l B l S 1 3 5 : 1 3 9 - 1 4 7

The function of aggressive chases by breeding Black and Red Kites Mfvus migrans and M. mifvus

during the post-fledging dependence period

JAVIER BUSTAMANTE & FERNANDO HIRALDO Estacidn Bioldgica de Dofiana, Pabelldn del Peru', Avda. Maria Luisa sln, E-1013 Sevilla, Spain

Black and Red Kites Milvus migrans and M . milvus chase other raptors approaching their nests. The study of this behaviour during the post-fledging period suggested that it reflects mainly, but not only, anti-predator behaviour. The frequency of vigilance and aggressive chases decreased through the post-fledging period as predicted by theoretical models of nest defence. Although predation risks were similar, Black Kites invested more time chasing intruders than did Red Kites. Black Kites, unlike Red Kites, chased away intruding juve- niles, which may be interpreted as a behaviour to avoid investment in unrelated fledglings. Black Kites usually nest in loose colonies where the risk of, and selection pressures against, accidental investment in unrelated fledglings is likely to be greater than for Red Kites. Differences in aggressive chases by Black and Red Kites are better related to this than to different predation risks.

In birds, reaction against intruders near their nests has been treated mainly as anti-predator behaviour (Andersson et al. 1980, Greig-Smith 1980, Buitron 1983, Knight et al. 1987, Montgomerie & Weatherhead 1988, Redondo 1989, Brun- ton 1990). Aggressive chases by raptors during the breeding period have been considered as either nest defence against predators (i.e. Andersson & Wiklund 1987, Wallin 1987, Wiklund 1990) or defence of territory, nest-site or mate from conspecifics or other raptors (Brown & Amadon 1968, New- ton 1979). In general, no attention has been given to the possibility that aggression could also serve to avoid misdi- rected parental investment in unrelated individuals, con- specifics or not, which could rob food from the nest or be adopted accidentally (Bustamante & Hiraldo 1990a. Dona- zar & Ceballos 1990).

In this paper, we present the results of a study of adult aggression towards intruders of different species by two me- dium-sized raptors, the Black Kite Milvus migrans and the Red Kite M. milvus. during their offspring's post-fledging dependence period. Our objectives were to determine why adults invested time and energy chasing away other raptors. whether this behaviour changed with fledgling age and if aggression against other raptors could be explained entirely as defence against predation. Both species of kites are of a similar size and have a similar feeding ecology, and their fledglings are at a similar risk of predation. The Red Kite tends to be a solitary nester while the Black Kite usually nests in loose colonies (Cramp & Simmons 1980) in which the risks of food piracy or accidental adoption of fledglings are greater.

Aggression by parents could function (1) to defend a feed- ing territory, nest-site or mate from competitors, (2) to pre- vent predation of fledglings or (3) to prevent food piracy or

accidental adoption of unrelated fledglings. Each of these functions leads to predictions regarding how aggressive re- sponses should change with time and how parents should react to different species or to individuals of different ages.

(1) If there is defence of a feeding temtory, the kites should be defending the area where they feed. Considering the de- fence of nest-site or mate, it is predicted that aggression should be directed mainly towards conspecifics, which are presumably the main competitors for nest-sites and the only competitors for mates. Juveniles should be attacked less ea- gerly than adults, as they are weaker competitors for food and do not compete for nest-sites or mates. Neither at the beginning nor at the end of the post-fledging period is it possible to start a new breeding attempt, so competition for a nest-site or a mate should be low and no changes in in- tensity of aggressive behaviour with time should be expected after the young leave the nest. If there are interspecific dif- ferences, the Red Kite should behave more aggressively, because it keeps its breeding territory, nest-site and mate during winter in the study area (F. Hiraldo, unpubl. data) while Black Kite pairs leave the territory and migrate in- dependently to Africa at the end of the post-fledging period (Bustamante & Hiraldo 1989). Black Kites would obtain no clear benefit from defending a nest-site or a mate they are obliged to leave and will have to regain the next breeding season.

