Great spotted cuckoos respond earlier to the arrival of feeding foster parents and perform less erroneous begging when hungry than their magpie host nest‐mates

In many bird species, parents usually feed the first nestling that starts to beg before its nest‐mates. The pressure to avoid missed feeds could trigger nestlings to perform in erroneous begging in absence of parents, which has the same costs as begging in the presence of parents but without any reward. So, nestlings should try to minimize both erroneous begging and missed feeds simultaneously. The threshold to start begging is predicted to be lower for hungry nestlings and for nestlings that are unrelated to their nest‐mates, because they suffer lower inclusive fitness costs when depriving nest‐mates of food. In line with this idea, we found that brood parasitic great spotted cuckoo nestlings responded sooner than their magpie nest‐mates when an adult arrived to the nest. Under laboratory conditions, nestlings of both species rarely incurred in erroneous begging when food was abundant, but under conditions of restricted food, magpie nestlings increased erroneous begging while cuckoo nestlings did not. Highly conspicuous begging in cuckoos results in an increased predation risk, which could have resulted in stronger selection pressures on cuckoos to avoid erroneous begging, probably resulting in better developed perceptual abilities, allowing cuckoos to perform better than their host nest‐mates.     

Facultative adjustment of pre‐fledging mass recession by nestling chimney swifts Chaetura pelagica

In species susceptible to mass‐dependent flight costs, mass recession prior to fledging may ensure that fledglings have appropriate wing loading. Our objectives were to determine if mass recession by chimney swift Chaetura pelagica nestlings is intrinsically controlled or facultatively adjusted by nestlings, and if mass recession is driven by changes in parental (i.e. reduced provisioning rates) or nestling (i.e. reduced begging) behavior. Nestling swifts (n = 50 in 17 broods) were divided into three treatment groups: controls, half‐weighted, or weighted. Half‐weighted and weighted nestlings had 0.6–0.7 or 1.2–1.3‐g lead weights, respectively, glued to body feathers on their backs during the period from 16 to 26 d post‐hatching. Weighted nestlings lost more mass than control and half‐weighted nestlings. After accounting for the added weights, control nestlings also had a higher wing loading than weighted nestlings. Video recordings revealed that provisioning rates of adult swifts did not vary throughout the nestling period, but the percent time nestlings spent begging increased slightly with age. Differences in mass recession among nestlings in different treatment groups resulted in convergence toward similar wing loading values likely optimal for flight efficiency. Mechanism(s) involved in this process remain unclear because provisioning rates were similar (from day 12 to 26 post‐hatching) whereas percent begging time by nestlings tended to increase with nestling age. However, weighted nestlings may have lost more mass than control nestlings by soliciting less food from adults than siblings, being more active, losing more water due to tissue maturation, or through some combination of two or more of these factors.     

Complex feeding behaviour by magpies in nests with great spotted cuckoo nestlings

Parent decisions about food allocation are usually based on simple time‐saving rules that optimize their own fitness; however, they can sometimes vary depending on the prevailing ecological conditions both outside and inside the nest. Parent–offspring interactions also become more complex when parents suffer from brood parasitism, which implies that they care for the parasite's eggs and unrelated young. The great spotted cuckoo Clamator glandarius is a specialist brood parasite that uses the magpie Pica pica as its primary host. Here, by filming food allocation by magpie parents in natural non‐parasitized and experimentally parasitized and non‐parasitized magpie nests, we have found that magpie provisioning behaviour is highly complex including two types of feedings apart from normal ones. First, false feedings, when the parent touched the chick's beak but did not leave any food, occurred more frequently when feeding a cuckoo than when feeding magpie nestlings. Second, two types of what we have called coax feedings: 2a) when magpie parents induce a nestling to beg by waking it up by touching it softly with the beak, and 2b) when parents disregard begging signals (always from brood parasitic great spotted cuckoos) while coaxing one non‐begging nestling (always one of their own) to feed it. We suggest that brood parasitism, involving selfish excessively begging nestlings, could have acted as a selective pressure for both false and coax feedings to evolve, as both imply ignoring nestlings that beg too much. We also discuss that these parental responses could have evolved either by a discrimination without recognition mechanism, or, more probably, by a recognition‐based discrimination mechanism.     

