ANIMAL BEHAVIOUR, 2006, 72, 539e544doi:10.1016/j.anbehav.2005.10.021

Sibling competition and the risk of falling out of the nest

PIERRE BIZE* & ALEXANDRE ROULIN†

*Division of Environmental & Evolutionary Biology, Institute of Biomedical & Life Sciences,

University of Glasgow

yDepartment of Ecology & Evolution, Biology Building, University of Lausanne

(Received 24 May 2005; initial acceptance 10 August 2005;

final acceptance 22 October 2005; published online 22 June 2006; MS. number: 8557R)

In birds, sibling competition encompasses several activities, one of which is jostling for position, that is,competing for the location in the nest where parents predictably deliver food items. We hypothesized thatnestlings that compete by jostling for position may fall out of the nest either accidentally or because sib-lings push each other to reduce brood size. This hypothesis predicts that in a competitive environmentneedy nestlings trade-off the benefit of being fed against the cost of falling out of the nest. As a first at-tempt to evaluate this hypothesis, we experimentally manipulated the number of young per brood inthe colonial Alpine swift, Apus melba. Nestlings fell out of their colony more frequently when reared inenlarged than in reduced broods. Because brood size manipulation affects not only the number of youngper nest but also their body condition, we analysed an extended data set to disentangle these two factors.This analysis showed that, independently of brood size, nestlings in poor condition and those reared inbroods where sibling differed markedly in weight were more likely to disappear from the colony. Nestlingdisappearance also occurred predominantly in nests close to the colony entrances. Although nestlingswifts can wander in the colony and become adopted in neighbouring nests, we found no evidencethat wandering per se increased the risk of falling out of the colony. Our study therefore highlights a novelcost of scramble competition.

� 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

In the face of sibling competition, nestlings often blockthe nest hole entrance (Stamps et al. 1985; Bryant & Tat-ner 1990; Kacelnik et al. 1995) or sit near the parents’landing site on the nest rim to monopolize parental atten-tion and thereby gain priority in feeding (Kilner 1995;Ostreiher 1997; Smiseth et al. 2003; Kolliker & Richner2004). Jostling for position where parents predictably de-liver food is not only time consuming and energeticallyexpensive (Rodrıguez-Girones et al. 2001; Chappell &Bachman 2002) but also dangerous because of the in-creased risk of falling out of the nest. Nestlings may fallout accidentally or siblings may push them in an attemptto reduce brood size. This might be particularly true inspecies that build open nests or for which the nest isin a burrow, on the edge of cliffs or in holes rather thanin deep cavities. The frequency of falls has been largelyignored by researchers, although each year many young

Correspondence: P. Bize, Division of Environmental & EvolutionaryBiology, Institute of Biomedical & Life Sciences, Graham Kerr Building,University of Glasgow, Glasgow G12 8QQ, U.K. (email: p.bize@bio.gla.ac.uk). A. Roulin is at the Department of Ecology & Evolution, BiologyBuilding, University of Lausanne, CH 1015 Lausanne, Switzerland.(email: alexandre.roulin@unil.ch).

50003e3472/06/$30.00/0 � 2006 The Association for the

birds are found out of their nest. For example, in 2004a Swiss rehabilitation station admitted 1249 live birds ofwhich 391 nestlings fell out of their nest (31%). Amongthese 391 nestlings, 11 were Alpine swifts, Apus melba,from a colony of 20e30 pairs in Lausanne, Switzerland(S. Maumary, personal communication).

In the present study, our objective was to examinea prediction of the hypothesis that sibling competitionincreases the risk of dying by falling out of nests. Whenparents arrive at the nest, offspring often start to begconspicuously, and there may be a non-negligible riskthat by pushing each other, in an attempt to get access toparents, one of the offspring falls out of the nest. Jostling forposition may therefore help maintain the honesty ofbegging (Godfray 1991, 1995), since only hungry nestlingsmay be willing to take the risk of falling. This implies thatnestlings have to resolve the trade-off between the benefitsof being nearer the location in the nest where parents pre-dictably deliver food and the risk of falling out of the nestif this location is near the nest cavity entrance, the rim ofthe nest or the location where parents enter a colony.

