards young Spanish Imperial Eagles nfar their offspring, and in the frequency of approachingfliyhts. For total observation time, see Table I

Age

75-85 392 146 0.4 I .0

86-95 342 1 5 2 96-105 262 I54

106-11 5 20 3 I06 I 16-1 2 5 138 74 126-135 112 25 I 36-150 60 73

F 5.136 0 .5 76 d.f. 6. 42 6 . 35 P <0.001 0.746

2.1 2.1 2.1 1.6 0.2

0. I

1.1 0.7 0 . 5 2.6 1.5 1.6

2.508 1.698 6. 42 6, 42 0.036 0.145

Table 4. Changes with the age of young Spanish lrnperial Eagles in the average number offeeds per day, distance offood deliveries and parent-ofspring attacks

Frequency of food Distance of food Attacks deliveries (per day) deliveries (m) parent-offspring Age

75-8 5 85-95 96-105

106-115 116-125 126-1 35 136-1 5 0

E' d.f. P

1.4 0.9 0.6 0.4 0.3 0.3 0. I

9 81

131

335 349

283

4.849 4. I6 0.009

0

0

0.3

1.3

0.6

0.2

1.0

3.756 6, 42 0.004

1 9 9 2 POSTFLEDGING DEPENDENCE I N S P A N I S H I M P E R I A L EAGLES 1 3 1

independently of adults during the dependence period. A r 35-day old bird was observed catching and eating a rabbit and a rz7-day old bird was seen parasitizing a year-old Egyptian Vulture Neophron percnoptrrus. which it displaced from sheep carrion.

Length of dependence period

No significant differences in the length of the dependence period were found between the three years of the study (Table 5). A significant relationship was found between the number of siblings and their age at independence: young from large broods became independent a t a younger age (Table 5). The length of the dependence period did not show any relation- ship either with the sex or with the order in which the brood hatched (Table 5). No significant relationship was found between the age of leaving the nest and the age at indepen- dence (rl3=o.3oz. P=o.379).

The age at which young became independent was related to the age at which they made their tirst soaring flight ( r , 3 = o . 5 6 ~ , P=o.o36). Thiscorrelation remains significant when analysed only for young from broods of three, avoiding the confounding effect of brood size (r6=o.y58, P < o . o o ~ ) . There was also a significant correlation between the length of the dependence period and the delay of the hatching date ( r i3=-o . j54 , I ’=0.039: where Day [ = t h e tirst hatch recorded over the 3 years).

If only the period from first soaring flight to the date of independence is considered. the correlation coefficient with the hatching date increased ( r , 3 = -0 .7or , P = o . o o 5 , Fig.

Averdge 17 b d.f. P ~~

Year I986 47.0 6 I987 58 i 6 2 280 2 , 22 0.126 1988 5 1 3 1 3

Broad size I 0 3 . 2 4 2 56.2 T I 19.992 2 . 2 2 <O.OOI 3 42.8 I 0

Hatching order I 2

I 40.0 3 2 42.2 4 0.957 2. 7 0.429 3 46.3 3

Sex I 2

0.884 I , 23 0.367 Male 49.0 Female 52.8 1 3

c r n I

. .

. 0 j

15 19 23 27 31 35 39

Period from first soaring flight to independence

Figure 2. Relationship between brood means of the length of the second part of the dependence period (from the first soaring flight to the date of independence) and hatching date (Day I =the first hatch recorded in the three years) in Spanish Imperial Eagles ( r , = -0.701. P = o . 0 0 5 ) .

2 ) . However the length of this period was not influenced by the age when the first soaring flight occurred (rl = - o. I 74, P=0.549) , by urea levels in the blood ( r l l = - 0 . 3 ~ 5 , P = o . ~ o 3 ) , by sex (Table 6) nor by the birth order (Table 6 ) , although it was affected by the number of siblings, being shorter in the case of three-chick broods and the same for one or two chicks (Table 6 ) .

The age at which the adult first showed aggression to the young was related to the age at independence (r6=o.85o, P=o.o15) as well as to age at the first soaring flight (r8 =o.720, P= 0.003).