(2) Defence against predators predicts that those species that pose a greater risk to fledglings should be attacked more eagerly by the parents (Montgomerie & Weatherhead 1988. Redondo 1989). There should be a decrease in aggressive- ness from nestling to post-fledging period because predation risk for juvenile raptors decreases with age (Montgomerie & Weatherhead 1988, Redondo 1989) as they become more

1 7 9

140 J . B U S T A M A N T E & F. H I R A L D O I B I S 1 3 5

M A F I S H (a)

0

eagle

.Black kite O W kite

oCommon kestrel *Spanish Knperial

\\ /

0

0. 0

I 4

II O J ABooted eagle

@Short toed eagle ,@Pine grove

1 km

Figure 1. plane used to estimate the density of raptors flying near each nest.

(a) Relative locations of nests of Red and Black Kites and other raptors in the study area: (b) Representation of the nest-observer

capable of defending themselves (Rosenfield & Kanvik 1983, Hiraldo et d. 1990). There should not be interspecific dif- ferences as fledglings of both species are of a similar size in relation to their potential predators, and there are no reasons to assume that the two species have evolved under different fledgling predation pressures.

(3) Defence against food piracy and adoption predicts an increase in aggressiveness during the post-fledging period, as the risk of accidentally investing in unrelated individuals increases. If nests are close, fledglings can intermingle, and recognition errors by parents might arise. If juveniles are capable of pirating food from nests, the number of possible

1993 A G G R E S S I V E C H A S E S B Y B R E E D I N G K I T E S 1 4 1

pirates is also increased as fledglings from neighbouring nests improve their flying skills. Juveniles should be chased more eagerly than adults because they may be seeking food more often or may be adopted accidentally. If adults behave ac- cording to actual risks of food piracy in the study area, there should be no interspecific differences in aggressiveness be- cause juveniles of both species pirate food from nests (Bus- tamante & Hiraldo 1990a) and both species nest at high density (Fig. la). However, the Red Kite, habitually a solitary nester, has probably evolved under a low risk of food piracy or accidental adoption and may not identify juvenile kites as possible food pirates.

STUDY AREA AND METHODS

Research was carried out in southwestern Spain, in the 6700 ha of the Biological Reserve of Doiiana (37"N. 6"30W) in the Dofiana National Park, where both kite species coexist. The three main habitats in the area are "marisma" or sea- sonally drying marshland. Mediterranean scrubland with scattered cork oaks Quercus suber and coastal sand dunes with stone pines Pinus pinea. For more detailed descriptions of the area see Valverde (1958) and Rogers & Myers (1980).

The breeding population of kites in the Reserve, around 80 pairs of Black Kites and 20 of Red Kites, is stable. Both species nest in trees in the Mediterranean scrubland and coastal sand dunes, but most nests are concentrated in a narrow strip of 250 ha in cork oaks close to the marisma (Fig. la). Like other raptors, both kites forage principally over the marisma, so most individuals frequently have to pass near other kite nests when flying to foraging areas.

Black Kite breeding pairs were observed in 1985 (pairs A, B and C during 8, 7 and 7 days, respectively), 1986 (pairs D, E, F, G and H during 4. 6. 8, 8 and 9 days, respectively) and 1987 (pairs I , J. K , L and M during 7, 8, 7.9 and 3 days, respectively). Five Red Kite breeding pairs (V. W, X, Y and Z) were observed in 1987 during 8. 5, 11, 9 and 7 days, respectively. Observations, which were regularly spaced throughout the post-fledging period, started some days be- fore the first juvenile fledged (average fledging age is 48 days after hatching for the Black Kite and 55 days for the Red Kite) and ended when all fledglings of the brood became independent (average 77 days after hatching for the Black Kite and 80 days for the Red Kite). Both species continued to feed their offspring at the nest during the whole post- fledging dependence period, and juveniles always remained near the nest (0-500 m). Detaiis of the post-fledging depen- dence period for the Black Kite have been published else- where (Bustamante & Hiraldo 1989, 1990b). Family groups were observed continuously from dawn to dusk (063&2030h GMT) from inside a vehicle parked at a mean distance of 235 m from the nest. As observation days on Black Kite pairs in 1987 were not always complete, data from these days were discarded when complete observations were required (Tables 3 and 4).