We investigated this novel hypothesis in the colonialAlpine swift. Nestling swifts are reared by both parents upto fledging 50e70 days after hatching. From 20 days

39Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

ANIMAL BEHAVIOUR, 72, 3540

onwards, young are frequently observed to move on footout of their nest to intercept parents (see Malacarne et al.1994 for behavioural observations on a closely related spe-cies, the pallid swift, A. pallidus) and to seek adoption inneighbouring families (Bize et al. 2003a). Our aim was toreduce nestling body condition to increase sibling compe-tition and, in turn, the likelihood that nestlings will com-pete for priority of access to parents. We manipulatedbrood size to create groups of nestlings in prime andpoor condition (i.e. experimentally reduced and enlargedbroods, respectively; Martins & Wright 1993; Cucco &Malacarne 1996). Our prediction was that nestlings fromenlarged broods should take more risks (Malacarne et al.1994; Leonard et al. 2000) and thereby be more likely todisappear from the colony by falling out in an attemptto be closer to an arriving parent. As nestlings may fall be-cause they wander in the colony rather than because theycompete for soliciting parental attention, we also reana-lysed data of an experiment (manipulation of ectoparasiteload) that did not alter body condition (Bize et al. 2003b)but induced nestlings to switch nests (Bize et al. 2003a).Here, we predicted that parasitized nestlings are not atrisk of falling out of the colony if the best strategy to re-duce the negative impact of ectoparasites is to switch nestsrather than compete for extra food. Swifts are particularlysuitable for studying a potential link between sibling com-petition and the risk of falling out of the nest (or colony inthe present case) since nestling swifts are frequentlyobserved to disappear from the colony (common swift,A. apus: Lotem 1998; pallid swift: Malacarne & Cucco1991; Alpine swift: this study).

METHODS

Study Species

The Alpine swift breeds in colonies in holes in cliffs orunder the roofs of tall buildings. Both parents build thenest which is open with a diameter of ca. 13 cm. A singlec1utch of one to four eggs is produced per year, and bothparents incubate the clutch and feed the offspring upto fledging. Nestlings are heavily infested by the blood-sucking louse fly, Craterina melbae (Hippoboscidae; Dip-tera), which negatively affects offspring development(Bize et al. 2003b, 2004a) and adult reproductive success(Bize et al. 2004b). Parents sometimes commit infanticidejust after nestlings hatch but this is rare (from 1999 to2004 we recorded only five cases in 805 broods in the Bi-enne and Solothurn colonies). In addition, daily observa-tions showed that parents do not remove nestlings thatdie in or near their nest, so the carcasses remain visible(N ¼ 116 carcasses recorded from 1460 nestlings hatchedbetween 1999 and 2004; P. Bize, personal observation).Therefore, nestling disappearance from the colony is notexplained by parents committing infanticide or removingthe dead bodies of offspring.

Study Site

The study took place in Switzerland between 1999 and2004 in two colonies under the roof of clock towers in

Bienne (ca. 100 breeding pairs) and Solothurn (ca. 50breeding pairs). In Bienne, adults reach their nest througha 5-cm open space which runs under the roof all round thebuilding, whereas in Solothurn they use 9-cm holeslocated every 1.5 m under the roof. The mean distance be-tween nests and colony entrances � SE was 94 � 1.7 cm(range 10e400 cm). In both colonies, nests are built onopen surfaces, which allows offspring to move on footout of their nest to intercept parents (Malacarne et al.1994) and to seek adoption in neighbouring families(Bize et al. 2003a). Although domestic cats Felis catus,rats, Rattus rattus, stone martens, Fouina fouina, and car-rion crows, Corvus corone, can depredate Alpine swift nests(Arn 1960), we never found any sign of predation in thetwo colonies during the study period. Swifts were usedto the daily presence of P.B. and there was no sign thathe induced nestling falls.

General Methods

Because younger nestlings are unable to walk, weanalysed the frequency of nestlings falling out of thecolony between days 20 and 50 after hatching (day0 ¼ hatching date), by which time they are mobile andsometimes join neighbouring families where they can bepermanently adopted (Arn 1960; Bize et al. 2003a). To pre-vent permanent adoption, we monitored nests daily, andin both colonies we returned each nest-switcher to its na-tal nest immediately after its discovery (Bize et al. 2003a).Nestlings that disappeared from their natal nest and thatwere not found adopted in a neighbouring family wereconsidered to have fallen down the tower. In 2004 fiveof 18 nestlings that disappeared were found alive at thefoot of the tower and two others were found dead. It is dif-ficult to prove, however, that nestlings that disappearhave fallen out of a colony because once they are on theground humans, cats or dogs can quickly remove them;furthermore, chicks can move by themselves and hide.We have sparse data for the period 1999e2003 becausewe did not systematically search the foot of the towersfor nestlings that fell out of the colony. Once on theground, nestlings are no longer fed by parents, and hencedie if human assistance is not provided.