Intensity of mouth coloration in Blackcap Sylvia atricapilla nestlings affects food distribution among siblings but not provisioning of the whole brood

Among various begging stimuli, mouth coloration has received increasing attention in recent years, and previous research has demonstrated that mouths of nestling Canaries Serinus canaria get redder with the extent of food deprivation and that parents preferentially feed nestlings of redder gapes. This study assesses whether the intensity of red mouth colour in nestling Blackcaps Sylvia atricapilla is a signal in parent–offspring communication. This is one of the few species with a naturally red gape in which the function of mouth redness has been tested. Three predictions were experimentally tested: (1) reddening the gape of a single nestling within a brood increases its provisioning in relation to other siblings; (2) reddening the gapes of all nestlings within a brood increases parental feeding rate; and (3) food deprivation increases nestling mouth redness. The effect of nestling quality on mouth redness was also assessed. The intensity of gape coloration affected food distribution, but in a way opposite to that expected: an increase in mouth redness of the nestling caused reduced feeding by parents. However, reddening the gapes of all nestlings had no effect on provisioning of the whole brood, suggesting that Blackcap parents use different cues for provisioning particular nestlings and the whole brood. Intensity of mouth redness in Blackcap nestlings was not affected either by food deprivation or by nestling quality in terms of mass and rank in the nest.     

Benefits of extra food to reproduction depend on maternal condition

The amount of food resources available to upper‐level consumers can show marked variations in time and space, potentially resulting in food limitation. The availability of food resources during reproduction is a key factor modulating variation in reproductive success and life‐history tradeoffs, including patterns of resource allocation to reproduction versus self‐maintenance, ultimately impacting on population dynamics. Food provisioning experiments constitute a popular approach to assess the importance of food limitation for vertebrate reproduction. In this study of a mesopredatory avian species, the lesser kestrel Falco naumanni, we provided extra food to breeding individuals from egg laying to early nestling rearing. Extra food did not significantly affect adult body condition or oxidative status. However, it increased the allocation of resources to flight feathers moult and induced females to lay heavier eggs. Concomitantly, it alleviated the costs of laying heavier eggs for females in poor body condition, and reduced their chances of nest desertion (implying complete reproductive failure). Extra food provisioning improved early nestling growth (body mass and feather development). Moreover, extra food significantly reduced the negative effects of ectoparasites on nestling body mass, while fostering forearm (a flight apparatus trait) growth among highly parasitized nestlings. Our results indicate that lesser kestrels invested the extra food mainly to improve current reproduction, suggesting that population growth in this species can be limited by food availability during the breeding season. In addition, extra food provisioning reduced the costs of the moult–breeding overlap and affected early growth tradeoffs by mitigating detrimental ectoparasite effects on growth and enhancing development of the flight apparatus with high levels of parasitism. Importantly, our findings suggest that maternal condition is a major trait modulating the benefits of extra food to reproduction, whereby such benefits mostly accrue to low‐quality females with poor body condition.     

Benefits of Extra Begging Fail to Compensate for Immunological Costs in Southern Shrike (Lanius meridionalis) Nestlings

Theoretical models aimed at explaining the evolution of honest, informative begging signals employed by nestling birds to solicit food from their parents, require that dishonest signalers incur a net viability cost in order to prevent runaway escalation of signal intensity over evolutionary time. Previous attempts to determine such a cost empirically have identified two candidate physiological costs associated with exaggerated begging: a growth and an immunological cost. However, they failed to take into account the fact that those costs are potentially offset by the fact that nestlings that invest more in begging are also likely to obtain more food. In this study, we test experimentally whether a 25% increase in ingested food compensates for growth and immunological costs of extra begging in southern shrike (Lanius meridionalis) nestlings. Three nestmates matched by size were given three treatments: low begging, high begging-same food intake, and high begging-extra food intake. We found that, while a higher food intake did effectively compensate for the growth cost, it failed to compensate for the immunological cost, measured as T-cell mediated immune response against an innocuous mitogen. Thus, we show for the first time that escalated begging has an associated physiological net cost likely to affect nestling survival negatively.     