Any flight by either or both parents towards an intruding

raptor near the nest-area was considered to be an aggressive chase when it included alarm calls and/or dives towards the intruder. Chases were timed in seconds with a stopwatch from the time they were detected until parent and intruder separated in different directions. Intruder species, distance to the nest when the pursuit started and number of times the parent dived towards the intruder were recorded for each chase. Because of the high density of breeding raptors in the area (Fig. la), there were individuals constantly flying near the nests we were monitoring. Chases began at a wide range of distances from the nest, so it was impossible to count the actual number of intruders and the reaction of the parents to each one of them. The number and population composition of raptors in the area changed through the come of the season, as some species (like the Black Kite) started migration and the proportion of independent juveniles of different species increased, potentially affecting the fre- quency of chases observed. During 1986 and 1987, to es- timate the probability of becoming an intruder for each spe- cies of raptor and age class, we recorded at each nest-site the number, species and age of all raptors crossing an imag- inary vertical plane going from the observer to the nest and extending 60 m high (hereafter referred to as the nest-ob- server plane) (Fig. lb). These counts estimated the density of flying raptors near each nest-site but were not the number of real intruders, since we were sampling a two-dimensional plane instead of the three-dimensional space around the nest defended by the kites. We assumed that the density of cross- ings of the nest-observer plane was an unbiased estimate of the probability for each species and age class of becoming an intruder each day at each nest site. The method assumes that different species and age classes did not avoid flying close to the nests. Although we could not test this assump- tion, the high density of nests in the area, the fact that most raptors had to cross the area when moving to and from their hunting areas and the fact that they never seemed to avoid areas close to the nests suggest that it is not unrealistic.

The distance between Red Kite nests was great enough to avoid confusion of the adults with those of other pairs. Their moulting patterns and individual characteristics usually al- lowed identification of both members of the pair, but indi- viduals could not be sexed. Although most Black Kite pairs studied had other pairs nesting nearby (range of nearest neighbor distance 30-630 m), most adults could be clearly recognized. In six Black Kite pairs, individuals were sexed. Three adult males were wing-tagged, while plumage col- ouration and moulting patterns in wings and tail allowed recognition of the other three.

Variables recorded in the field were daily total time at least one parent was near the nest-site (minutes), daily total number of aggressive chases, daily total time spent in ag- gressive chases by each pair (seconds), species and age of intruder, chase duration (seconds), number of times the pur- suer dived towards the intruder and estimated distance be- tween the intruder and the nest when the chase started (to the nearest 10 m). As most variables did not seem to follow a normal distribution, the median was used as an estimator

1 4 2 J . B U S T A M A N T E & F. H I R A L D O I B I S 1 3 5

Table 1. Ranges of the medians are shown in parentheses. n = number ofpairs. All years combined

Characteristics of aggressive chases of Black and Red Kites. Data are medians of the median values for the diflerent pairs observed.

Black Kite Red Kite (n = 13) (n = 5) 2’ P

Duration of a chase (seconds) Number of dives per chase Distance to the nest (m)

23 (10-64) l(1-2)

70 (35-100)

33 (1468) 2 (0-3)

50 (20-100)

1.78 0.075 0.72 0.470 1.51 0.131

Wilcoxon test for differences between Black and Red Kites.

of central tendency and nonparametric statistics were used following Siege1 & Castellan (1988).

RESULTS

Aggressive chases

Breeding adults of both Red and Black Kites usually behaved aggressively towards conspecifics or other raptors that came close to their nest or their fledglings. One or both members of the pair flew close to the intruder giving alarm calls or dived at the intruder several times until it left the area. Both species search for food over wide areas, and breeding adults may travel up to 20 km from the nest in their feeding trips (Veiga & Hiraldo 1990). so nests were sometimes unattend- ed.

No significant differences between species were found in median chase duration, median number of dives per chase or median distance to the nest when the chase was started (Table 1).

For all pairs of both species studied, both parents chased intruders. Only in six Black Kite pairs were sexes easily recognizable. For four of these pairs, more territorial chases were recorded for females than for males, whereas the re- verse was true for the other two pairs.