Brood Size Manipulation

In Bienne we matched 16 and 14 pairs of nests witha similar clutch size, hatching date and brood sizeat hatching in 2003 and 2004, respectively. Within pairsof nests we exchanged two hatchlings from one nest(i.e. reduced broods) for one in the other nest (i.e. enlargedbroods) 5 days after hatching in 2003 and 2 daysafter hatching in 2004. Prior to manipulation each nestcontained two to three nestlings, and thus brood sizeafter the manipulation remained within the naturalrange (1e4 nestlings). At the time of cross-fostering,exchanged hatchlings were similar in size to their residentsiblings (paired t tests: wing length: t44 ¼ �0.43, P ¼ 0.66;body mass: t44 ¼ �1.00, P ¼ 0.32). Enlarged and re-duced broods did not differ in size before manipulation

BIZE & ROULIN: COSTS OF SIBLING COMPETITION 541

(X� SE ¼ 2:5� 0:1; Wilcoxon signed-ranks test: Z ¼ 0.25,N ¼ 30, P ¼ 0.80) or in distance to the closest entrance inthe colony (Student’s t test: t60 ¼ 0.23, P ¼ 0.82). For anunknown reason, in 2003 one brood was abandonedsoon after its size was reduced, and thus sample size was29 reduced broods and 30 enlarged broods.

Ectoparasite Load Manipulation

In Solothurn we matched 11, 20 and 19 pairs of nestswith a similar clutch size, brood size 10 days afterhatching and number of louse flies 10 days after hatching,in 1999, 2000 and 2001, respectively. Within pairs of nestswe manipulated louse fly load by transferring ectopara-sites from a donor brood (referred to as ‘deparasitizedbrood’) to a recipient brood (‘parasitized brood’). Manip-ulations of louse fly load started 10 days after hatchingbecause ectoparasites are rarely found on offspring beforethis age (Bize et al. 2003b). As C. melbae is highly mobileand deparasitized nests were frequently reinfested by ecto-parasites, we transferred ectoparasites every 5 days untilnestlings were 50 days old. More information on this ex-periment can be found in Bize et al. (2003a, b).

Colony, Year and Other Brood Characteristics

To investigate the relative contributions of colony, yearand other brood characteristics on nestling disappearance,we carried out a single logistic regression on data fromBienne and Solothurn. The dependent variable was nes-tling disappearance (coded 1 when at least one chickdisappeared from the nest and 0 when no chick disap-peared) and the explanatory factors were colony, year,brood size, distance between nests and the closest colonyentrance, mean nestling body mass and ectoparasite load20 days after hatching, and coefficient of variance (CV) innestling body mass 20 days after hatching. A high CV inbody mass indicates that nestmates differ strongly in bodymass, and hence that one nestling may be outcompeted.Year was included in the model as a random factor nestedwithin colony. Because some of the explanatory variablescould covary, we fitted a backward stepwise logistic re-gression in which we sequentially removed nonsignificantterms in the model until only significant terms wereretained. The result is the most adequate model forexplaining the variability in nestling disappearance.Each nest appears only once in the analysis.

Ethical Note

We tried to minimize disturbance of the colonies asmuch as possible by installing curtains under the roof toavoid birds’ seeing us. We made no attempt to catchnestlings that fell out of the colonies, using nets forinstance, because of the difficulty of installing them.Although in our ectoparasite load manipulation we addedparasites to nests already naturally infested, the number ofectoparasites per parasitized nestling remained within thenatural range of infestation, as observed in previous years

in Solothurn and other Swiss colonies (Roulin et al. 1998;Bize et al. 2003b; P. Bize, personal observation). In the Al-pine swift, nestling mortality can be extremely high withyearly mortality rates ranging between 5 and 98% in Solo-thurn (Arn 1960). From 1999 to 2004, the mortality in-duced by our experiments was similar (ManneWhitneyU tests: P > 0.31) to mortality observed in other Swiss col-onies in the same years (Swiss Ornithological Station, per-sonal communication). The experiments were approvedby the veterinary services of the Canton Berne and Solo-thurn and birds were caught and marked under the legalauthorization of the Swiss Agency for the Environment,Forests and Landscape.