Why brown-headed cowbirds do not influence red-winged blackbird parent behaviour

Brown-headed cowbirds, Molothrus ater, frequently parasitize red-winged blackbirds,Agelaius phoeniceus . The presence of a brood parasite, unrelated to both host nestlings and parents, has provoked speculation regarding within-brood food allocation and parental provisioning. This study is the first to compare directly the effect of brood parasitism on host parent and offspring behaviour in younger and older broods. We videotaped 28 unparasitized red-winged blackbird broods and compared them to 22 parasitized broods. Red-winged blackbird nestling begging appears largely unaffected by cowbird parasitism. The presence of the cowbird in the nest affected neither the latency nor duration of host nestling begging, but stimulated more frequent begging by red-winged blackbird nestlings following food distribution. Begging by cowbirds was unique in two ways: (1) cowbirds maintained a consistent begging effort throughout the nestling period (but did not receive a consistent food share); and (2) cowbirds begged longer and more frequently following the allocation of food. Persistent begging by the cowbird following the allocation of food has implications for the division of parental care, if by doing so the brood parasite is able to provoke the foster parent to increase provisioning, at the expense of brooding. We found no evidence for the adjustment of parental care. Neither the foraging rates nor the lengths of the parental feeding visits differed markedly between parasitized and unparasitized broods. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Parental allocation of food to nestling tree swallows: the influence of nestling behaviour, sex and paternity

Parents are expected to invest more in young that provide the greatest fitness returns. The cues that parents use to allocate resources between their offspring have received much recent attention. In birds, parents may use begging intensity, position in the nest or nestling size as cues to provision the most competitive young or those most likely to survive. It may also benefit parents to invest in young differentially by sex or relatedness if the fitness returns of sons and daughters differ or broods are sired by multiple males. We examined the allocation of food to tree swallow, Tachycineta bicolor, nestlings in relation to their begging behaviour, size, sex and paternity. Provisioning by parents was not related to nestling size, sex or paternity. The begging behaviour of nestlings did not differ with respect to sex or paternity. Both parents were more likely to feed nestlings that begged first or were closer to the nest entrance, suggesting that parents allocate food resources in response to cues that nestlings control. As a consequence, brood reduction was facilitated by biased provisioning within the brood in addition to the nestling size hierarchies created by hatching asynchrony. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Stimuli for nestling begging in blue tits Cyanistes caeruleus: hungry nestlings are less discriminating

In altricial birds, nestlings usually respond to the sound and appearance of the provisioning adults by begging for food when the adults arrive at the nest. Nestlings can, however, also beg incorrectly on hearing misleading sounds in the environment and fail to beg when the adult arrives. This study uses the blue tit Cyanistes caeruleus to test the hypotheses that nestling begging strategies are influenced by the reliability of the stimulus to beg, and that nestling motivational state affects the response to different stimuli. Here, we show experimentally that nestling hunger strongly influences the response to stimuli that vary in their reliability. While hunger increases begging rate, it also increases the likelihood that nestlings will beg when the parent is absent. This is in agreement with both the predictions of signal detection theory and recent empirical work on other species. We found, however, no evidence that age-related perceptual constraints influence the begging response of ten day old nestlings to different stimuli.     

Do parasites and antioxidant availability affect begging behaviour,growth rate and resistance to oxidative stress?

Early‐life trade‐offs faced by developing offspring can have long‐term consequences for their future fitness. Young offspring use begging displays to solicit resources from their parents and have been selected to grow fast to maximize survival. However, growth and begging behaviour are generally traded off against self‐maintenance. Oxidative stress, a physiological mediator of life‐history trade‐offs, may play a major role in this trade‐off by constraining, or being costly to, growth and begging behaviour. Yet, despite implications for the evolution of life‐history strategies and parent–offspring conflicts, the interplay between growth, begging behaviour and resistance to oxidative stress remains to be investigated. We experimentally challenged wild great tit (Parus major) offspring by infesting nests with a common ectoparasite, the hen flea (Ceratophyllus gallinae), and simultaneously tested for compensating effects of increased vitamin E availability, a common dietary antioxidant. We further quantified the experimental treatment effects on offspring growth, begging intensity and oxidative stress. Flea‐infested nestlings of both sexes showed reduced body mass during the first half of the nestling phase, but this effect vanished short before fledging. Begging intensity and oxidative stress of both sexes were unaffected by both experimental treatments. Feeding rates were not affected by the experimental treatments, but parents of flea‐infested nests fed nestlings with a higher proportion of caterpillars, the main source of antioxidants. Additionally, female nestlings begged significantly less than males in control nests, whereas both sexes begged at similar rates in vitamin E‐supplemented nests. Our study shows that a parasite exposure does not necessarily affect oxidative stress levels or begging intensity, but suggests that parents can compensate for negative effects of parasitism by modifying food composition. Furthermore, our results indicate that the begging capacity of the less competitive sex is constrained by antioxidant availability.     