To test if different species of intruders (considering only four groups: Black Kites, Red Kites, Booted Eagles Hieraaetus pennatus and other raptors) were chased more or less fre- quently than expected (according to their frequency of flying near each particular nest-site), we calculated, for each mon- itored kite pair, the difference between the number of chases recorded towards each group and the number of chases expected from the counts of raptors crossing the nest-ob- server plane near the nest during the same observation days (Table 2). For each breeding pair, all observation days had to be pooled. Performing a Friedman two-way analysis of variance by ranks on these differences, it was not possible to reject the null hypothesis that the four groups were chased in proportion to their abundance in the area near the nest: Black Kite pairs (F, = 1.05, n = 10. k = 4, P > 0.05). Red Kite pairs (F, = -2.76. n = 5. k = 4, P > 0.05).

The Spanish Imperial Eagle Aquila adalberti, which was a very uncommon raptor in the area and had to be pooled

with “other raptors” in the previous analysis, seemed to be very vigorously chased when it appeared. However, it was chased at only one Black Kite and one Red Kite nest-site that were less than 300 m from the eagle nest and were the only sites at which the eagle was observed flying. In both cases, the eagle was chased more frequently than expected (Table 2). Unfortunately it was an infrequent intruder. and we did not know if other pairs would have behaved in the same way.

Decrease in aggressiveness

For all the variables considered, the investment in vigilance and aggressive chases by both species decreased throughout the post-fledging dependence period. In all pairs of both kites, the daily total time for which at least one parent was near the nest-site, daily total time spent in aggressive chases, daily total number of aggressive chases and daily total num- ber of dives decreased with age of the eldest chick (Table 3). Only in one Black Kite pair (0). with a small sample size, did the number of chases not decrease with age. The number of pairs in which vigilance and aggressive chases decreased was statistically significant (one-tailed binomial test. P =

0.002 for daily number of chases and P < 0.001 for the other variables).

The observed decrease in aggression could be due to a decrease with time in the number of raptors in the area. We calculated, for each pair, the correlation between raptors counted crossing the nest-observer plane and the different variables measuring vigilance and defence. Only some pairs showed positive correlations between the variables measur- ing vigilance and defence and the number of raptors in the area. For those in which the correlation was positive, we calculated Kendall partial rank-order correlation coefficients with the age of the eldest chick, keeping constant the variable “raptors crossing”. All variables measuring aggressiveness in all pairs showed a negative partial correlation with chick age, except for daily total time at least one parent was near the nest-site in one Black Kite pair (H).

For both species of kites, the decrease in total time spent in aggressive chases did not merely reflect the decrease in the time that adults spent near the nest-site. The percentage of the time parents were present near the nest-site that was

1 9 9 3 A G G R E S S I V E C H A S E S B Y B R E E D I N G K I T E S 1 4 3

Table 2. Number of aggressive chases by Black and Red Kites towards different species of intruders. The number of chases expected according to relative abundance of each species crossing the nest-observer plane at each nest-site is shown in parentheses. In 1985 no counts were made of raptors crossing the nest-observer line. - represents missing values

Intruders chased

Spanish Imperial No. of raptors

Pairs Black Kite’ Red Kite Booted Eagle EagleZ Other raptors’ crossing‘

Black Kite A B C D E F G H r I K L M

Total chases

Red Kite V W X Y 2

Total chases

38 24 17 6 (13.6)

16 (16.0) 17 (15.7) 34 (25.8) 14 (12.0) 13 (13.7) 13.5 (17.8) 8 (10.4)

18 (17.8) 1 (0.6)

219.5

18 (17.1)

12 (11.7) 13 (10.8) 15 (17.1)

59

1 (1.0)

0 0 3

0 (0.7) 0 (0.4) 1 (6.6)

2 (0.7)

0 (0.8)

1 (1.9)

5.5 (0.8) 2 (0.1) 2 (2.2) 0 (0.0)

16.5

1 (0.3) 0 (0) 1 (0.2) 0 (0.3) 1 (0.5)

3

2 2 3

6 (3.6) 2 (0.9) 0 (2.5)

0 (0.0)

0 (1.2) 0 (0.8) 0 (0.5) l(O.1)

0 (0.2) 0 (0.4)