Statistical Procedure

For statistical analyses we used JMP IN version 5.1. (SASInstitute Inc., Cary, NC, U.S.A.). Mean values are quoted�1 SE, statistical tests are two tailed and P values less than0.05 are considered significant.

RESULTS

Brood Size Manipulation

Twenty days after the manipulation of brood size therewere fewer (1.4 � 0.1 versus 3.3 � 0.1; ManneWhitneytest: Z ¼ 6.61, N1 ¼ 29, N2 ¼ 30, P < 0.0001) but heaviernestlings (85.8 � 2.3 g versus 74.4 � 2.3 g; Student’s ttest: t57 ¼ 3.47, P ¼ 0.001) in reduced than in enlargedbroods.

Among 148 hatchlings from 59 manipulated nests, six(4.1%) offspring died before 20 days of age and 24 (16.2%)between 20 and 50 days. Among the 24 offspring that diedafter 20 days, 11 (45.8%), from 10 of 30 enlarged broods,disappeared from the colony before day 50 (mean age29.5 � 3.7 days). The 13 (54.2%) other offspring that diedafter 20 days were found in or close to their natal nest, andtheir carcasses were not removed by parents. Nestling dis-appearance never occurred in reduced broods (N ¼ 29broods; Fisher’s exact test comparing number of enlargedand reduced broods with at least one nestling having dis-appeared from the colony: P ¼ 0.0008). Cross-fosterednestlings disappeared from the colony as often (7 of 86,8.1%) as noncross-fostered nestlings (4 of 56, 7.1%; Fish-er’s exact test: P ¼ 1.00).

Ectoparasite Load Manipulation

Among 194 hatchlings from the 78 manipulated nests,nine (4.6%) died before 20 days of age and 51 (26.3%)between 20 and 50 days. Among these 51 offspring thatdied after 20 days, 14 (27.5%), from 14 (17.9%) manipu-lated nests, disappeared from the colony. Nestling disap-pearance occurred as frequently in deparasitized (7 of 40broods, 17.5%) as in parasitized nests (7 of 38 broods,18.4%; Table 1) and at a similar mean age (31.4 � 2.6 daysversus 28.6 � 3.4 days; Student’s t test: t12 ¼ 0.67,P ¼ 0.52).

ANIMAL BEHAVIOUR, 72, 3542

Colony Site, Year and Brood Characteristics

Nestling body mass, coefficient of variation in nestlingbody mass and distance to the colony entrance signifi-cantly explained offspring disappearance (Table 1).Nestlings that were light (Fig. 1a), reared in broods withlarge differences in body mass between nestmates(Fig. 1b) and located close to colony entrances (Fig. 2)were more likely to disappear from the colony. Colony,year, brood size and offspring ectoparasite load did not sig-nificantly explain offspring disappearance (Table 1).Within broods, at day 20 after hatching there was no dif-ference in body mass and ectoparasite load betweennestlings that disappeared from the colony and their nest-mates that did not disappear (body mass: 66.5 � 3.1 g ver-sus 68.6 � 2.4 g; paired t test: t18 ¼ �0.56, P ¼ 0.58;ectoparasite load: 15.6 � 2.6 versus 15.1 � 2; t18 ¼ 0.22,P ¼ 0.83).

DISCUSSION

Our results are consistent with our prediction that becausebrood size manipulation alters nestling body condition,individuals from enlarged broods should compete to in-tercept parents (Malacarne et al. 1994) and thereby be morelikely to fall out of the colony than nestlings from reducedbroods. Furthermore, a logistic regression showed that, in-dependently of brood size, mean nestling body mass andvariation in body mass between nestmates was associatedwith the probability of disappearing. Although brood sizeper se did not explain nestling disappearance, we foundthat young reared close to colony entrances were morelikely to disappear from the colony, implying that nestlingsreared in a restricted space may be more likely to fall fromthe nest. The finding that nestling disappearance is linkedto nest location highlights a potential role of sibling compe-tition in the evolution of nest design and location as well asof parental behaviour. For instance, swifts usually buildtheir nest at the bottom of cavities and, although nests areopen and nestlings can move out of them, parents preferen-tially feed offspring that are in the nest cup or close to it(Malacarne et al. 1994; Lotem 1998).