Carotenoid-based nestling colouration and parental favouritism in the great tit

While elaborate carotenoid-based traits in adult birds may have evolved as honest signals of individual quality in the context of sexual selection or other social interactions, the function of carotenoid-based colours in juveniles is less well understood. We investigated the hypothesis that carotenoid-based nestling colouration has evolved in response to parental preference of intensely coloured offspring during food provisioning. In a field experiment, we manipulated nestling plumage colouration by a carotenoid-supplementation and analysed the parental food provisioning behaviour before feather appearance and at the end of the nestling stage. Carotenoids per se did not influence the nestlings begging behaviour or parental feeding decisions and we found no evidence that carotenoid-based colouration in nestling great tits has a signalling function in parent-offspring interactions. Parents did not discriminate between intensely coloured and control offspring in their food provisioning and in accordance with this finding intensely coloured nestlings were not heavier or larger at the end of the nestling stage. Alternative explanations for the evolution of carotenoid-based colours in nestling birds are discussed.     

With a little help from my kin: barn swallow nestlings modulate solicitation of parental care according to nestmates' need

In altricial species, offspring competing for access to limiting parental resources (e.g. food) are selected to achieve an optimal balance between the costs of scrambling for food, the benefits of being fed and the indirect costs of subtracting food to relatives. As the marginal benefits of acquiring additional food decrease with decreasing levels of need, satiated offspring should be prone to favour access to food by their needy kin, thus enhancing their own indirect fitness, while concomitantly reducing costs of harsh competition with hungry broodmates. We tested this prediction in feeding trials of barn swallow (Hirundo rustica) nestlings by comparing begging behaviour and food intake of two similar-sized nestmates, one of which was food-deprived (FD). Non-food-deprived (NFD) offspring modulated begging intensity depending on their nestmate's need: when competing with FD nestmates, NFD nestlings reduced both the intensity and frequency of begging displays compared to themselves in the control trial before food deprivation. Hence, NFD nestlings reduced their competitiveness to the advantage of FD nestmates, which obtained more feedings and showed a threefold larger increase in body mass. Moderation of individual selfishness can therefore be adaptive in the presence of a needier kin, because the indirect fitness benefits of promoting its condition can outweigh the costs of forgoing being fed, and because it limits the cost of begging escalation against a vigorous competitor.     

Are offspring begging levels exaggerated beyond the parental optimum? Evidence from a bidirectional selection experiment

Parental care involves elaborate behavioural interactions between parents and their offspring, with offspring stimulating their parents via begging to provision resources. Thus, begging has direct fitness benefits as it enhances offspring growth and survival. It is nevertheless subject to a complex evolutionary trajectory, because begging may serve as a means for the offspring to manipulate parents in the context of evolutionary conflicts of interest. Furthermore, it has been hypothesized that begging is coadapted and potentially genetically correlated with parental care traits as a result of social selection. Further experiments on the causal processes that shape the evolution of begging are therefore essential. We applied bidirectional artificial selection on begging behaviour, using canaries (Serinus canaria) as a model species. We measured the response to selection, the consequences for offspring development, changes in parental care traits, here the rate of parental provisioning, as well as the effects on reproductive success. After three generations of selection, offspring differed in begging behaviour according to our artificial selection regime: nestlings of the high begging line begged significantly more than nestlings of the low begging line. Intriguingly, begging less benefitted the nestlings, as reflected by on average significantly higher growth rates, and increased reproductive success in terms of a higher number of fledglings in the low selected line. Begging could thus represent an exaggerated trait, possibly because parent–offspring conflict enhanced the selection on begging. We did not find evidence that we co‐selected on parental provisioning, which may be due to the lack of power, but may also suggest that the evolution of begging is probably not constrained by a genetic correlation between parental provisioning and offspring begging.     