16

0 (0.6) 0 (0) 0 (0.2) 0 (0.5) 0 (0.5)

0

2 0 1 0 (2.8) l(2.7) 2 (4.0) l ( l . 1 ) l(O.9) 2 (1.8) 2 (1.9) 0 (0.4) l(O.1) 0 (0.2)

13

0 (1.1) 0 (0) 0 (0.9) 0 (1.4) 0 (1.4)

0

- 50

128 142 130 137 124 131 338 151 23

198 406 336 244 157

More than one individual of different species were occasionally chased simultaneously so values are not necessarily integers. The Spanish Imperial Eagle had to be pooled with “other raptors” for statistical analyses. Other raptors chased by Black Kites were Marsh Harrier Circus aeruginosus (4 t ies), Montagu’s Harrier C. pygargus (2). Common Kestrel

Raptors occasionally seen crossing but never chased by kites were Hobby Falco subbuteo, Peregrine Falcon Falco peregrinus and Short-toed Falco tinnunculus (2). Buzzard Buteo buteo (3). Griffon Vulture Gypsfulvus (1). Black Vulture Aegypius monachus (1).

Eagle Circaetus gallicus.

spent in aggressive chases was also negatively correlated with the age of the eldest chick (binomial test, P < 0.001).

The chase itself did show a slight decrease in aggressive- ness with chick age. The daily median chase duration was negatively correlated with chick age in only half of the pairs of both species (8 out of 16. Table 4). but median number of dives per chase decreased in all but four pairs (one-tailed binomial test, P < 0.011).

Interspecific differences in chasing behaviour

Before the first flight of the eldest chick, the adults of both species spent similar time near the nest-site, and a similar percentage of this time was spent in aggressive chases. Dur- ing the post-fledging period, Red Kite pairs spent signifi-

cantly less time near the nest-site and in aggressive chases than Black Kite pairs, but the percentage of time near the nest-site chasing away intruders remained not significantly different (Fig. 2). Differences in nest-site vigilance cannot be attributed to foraging distance from the nest, as Red Kites forage on average closer to the nest than Black Kites (Veiga & Hiraldo 1990). nor to differences in the duration of the post-fledging dependence period, which is very similar in both species (Black Kite 28 days, s.d. = 5.49, n = 13 [Bus- tamante & Hiraldo 19891; Red Kite 26 days, s.d. = 6.32, n = 12 [Bustamante 19901).

Black Kites also readily chased away any Black Kite ju- veniles that came near their nest. Juvenile kite intruders were chased by all Black Kite pairs but were never chased by Red Kite pairs (Table 5). Black Kite juveniles became more frequent intruders at nests in the second half of the

1 4 4 J . B U S T A M A N T E & F . H I R A L D O I B I S 1 3 5

Table 3. Kendall rank-order correlation coefficients (T) for each pair of Black and Red Kites for (a) the daily total time that at least one parent was near the nest-site, (b) the daily total time spent in aggressive chases, (c) the daily total number of aggressive chases and (d) the daiJy total number of dives, in relation to the age of the eldest chick in the brood (AGE) and the daily number of raptors crossing the nest-observer plane (RAPTORS). For those pairs with a positive correlation with RAFTORS, we have estimated the Kendall partial rank-order correlation coefficient with AGE. keeping ~ R S constant (TdGE, - represents missing values, * P < 0.05, ** P < 0.01. *** P < 0.001

(a) Daily total time adults present at nest

(c) Daily total no. of aggressive chases (b) Time spent in aggressive chases

Pairs TAG1 TRA,ORS TAGE.R4PTORS TAGS TRAPro, TAGE.RAPTORS TAGE T m o m TAG, RAPMRS

Black Kite A (n = 8) B (n = 7) C (n = 7) D (n = 4) E (n = 6)

G (n = 8) H (n = 8)

F (n = 8)

Red Kite V (n = 8) W (n = 5 ) x (n = 11) Y (n = 9) z (n = 7)

