Behavioral Ecology

Intrasexual competition, the intense competition between individuals of the same sex, is a strong evolutionary force that is well known to select for elaborate and spectacular traits in males. In contrast, female-female competition is poorly understood. Although historically expected to be weak, evidence for female-female competition is fast accumulating. Yet, systematic studies of such competition are rare. Here, we frame and test for a strategy of competition in females in a wild population of a classically polygynous lizard, Psammophilus dorsalis. We used models of female P. dorsalis to simulate a gradient in perceived competitor threat to individually tagged, wild females on their territories, and comprehensively measured their signalling and aggression responses. Similar experiments were performed with males. Our results clearly demonstrate a complex, threat-sensitive strategy of direct competition among females in this species with conventional sex roles. Females systematically escalate their use of diverse signals along a gradient in threat and show surprisingly high levels of physical aggression. Comparisons with the responses of males to simulated threats reveal distinct sex differences in competition strategies that match theoretical expectations derived from life history. Using these findings, we argue that female-female competition may be complex but cryptic, requiring experimental manipulations to uncover.

Diel rhythms are driven by genetic and environmental components. These rhythms are mediated by the circadian clock, and entail rhythmicity of various physiological and behavioural traits. Although individuals show to some extent repeatable timing (i.e., chronotype), there is ample variation of diel timing observed within and between individuals of the same species. Here, we investigated various environmental factors, including timing of the social partner, that could explain day-to-day variation within individuals. Synchronisation with the social partner during provisioning timing could increase breeding success, and decrease extrapair paternity opportunities for females during the fertile period in case of consistent timing across the breeding stages. Therefore, we also investigated fitness consequences of between-individual variation. We first assessed the magnitude of between- and within-individual variation in the timing of nest visits by great tits (Parus major). We monitored nest visits of males and females in 37 broods during chick provisioning in 2020 and 2021. Next, we explored the responsiveness of the diel timing to environmental variables, specifically comparing abiotic and social factors. The onset of nest visits varied significantly with day within the breeding season, rainfall and the diel timing of the breeding partner but not with night temperature. In response to the partners onset, females responded stronger compared to males. By contrast, offset was generally more variable within individuals and less variation was explained by the environmental variables. Both males and females delayed their activity offset with the progressing season and females also had a later onset with more daytime rainfall. Further, the reproductive output and extrapair paternity were independent of parental chronotype and their synchronisation within pairs. It is possible that consistency of chronotypes is less important for reproductive success than the ability to plastically respond to changing environmental conditions. Thus, the next step could be to investigate potential individual differences in plasticity which could be even linked to specific chronotypes. This information might be crucial to predict how species can cope with unpredictable environmental conditions.

In over a thousand avian species, pairs coordinate vocalizations to form duets. Songbirds may use calls (simple, non-learned vocalizations) and songs (complex, learned vocalizations) to answer their partners. In Galapagos Yellow Warblers (Setophaga petechia aureola), song answers in duets appear to have evolved recently in addition to older call answers. Here, we explored the structure and functions of duets in an observational study to understand why song answers in duets may have evolved. We first described the temporal structure of duets initiated by either sex. We then tested whether the distance between pair members, countersinging with neighbours, and timing relative to aggressive encounters predicted answering probability for each sex. Finally, we tested whether the behavioral context of call and song answers differed. We found that most duets were initiated by males, and call answers were almost exclusively given by females. Female answers were also more tightly coordinated than those of males. Female answering probability decreased with increasing distance between pair members, whereas male answering probability did not. In addition, females were less likely to answer with songs when farther from their mate. Females answering probability also increased slightly when their mates initiation was preceded by a neighbors song. Our results show some evidence in support of the joint resource defence hypothesis for female answers. On the other hand, the significance of song in female answers may be related to some other functions that we did not test here, such as signaling quality or commitment to the pair bond. Lay summaryWho turns a song into a duet? In Galapagos Yellow Warblers, it is usually the female: she answers her mates song with a tightly timed call or song. Male replies were rarer and less coordinated. While male replies didnt seem to have a function, female replies were used in territorial disputes with neighbours and may indicate joint ownership of a territory. Female song replies, on the other hand, may signal commitment to the pair bond.