Mouth colour components of begging are dynamic signals of quality in European starling nestlings

Offspring solicit food from their parents through begging signals. Nestling skin and flange coloration are begging signals that appear to convey information about nestling need or condition, and several experiments have shown that modifications of nestling coloration affect parental allocation decisions. However, it is important to examine the short‐term changes in these signalling components in response to food constraints since such dynamic changes are required for signals to indicate condition or need. Using a food deprivation experiment, we tested whether flange and skin reflectance in European starling Sturnus vulgaris nestlings change after a three‐hour interval. We investigated whether flange and skin reflectance changed according to the predictions arising from the ‘signal of quality’ or ‘signal of need’ hypotheses on the function of begging signals. We found that flange carotenoid and UV reflectance changed according to the signal of quality hypothesis with nestlings in good condition increasing their signal expression in response to the food deprivation, whereas those in poor condition decreased their signal expression. With the use of vision modelling, we show that changes in flange reflectance are detectable by starling parents. In contrast, we found a correlation going in the opposite direction for changes in skin UV reflectance. Nestlings with low lipid reserves increased their reflectance compared to nestlings with high reserves. However, vision modelling showed that short‐term changes in skin UV reflectance are not large enough to be detectable by the parents. Our study shows that flange carotenoid and UV reflectance are dynamic components of begging with short‐term variations that can be used by parents as signals of nestling quality.     

Begging in Common Redstart Nestlings: Scramble Competition or Signalling of Need?

Begging behaviour by the young affects parental food distribution among nestlings of altricial birds. We present an analysis of two types of begging behaviour (assuming the front nest positions and gaping) based on videotaped natural nestling feeding in European common redstart (Phoenicurus phoenicurus). We test whether these types of begging support the predictions of two mathematical models: scramble competition with competitive asymmetries between nestlings [Anim. Behav. 27 (1979) 1210] or honest signalling model [Nature 352 (1991) 328]. None of the measured variables of nestling or parental behaviour were affected by body weight differences between siblings. In contrast, both gaping and nest positioning were affected by individual differences in nestling hunger. In agreement with the honest signalling model, hungrier nestlings gaped with higher probability and started to gape sooner after the arrival of the parent than did their less hungry nestmates. Those nestlings with the shortest latency to gape also received food more often. Nest positioning was related to nestling hunger in a way unforeseen by the existing models. The intervals between nestling position changes were several times longer than the intervals between parental feeding visits, and parents preferred to feed nestlings in front positions, so nestlings in front positions were always less hungry than nestlings in back. Hence the pattern of movements influenced the feeding decision in favour of the more satiated nestlings and acted against the effect of gaping. Nestling movement seemed to be caused by the less hungry nestlings moving actively from front to rear positions. Low mortality of individual nestlings within broods that survived to fledging and small within‐brood variation in fledging weights indicated low competition among nestmates. We suggest that there are two behavioural mechanisms that contribute to the equalization of fledging weights in common redstart nestlings: the signalling of need through gaping and the regular turnover of nestlings at front positions.     

Begging at high level simultaneously impairs growth and immune response in southern shrike (Lanius meridionalis) nestlings

Theoretical models suggest that begging should be costly in order to be evolutionarily stable. However, evidence for such a cost is contradictory (e.g. for growth costs) or scant (e.g. for immunological costs). Here, we experimentally test the existence of both costs in southern shrike (Lanius meridionalis) nestlings. Nestlings were paired by nest of origin and similar body mass. In each pair, a nestling was forced to beg for about 30 s h(-1) , whereas the other begged for only 2 s, both nestlings receiving the same quantity of food. At the same time, the nestling response to an antigen (phytohaemagglutinin) was measured. Nestlings forced to beg for longer showed a reduction in growth rate and in immunocompetence when compared to control chicks. The two costs occurred independently of each other and were negatively correlated to time begging. These results strongly support models of honest signalling as well as scramble competition, which predict that begging should be costly in order to be evolutionarily stable.     