-0.71' -0.24 -0.71' -0.67 -0.20 -0.50 -0.79** -0.07

-0.98*** -0.8W -0.82*** -0.83** -0.71

~~ ~~

- - -

-1.00 -0.47 0.00

-0.04 0.11

0.25 0.60 0.71" 0.46 0.47

-0.79** -0.71* -0.49 -0.33 -0.20 -0.40 -0.36

0.05 -0.44

-0.98"' -0.71' -0.67 -0.32 -0.59** -0.74*** -0.82** -0.56*** -0.83** -0.62

~~

- - -

-0.67 0.07 0.18 0.25 0.22

0.50 0.32 0.69*** 0.18 0.47

~~

-0.37 -0.36 -0.40 -0.50

-0.72** -0.11 -0.42 -0.54* -0.68*

-0.96*** -0.75* -0.15

0.33 -0.23 -0.42 -0.641 -0.48'

-0.74'* -0.32 - 0.74**' -0.64*** -0.78'

- - - 0.00

-0.39 0.19 0.08 0.23

0.52 0.32 0.79'** 0.09 0.20

~~

-0.38 -0.66" -0.55"

-0.75** -0.11 -0.27 -0.65" -0.81"*

post-fledging period. To test if juvenile Black Kites were chased more frequently than adults, we calculated for each Black Kite pair the difference between the number of chases towards Black Kite juveniles recorded and the number ex- pected from their abundance in relation to adult Black Kites, using the counts of raptors crossing each nest-observer plane. To compensate for the fact that adults decreased the inten- sity of vigilance and chases during the post-fledging period while juvenile intruders increased their frequency of flying in the area, we used for this analysis only those observation days after the first intruder Black Kite juvenile was observed at each nest-site area for each pair. Black Kite juveniles were chased significantly more often than adult Black Kites (dif- ferences between observed and expected number of chases towards juvenile Black Kites for each pair tended to be pos- itive, two-tailed Wilcoxon signed rank test, P = 0.049). Red Kite juveniles were not seen flying often enough near any of the nests to determine whether juvenile and adult Red Kites were chased by Black Kites with similar frequency (two-tailed Wilcoxon signed rank test, P = 0.5).

Red Kites were never observed chasing away Black Kite or Red Kite juvenile intruders. When juveniles started in- truding near the nests, most Red Kite pairs had reduced their vigilance and aggressiveness to very low levels and no chases were recorded (Table 5). Two Red Kite fledglings were adopted soon after fledging at two of the five monitored Red Kite nests and were never attacked by the adults (Busta- mante & Hiraldo 1990a). These adopted fledglings were not recorded when counting the raDtors crossing the nest-ob-

server plane and were not considered when estimating the probability of intrusion (Table 5).

DISCUSSION

Although theoretical models predict a drop in the intensity of nest defence after fledging (Montgomerie & Weatherhead 1988. Redondo 1989). most studies involving temporal pat- terns of nest defence do not extend over the entire period of parental care and, consequently, the predicted drop is not observed. Our observations on the reduction in vigilance and defence during the post-fledging dependence period of Black and Red Kites agree with the predictions of these models based on offspring vulnerability.

Although adult vigilance near the nest during post-fledg- ing could be mainly an anti-predator behaviour, most ag- gressive chases were directed towards Black Kites that posed no risk to kite fledglings of either species. Aggressive be- haviour by kites could be interpreted as territory, nest-site or mate defence (Brown & Amadon 1968, Newton 1979). The area defended by kites is not a feeding territory, as most of the kites search for food far from the nest. The average area defended by both kites, 1-3 ha, is very small in relation to the home-range of breeding pairs, usually more than 2000 ha (Heredia et al. 1991, unpubl. data). Mate or nest-site defence against conspecifics or other competitors is unlikely when the breeding season is coming to an end and there is insufficient time to initiate a new breeding attemd. Nev-

1 4 5 1 9 9 3 A G G R E S S I V E C H A S E S B Y B R E E D I N G K I T E S

Table 3. Continued

(d) Daily total no. of dives

TAG, TRAPTORS TACE.KAPT"RS

-0.79** -0.15 -0.24 -0.33 -0.36 -0.57* -0.40 -0.26

- -0.67 -0.64

0.34 -0.49 0.37 -0.48 0.43 -0.38

-0.76" 0.47 -0.77'* -0.32 0.32 -0.11 -0.74"* 0.79*** -0.27 -0.65*** 0.09 -0.65** -0.79" 0.16 -0.81"

ertheless, if defence of nest-site or mate long before the breeding season is advantageous and was the reason for aggressive chases, sedentary Red Kites, which retain their mates and use their nest-site as a roost in winter (Heredia et al. 1991), should behave more aggressively or at least show a smaller decrease in aggressiveness than migratory Black Kites, which leave their nest site and migrate inde- pendently of their mates (Brown & Amadon 1968). However, our data show the opposite trend.