Table 1. Stepwise logistic regression on the likelihood that at leastone nestling per nest disappeared from the colony (coded 1/0) in re-lation to colony (Solothurn/Bienne), year, brood size (1e4 chicks),distance to colony entrance, mean offspring body mass and ectopar-asite load at day 20 after hatching and coefficient of variation in nes-tling body mass at day 20 after hatching

Source of variation df Wald c2 P

Mean body mass (20 days) 1 10.13 0.002CV body mass (20 days) 1 10.55 0.001Distance to entrance 1 4.73 0.03Rejected terms:

Year (colony) random 3 2.52 0.47Brood size 1 1.43 0.23Mean ectoparasite load (20 days) 1 1.72 0.19Colony 1 1.90 0.17

Year was included in the model as a random factor nested within col-ony. Each nest appears only once in the analysis.

Because one could also argue that nestlings in poorcondition fell out of the colony because they wandered insearch of a foster family where rearing conditions werebetter, we reanalysed data of an experiment (manipulationof ectoparasites) that induced nest switching (Bize et al.2003a) but had no effect on nestling body mass (Bizeet al. 2003b). We predicted that although nestlings wandermore often when experimentally parasitized than whendeparasitized (Bize et al. 2003a), they should not have dis-appeared more often from the colony even though para-sitized chicks died more often than deparasitized chicks(Bize et al. 2005). This is exactly what we found in Solo-thurn in 1999e2001. In agreement with these findings,nestling infestation load at day 20, but not brood size ma-nipulation, significantly predicted the probability of nes-tlings switching nests in Bienne in 2003 and 2004 (Bize& Roulin, in press). Therefore, nest switching appearsto be driven by selection on young to reduce ectoparasiteload, whereas nestling disappearance is triggered bysibling competition. Nestling disappearance occurred as

60

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90(a)

(b)

Bod

y m

ass

(g)

0

5

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25

Solothurn BienneColony

CV

bod

y m

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(%)

Solothurn Bienne

Figure 1. Mean � SE (a) body mass and (b) coefficient of variation in

body mass of 20-day-old nestlings from broods where at least one

young disappeared from the colony (,) and where no young disap-

peared (-).

BIZE & ROULIN: COSTS OF SIBLING COMPETITION 543

frequently in Solothurn (17.9%) as in Bienne (16.9%)which validates the comparison between the experimentscarried out at two different sites.

In conclusion, based on the facts that predation wasabsent, our presence in the colony apparently did notdisturb nestlings and seven of 18 disappearing nestlingswere found at the foot of the towers where they werereared, we suggest that nestlings in poor condition weremore likely to fall out of the colony because they competefor priority of access to an incoming parent with food.Because in many birds parents do not feed nestlings thathave fallen out of the nest, this novel cost of scramblecompetition may account for a large portion of the overallcost. Further behavioural observations are required todisentangle whether nestlings in poor condition fall outof their nest because they take more risks to be fed orbecause they are evicted from nests by stronger siblings(Mock & Parker 1997). In the present study, within neststhere was no difference in body mass between nestlingsthat disappeared and their nestmates that did not disap-pear, which suggests that nestlings may have fallen fromthe colony accidentally. Although adaptive suicide hasbeen proposed as a potential outcome of sibling competi-tion (O’Connor 1978), this hypothesis is unlikely in Al-pine swifts, because natural selection should favournestlings that seek adoption in neighbouring familiesover those that commit suicide.

Acknowledgments

We thank Ben Brilot, Roni Ostreiher and two anonymousreferees for useful comments, Stanley Maumary for kindlyproviding data from the ‘La Vaux-Lierre’ rehabilitationstation, and the Swiss Ornithological Station for kindlyproviding data about swift reproduction in other colonies.The Swiss Science Foundation supported this work (grantno. PP00A-109009 to P.B. and no. PP00A-102913 to A.R).

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Solothurn Bienne

Colony

Dis

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Figure 2. Mean � SE distance to colony entrance of broods where at

least one young disappeared from the colony (,) and where no

young disappeared (-).

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