Competition with a host nestling for parental provisioning imposes recoverable costs on parasitic cuckoo chick's growth

Chicks of the brood parasitic common cuckoo (Cuculus canorus) typically monopolize host parental care by evicting all eggs and nestmates from the nest. To assess the benefits of parasitic eviction behaviour throughout the full nestling period, we generated mixed broods of one cuckoo and one great reed warbler (Acrocephalus arundinaceus) to study how hosts divide care between own and parasitic young. We also recorded parental provisioning behaviour at nests of singleton host nestlings or singleton cuckoo chicks. Host parents fed the three types of broods with similar-sized food items. The mass of the cuckoo chicks was significantly reduced in mixed broods relative to singleton cuckoos. Yet, after the host chick fledged from mixed broods, at about 10-12 days, cuckoo chicks in mixed broods grew faster and appeared to have compensated for the growth costs of prior cohabitation by fledging at similar weights and ages compared to singleton cuckoo chicks. These results are contrary to suggestions that chick competition in mixed broods of cuckoos and hosts causes an irrecoverable cost for the developing brood parasite. Flexibility in cuckoos' growth dynamics may provide a general benefit to ecological uncertainty regarding the realized successes, failures, and costs of nestmate eviction strategies of brood parasites.     

Avoidance,tolerance, and resistance to ectoparasites in nestling and adult tree swallows

We examined avoidance, tolerance, and resistance strategies of nestling and adult tree swallows Tachycineta bicolor in response to ectoparasitic blowflies Protocalliphora sialia. Tree swallows avoided settling in north‐facing nest boxes early in the breeding season. These boxes were more likely to be parasitized later in the season, suggesting that box selection may facilitate blowfly avoidance. After experimentally manipulating blowfly intensity, we found that nestlings were generally tolerant of parasitism. Parasites significantly reduced nestling blood hemoglobin but had no effect on nestling body mass, primary feather growth, age at fledging, or fledging success. Parents of parasitized nestlings did not increase their food provisioning rate to promote nestling tolerance. Adult female tree swallows demonstrated both tolerance and resistance: blowfly parasitism had no effect on adult hemoglobin and body mass, and those with higher P. sialia‐binding antibody levels had fewer blowfly larvae in their nests. Nestling antibodies were unrelated to blowfly intensity. Despite considerable variation among years, our results suggest that the costs of blowfly parasitism to nestling and adult tree swallows are modest, and limited to blood loss in nestlings. Future work should examine the effects of reduced blood hemoglobin on fledgling survival and the importance of parasite‐specific antibodies.     

Features of begging calls reveal general condition and need of food of barn swallow (Hirundo rustica) nestlings

Altricial offspring of birds solicit food provisioning by complexbegging displays, implying acoustic and visual signals. Differentcomponents of begging behavior may function as reliable signalsof offspring state and thus reproductive value, on which parentsbase optimal parental decisions about allocation of criticalresources (e.g., food). We experimentally manipulated componentsof general condition of nestling barn swallows (Hirundo rustica)by (1) altering brood size by cross-fostering an unbalanced number of nestlings between pairs of synchronous broods andthus manipulating the level of within-brood competition forfood, (2) injecting some nestlings with a harmless immunogen,simulating an infection, and (3) preventing part of the nestlingsfrom receiving food for a short period while establishing controlgroups. We recorded rate of begging response by individual nestlings as parents visited the nest and recorded begging calls usinga DAT recorder to analyze six sonagraphic features of vocalizations.Our factorial experiment revealed that nestlings deprived offood begged more frequently when parents visited the nest comparedto their non—food-deprived nest mates. Food deprivationincreased duration of syllables forming begging calls, whereas brood size enlargement resulted in increased latency of responseto parental calls. Heavy nestlings in good body condition vocalizedat a relatively low peak frequency. To our knowledge, thisis the first study in which begging rate and sonagraphic structureof begging calls are shown to reliably reveal a diverse setof components of offspring general state, on which parental decisions may be based.     

Maternally derived yolk testosterone enhances the development of the hatching muscle in the red-winged blackbird Agelaius phoeniceus

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red-winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mass) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. Moreover, direct injections of testosterone into egg yolks resulted in an increase in relative complexus mass, while injections of flutamide, a testosterone antagonist, resulted in a decrease in relative complexus mass. Neither yolk testosterone concentration nor relative complexus mass differed between male and female nestlings.