Song learning is generally assumed to be beneficial for a young songbird, but merely incidental, without costs or benefits, for the older song tutors. In the present study we contrast two mutually exclusive hypotheses about the tutor/tutee relationship: (1) that it is cooperative, or at least mutually tolerant, with tutor and tutee mutually benefiting from their relationship, vs. (2) that it is competitive, with tutor and tutee competing over territory, so that one or the other suffers negative fitness consequences of their relationship. In a field study of three consecutive cohorts of song sparrows (Melospiza melodia) we determined the older bird (primary tutor) from whom the young bird (tutee) learned most of his songs, and how long tutee and primary tutor survived subsequently. We found that the more songs a tutee learns from his primary tutor, the longer their mutual survival on their respective territories. This study provides the first evidence of a mutual benefit of bird song learning and teaching in nature.

Individually distinctive vocalizations are widespread in nature, although the ability of receivers to discriminate these signals has only been studied through limited taxonomic and social lenses. Here we ask whether anuran advertisement calls, typically studied for their role in territory defense and mate attraction, facilitate recognition and preferential association with partners in a pair bonding poison frog. Combining no- and two-stimulus choice playback experiments, we evaluated behavioral and physiological responses of females to male acoustic stimuli. Virgin females oriented to and approached speakers broadcasting male calls independent of caller identity, implying that females are generally attracted to male acoustic stimuli outside the context of a pair bond. When pair bonded females were presented with calls of a mate and a stranger, they showed a slight preference for calls of their mate. Moreover, behavioral responses varied with breeding status: females with eggs were faster to approach stimuli and spent more time in the mate arm than females that were pair bonded but did not currently have eggs. Our study suggests a potential role for individual vocal recognition in the formation and maintenance of pair bonds in a poison frog and raises new questions about how acoustic signals are perceived in the context of monogamy and biparental care.

Testosterone mediates suites of physical and behavioral traits across vertebrates, and circulation varies considerably across and within taxa. However, an understanding of the causal factors of variation in circulating testosterone has proven difficult despite decades of research. According to the challenge hypothesis, agonistic interactions between males immediately prior to the breeding season produce the highest levels of testosterone measured during this period. While many studies have provided support for this hypothesis, most species do not respond to male-male competition by elevating testosterone. As a result, a recent revision of the hypothesis ( challenge hypothesis 2.0) places male-female interactions as the primary cause of rapid elevations in testosterone circulation in male vertebrates. Here, we offer a test of both iterations of the challenge hypothesis in a tropical bird species. We first illustrate that male White-shouldered Fairywrens (Malurus alboscapulatus) differ by subspecies in plasma testosterone concentrations. Then we use a social network approach to find that males of the subspecies with higher testosterone are characterized by greater social interaction scores, including more time aggregating to perform sexual displays. Next, we use a controlled experiment to test whether males respond to simulated territorial intrusion or courtship interaction contexts by elevating testosterone. Males sampled during courtship had greater plasma testosterone both relative to flushed controls and males sampled during simulated intrusion. Ultimately, our results are consistent with challenge hypothesis 2.0, as males rapidly elevated testosterone following interactions with females, but not during territorial challenges.

The timing of reproductive processes can influence how mating interactions shape reproductive strategies. We examined how female age correlates with receptivity and preference in the jumping spider Habronattus formosus, a system characterized by elaborate male courtship and strong female choice. To test how receptivity changes across the post-maturation period, we paired females of different ages with males and quantified male courtship displays and mating outcomes. Females were not receptive immediately after maturation and instead exhibited receptivity primarily 2-3 weeks after maturation. We further found that the full male courtship predicted mating success in older females, whereas only the second stage of the courtship predicted success in younger females. The delay in female receptivity is consistent with expectations for strong female choice systems, where females may have a greater opportunity to evaluate male courtship before mating. These results highlight the importance of age-dependent changes in female reproductive state and suggest that the timing of receptivity may shape how different components of male courtship influence mating success.