DISCUSSION

The results obtained in this study support the view that the dependence period is composed of two periods controlled by different factors. The first period, starting from the time when chicks leave the nest to the first soaring flight, represents an average of 5 1% of the total length of the dependence period. The length of this period, during which time development of the different types of flight occurs. is related to the physical condition of the young; nestlings with higher urea levels took longer to reach the necessary development to make their first soaring flight. These are birds that have suffered food shortage and have had to catabolize body tissue, thereby increasing nitrogen residues and, in consequence, the urea level in the blood (Garcia-Rodriguez et al. I 987a. Ferrer r t al.

1 3 2 MIGIIEL F E R R E R I B I S I34

Table 6 . E@cts ~Jdiflrrerit Jurtors or1 thP length (in dizys) of' the prritid /rani .first soaring flight to indqwndrncc in the Spanish lmprrial Eagle

Average - ~ _.__

Year 1986 20 .5 1987 29.2 I988 2 j .2

Brood size I 27.5 2 28.8 3 17.6

Hatching order I 3 0 . 0

2 27.8

I 17.0 2 16.6

3 19.5

Sex Male 22.8 Female 27.2

n -

h 6

L O

4

7 I 1

5 6

2

3 2

I I

I1

I: __

2.0 I 5

7.5 32

0.177

2 .30 I

1.764

d.f

2. 19

2. I 9

1. 9

2 , 4

I . 20

ry8 7 , Ferrer rgyoa) . This delay produced an increase of the total length of the dependence period. The length of this period was not related to hatching date. The effect of brood size on the length of this period could be due to a clear trend of a decrease in the urea level as the sibling number increases (Ferrer rgyob).

The second stage of the dependence period. from the first soaring flight to the date of independence, represents on average 49% of the total length of the dependence period. The length of this period is related to hatching date, being longer for young hatched earlier. This could be due to the necessity for adults to devote their whole time to foraging and recovering before starting a new reproductive cycle. Later parents have less time to prolong the second period of the dependence. It is known that the laying date of birds of prey (Newton 1979) is conditioned by the rate with which the adult is able to reach a threshold weight. so physical condition of the parents could be the ultimate cause of the length of the second period of the dependence. The physical condition of the young, however, is not related to the length of this period. Increased sibling number decreased the length of this period in the case of a triple brood but no differences were found between broods of two and one. Hroods of three could require more investment and. consequently, the stress on the parents might increase, so they would impose shorter length on the second period of dependence.

Thus, the dependence period consists of two separate phases, each representing about half of the total period, so that variations occurring in either phase affects the overall length. Parental choice of whether to prolong or reduce the

dependence period probably depends on the physical condi- tion of the adults, and is limited to the stage when the young have already become skilled in flight. Thus, the appearance of a conflict between the interests of parents and chicks is only possible during this period. However, the total length of the dependence period is determined, in addition to any parental decision, by the physical condition of the young, which determines the age when the first soaring flight occurs. The existence of this double control could explain the low correlations found by other authors between the overall length of the dependence period and variables such as the hatching date and maturation of flying (Alonso et al. 1987, Bustamante & Hiraldo 1989, 1990).

The decrease in adult presence and parental feeding observed in the Spanish Imperial Eagle, especially after the first soaring flight. indicates an overall decrease of the parental investment, as has been described for numerous bird species (Moreno I 984, Edwards I 98 5 , Bustamante & Hiraldo I 9 yo). Adult aggressive behaviour towards their young has been proposed as the mechanism used to end the dependence period (Alonso et ul. r987), yet in the present study the frequency of attacks recorded was shown to be of the same intensity at beginning of the first soaring flights as near the independence date: moreover, attacks occurred before paren- tal feeding was over. Differences between aggression to their own young and to other eagles, as well as the fact that aggression was seen only when their own young were soaring, support the view that aggressive behaviour allows the parent to evaluate the flight capacity of the young.

I am indebted to J. j. Negro and C. Vila for helping in the field. I thank J . A. Donazar. F. Hiraldo and P. J . Jones for suggestions on the manuscript. This study was supported by DGICYT. project number PB87-0405 and by a post-doctoral grant from Consejo Superior de lnvestigaciones Cientificas.

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Ferrer. M. 199oa. Hematological studies in birds. Condor 92: 1085- 1086.

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1992 I’OSTFLEIIGING DEPENDENCE I N S P A N I S H I M P E R I A L EAGLES 1 3 1

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Konrad. P.M. & Gilmer. D.S. 1986. Post-fledging behavior of Ferruginous Hawks in North Dakota. Raptor Res. 20: 35-39.

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