Predation risk may explain why Spanish Imperial Eagles were chased more actively than other raptors in the few instances when they appeared. Only aerial predators can be a real risk for fledglings, and the Spanish Imperial Eagle is the only one that can prey on adult and juvenile kites in the area. Gonzalez (1989) found Black Kite feathers in pellets of Spanish Imperial Eagles in Dofiana, and the metal ring of a recently fledged Red Kite was found while searching for prey remains under a habitual perch of a Spanish Imperial Eagle in 1987. Chases towards other raptors posing little risk to fledgling kites could simply be a behaviour persisting from earlier in the breeding cycle, when these raptors. especially kites, may prey upon eggs or young nestlings (Jones & Maiiez 1990, unpubl. data) and may also be competitors for mates or for nest-sites.

However, neither defence of offspring against predators

Table 4. Kendail rank-order correiation coefficients (T) for each pair of Black and Red Kites (a) between the percentage of time parents were present at the nest-site spent in aggressive chases and the age of the eldest chick in the brood (AGE), and the daily number of raptors crossing the nest-observer plane (RAPTORS). For those pairs with a positive correlation with RAPTORS, we have estimated the Kendail partial rank-order correlation coefficient with AGE, keeping RAPTORS constant: (b) between the daily median duration of a chase and AGE: (c) between the daily median number of dives per chase and AGE. - represents missing values, + P < 0.05, +* P < 0.01 +** P < 0.001

(a) % time parents spent in chases

Pairs TAGB TKA,,, TA'E.RAFTORS

(b) Chase (c) Dives per duration chase

T A G E TAGE

Black Kite A (n = 8) B (n = 7) C (n = 7) D (n = 4) E (n = 6) F (n = 7) G (n = 8) H (n = 7) r I K L M

Red Kite V (n = 7) W (n = 4) X (n = 11) Y (n = 8) Z (n = 7)

-0.57' -0.62 -0.33 -0.33 -0.20 -0.11 -0.36 -0.48

-0.14 -0.24 -0.65*** -0.58" -0.43

- -0.67

0.07 0.18 0.25 0.29

0.24 0.24 0.60*** 0.11 0.14

-0.37 -0.02 -0.40

0.57

-0.10 -0.08 -0.34 -0.57' -0.44

(n = 6)

(n = 5) (n = 8) (n = 6) (n = 6) (n = 1)

(n = 5) (n = 1) (n = 4) (n = 2) (n = 6)

-0.40 -0.52 -0.52 -0.67 -0.07

0.55 -0.32

0.19 -0.40

0.07 0.07

-0.20 -

0.00

0.55 1.00 0.28

-

-0.59* -0.26 -0.48 -0.91' -0.36 -0.36

0.15 0.55

-0.40 -0.45

0.07 -0.28 -

-0.36

0.67 -1.00 -0.43

-

146 1. B U S T A M A N T E & F . H I R A L D O I B I S 1 3 5

6 -

5 - - C : 4 - 2 P

.- c 3 - .., * 2 -

1 -

(a)

I

1 0 0

Z= -0.16 n.s. 2. 2.85 *+ 2. 2.77 **

(b)

1200 I 4 O 0 L 7

4001 + f i

0 A i r , Z= 1.19 n.s. 2. 2.56 ** 2. 2.29 **

Table 5 . Number OJ aggressive chases towards adult and juvenile kites by each pair of Black and Red Kites, and number of crossings of the nest-observer plane during the same days. Only those obser- vation days after thefirst juvenile kite was observed near each nest- site are considered. - represents missing values

