Barn swallow chicks beg more loudly when broodmates areunrelatedG. BONCORAGLIO & N. SAINODipartimento di Biologia, Universita` degli Studi di Milano, Milano, ItalyIntroductionKin selection theory predicts that individuals should bemore willing to subtract limiting resources to conspecificsas genetic relatedness to the competitors decreases(Hamilton, 1964). This is expected because the indirectfitness cost of a selfish act will decrease with decreasingrelatedness to the recipient.Offspring in a family often compete among themselvesfor depreciable parental care (Clutton-Brock, 1991).Because of the extensive occurrence of extra-bondfertilizations and brood parasitism in diverse animal taxa(Birkhead & Møller, 1998), however, the level of scram-ble competition among family members can be expectedto vary according to their actual genetic relatedness,decreasing as average relatedness increases. In fact,Briskie et al. (1994) showed that the loudness of vocal-izations that nestling birds use to solicit care increasesacross species with the frequency of extra-pair paternity.Begging displays are common among birds (Kilner,2002) and include visual and vocal signals (e.g. gaping,loudness of calls) which increase in intensity according tohunger (Johnstone & Godfray, 2002). As the marginalbenefit of obtaining additional food is expected todecrease with increasing satiation (Parker & Macnair,1979), each offspring should strike the optimal balancebetween the direct fitness benefits of obtaining more foodby increasing begging intensity, and the indirect fitnesscosts of subtracting food to nestmates, which in turndepends on relatedness to the competitor.In the barn swallow population we studied, extra-pairpaternity is frequent, and approximately half of thebroods contain extra-pair offspring (Saino et al., 1997a).Brood parasitism is relatively rare (this study; Møller,1994). Food is limiting, as shown by correlational andmanipulative studies where feedings received by indi-vidual nestlings, body mass and feather growth, andimmune response have been shown to decline withbrood size, and high-quality food supplementationenhances offspring phenotypic quality (e.g. Saino et al.,1997b, 2000). In another study on a different samplefrom the same population, we found that nestlingsincrease the loudness of their begging calls after a shortperiod of food deprivation and that loudness of beggingcalls of individual nestlings positively predicts theirsubsequent short-term body mass gain as a result ofincreased parental food provisioning (Boncoraglio et al.,Correspondence: Giuseppe Boncoraglio, Dipartimento di Biologia, Univer-sita`degli Studi di Milano, Via Celoria 26, I-20133 Milano, Italy.Tel.: +39 0250314716; fax: +39 0250314713;e-mail: [email protected]ª 2007 THE AUTHORS. J. EVOL. BIOL. 21 (2008) 256–262256 JOURNAL COMPILATION ª 2007 EUROPEAN SOCIETY FOR EVOLUTIONARY BIOLOGYKeywords:begging;brood parasitism;extra-pair paternity;Hirundo rustica;kin selection;sibling competition.AbstractParents of a variety of animal species distribute critical resources among theiroffspring according to the intensity of begging displays. Kin selection theorypredicts that offspring behave more selfishly in monopolizing parental care asrelatedness with competitors declines. We cross-fostered two eggs betweenbarn swallow (Hirundo rustica) clutches and compared the loudness of beggingbetween mixed and control broods under normal feeding conditions and aftera period of food deprivation. Begging loudness was higher in mixed broodsunder normal but not poor feeding conditions. Survival was reduced in mixedthan control broods. Call features varied according to parentage, possiblyserving as a cue for self-referent phenotype matching in mixed broods. This isthe first evidence within a vertebrate species that competitive behaviouramong broodmates depends on their relatedness. Thus, kin recognition andrelatedness may be important determinants of communication among familymembers, care allocation and offspring viability in barn swallows.doi: 10.1111/j.1420-9101.2007.01441.x2007). Based on kin selection theory, these findings ledus to predict that scramble competition for food withinbarn swallow broods, as reflected by the overall level ofcollective begging loudness during feeding events, shouldincrease as the average level of relatedness among theoffspring declines.In the present study we produced clutches withexperimentally lowered average relatedness by recipro-cally cross-fostering eggs between synchronous nests totest whether broods of mixed origin compete moreharshly than natural broods. This approach was adoptedbecause decreasing the average relatedness of brood-mates by manipulating the frequency of extra-pairpaternities in a free-living breeding population of birdswould be technically very difficult. Loudness of beggingcalls of whole broods was used as an index of the overalllevel of scramble competition among broodmates duringfeeding events, and was recorded both before and after aperiod of food deprivation to investigate whether thepredicted increase in loudness after food deprivationdiffered between the two groups of broods. Finally, werecorded individual nestlings in mixed broods to testwhether sonagraphic features of begging calls variedaccording to the origin of the nestlings, thus possiblyserving as a self-referent phenotypic cue to non-kindiscrimination (see Hauber & Sherman, 2001).MethodsWe studied barn swallows at eight colonies east of Milano(Northern Italy) during spring 2005. The adults werecaptured, marked and subjected to blood sampling. Thecolonies were visited daily to record breeding events andidentify social pairs by observation.We identified pairs of synchronous clutches (Appen-dix S1 for supplementary methods) with four to sixeggs and located in different colonies. Two randomlychosen, marked eggs were reciprocally cross-fosteredbetween paired nests by day 8 of incubation (AppendixS1 for details). Each ‘mixed’ clutch was matched to acontrol clutch (thus forming a ‘dyad’ including acontrol and a mixed clutch) with four to six eggs fromthe same colony and therefore exposed to the sameecological conditions.Control clutches were subjected to the same manipu-lations as mixed clutches, except that the eggs werereturned to their original nest. We created 44 mixedclutches. We decided to record begging calls only indyads where at age 12 each brood contained at least fournestlings, and final