Black Kite Red Kite intruders Black intruders Red chased Kite chased Kite

ad/juv ad/juv Pairs ad juv crossing ad juv crossing

Black Kite A 3 4 B 1 1 C 8 4 D 1 1 E 2 1 F 12 2 G 11 2 H 10.5 3.5 r 7 5 I 3 1 K 1 2 L 4 0 M 0 1

Red Kite

-1- -1- -/- 1111 2112 71/18 40/7 79/24 2019 14/2 32/15 1312

714

0 0 --- 0 0 -1- l l -4- 0 0 210 0 0 311 0 0 2/ 1

18/6 1 0 1 0 5/11 0 2 4/1 0 5.5 4/4

112 2 0 1 1 9/7 0 0 0/1

o/o V 0 0 3012 W 0 0 010 0 0 010

010 X 5 0 3515 Y 0 0 1917 0 0 010

0/1 2 1 0 2810

0 0

0 0

0 0

( C )

t c

nor competition for nest-sites or mates can explain why intruding Black Kite juveniles were frequently chased away by adult Black Kites, while Red Kites reacted unaggressively towards unrelated juvenile kites. Neither can explain why Black Kites maintained a relatively high level of vigilance near the nest during the post-fledging period while Red Kites markedly reduced vigilance after their offspring fledged. The relative sizes of fledglings of the two kites and their aerial skills in relation to the eagle as a predator were very similar. Other kites and mammal predators, both abundant in the area, were no longer a risk for fledglings. The sharp decrease in vigilance by Red Kites after fledging probably followed the decrease in predation risk.

T

! i ~

i

Figure 2. Change with chick age in vigilance and defence by adult Black Kites (solid circles) and Red Kites (open circles) during the post-fledging dependence period. (a) Daily total time that at least one parent was near the nest-site: (b) Daily total time spent in aggressive chases: (c) Percentage of time that parents were present at the nest-site spent in aggressive chases. As there was more than one observation day for most of the pairs in each period considered,

we first calculated the median for each pair. Data represented are medians of the median values of different pairs and the range of medians. Sample sizes are: Red Kite: 5 pairs: Black Kite: 6 pairs before first flight. 8 pairs during 1st to 15th days of post-fledging period and 7 pairs after 16th day of post-fledging period. z = Wil- coxon test for the difference between Black and Red Kites. n.s. =

not significant, * P < 0.05, ** P < 0.01.

1993 A G G R E S S I V E C H A S E S BY B R E E D I N G K I T E S 1 4 7

Parental investment in unrelated juveniles and food piracy from nests by juveniles have been recorded in various species of raptors (Ellis 81 Groat 1982, Poole 1982, Donazar & Ce- ballos 1990). Fledglings of both species of kites have been observed pirating food from neighbouring nests in Doiiana once they had sufficient skill in flying, and accidental adop- tions of fledglings have been recorded in both species, al- though significantly more frequently in the Red Kite (Bus- tamante & Hiraldo 1990a). Food piracy is possible because adults continued to bring food to the nest itself during the whole post-fledging period, and kite fledglings never re- sponded aggressively to intruders. Although adult kites could also pirate food from nests, they were never observed doing so. Adult Black Kites, by attacking juvenile kite intruders, might reduce the risk of adopting unrelated juveniles and/ or suffering food piracy from their nests. Although the risks in Doiiana are similar for both species, and both suffer food piracy and adoptions (Bustamante & Hiraldo 1990a), selec- tive pressure may be stronger in the Black Kite, which nests in loose colonies (Cramp & Simmons 1980), compared to the Red Kite, which usually nests solitarily.

Funding was provided by projects 944 CSIC-CAICYT and PB87- 0405 DGICYT and a Predoctoral Fellowship of the PFPI (Mdsterio de Education y Ciencia) for the first author. We thank M. Hualdo. G. Doval, F. Martinez. F. Dominguez. M. Aragoneses. J. Jaiiez and P. Ferreras for their assistance with field work, and A. M. Jones and S. Nadeau for corrections on the English text. E. Aguilera. D. Buitron. C. G. Wiklund. P. W. Edwald and four anonymous referees provided helpful comments on earlier versions of this manuscript.

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Submitted 28 lanuam 1992: revision acceuted 26 May 1992