Behavioural variation is a hallmark of animal societies, which commonly contain breeders and nonbreeders, and helpers and nonhelpers. In some cases labour is divided with nonbreeders \"helping\" - gaining indirectly, via genetic benefits, or directly, e.g. by augmenting group size. Conversely, they may benefit by not helping, conserving energy for breeding later. However, subordinate behaviour after inheriting a breeding position is rarely evaluated.\n\nIn the Australian interior, Acacia thrips Dunatothrips aneurae (Thysanoptera) glue Acacia phyllodes together into \"domiciles\". Foundresses, usually sisters, build domiciles singly or communally. Some co-foundresses are nonreproductive, and their role is currently unknown. I experimentally rejected the idea that they substantially \"help\" by contributing to domicile repair. Nonreproductives were less likely to repair damage than reproductives. Alternatively, they may be waiting to inherit the domicile, or simply of too poor quality to reproduce or help. To test these alternatives, in the field, I allowed repairer or nonrepairer females to \"inherit\" a domicile by removing their nestmate(s). Thus isolated, \"nonrepairer\" females took much longer to repair domiciles than \"repairers\", control singletons or pairs. Although ovarian condition was equivalent across groups, after 21 days nonrepairers actually laid fewer eggs compared to other groups.\n\nThus, labour was not divided: instead reproduction and helping covaried, probably depending on female quality and the outcome of intra-domicile competition. Nonreproductive nonhelpers were not waiting to breed. Their role, and their net effect on colony productivity, remains to be shown. They are likely subfertile, and may make the \"best of a bad job\" by gaining indirect benefits to the best of their limited ability.

Male bees navigate complex trade-offs between energy acquisition and reproductive signalling, yet their movement strategies remain understudied. Unlike workers that optimise foraging to support the colony, males forage independently to collect nectar and deposit sex pheromones on selected plants. Using high-resolution 3D tracking in an indoor flight cage, we investigated how the spatial arrangement of nectar and scent-marking sites, along with nectar availability, influence male movement patterns. We manipulated the distribution of feeders (artificial flowers) and scent-marking locations (branches), and varied nectar delivery rates, to assess effects on foraging, scent-marking, and patrolling. Males responded strongly to spatial structure: in clumped arrays with evenly spaced resources, movements between consecutive visits were shorter and more localised, while in dispersed arrays with irregular spacing, transitions were longer and more variable. The combination of dispersed spacing and low nectar availability imposed the highest foraging demands, resulting in fewer feeding events and reduced total feeding time. Despite these increased costs, males maintained consistent investment in reproductive behaviours, suggesting a prioritisation of mate-seeking over energy gain. Rather than reducing signalling, males adjusted their foraging strategy--favouring fewer but prolonged feeding bouts when nectar availability allowed. These findings reveal a unidirectional behavioural adjustment, in which foraging is modulated to sustain reproductive effort, and show how spatial resource structure and nectar availability together shape movement decisions in male pollinators. LAY SUMMARYMale bumblebees keep searching for mates even when food is scarce, adjusting how they forage to conserve energy. Using 3D tracking in a controlled environment, we found that males reduce foraging effort but maintain reproductive behaviours. This shows how animals balance survival and reproduction in changing environments, offering new insights into pollinator behaviour and resilience.

Social reciprocity is thought to be the most important driver of cooperation among non-kin, but its effects within complex social networks where individuals choose their partners have not been studied. Here, we test whether reciprocation can explain social dynamics in a bird species where males form coalitions to court females, the wire-tailed manakin (Pipra filicauda). In our study population, approximately a third of male-male social interactions involve one male leaving his territory to interact and display with the owner of another territory; this directional behaviour provides benefits to the recipient at a cost to the visiting male. We characterize social ties among pairs of territory holders according to the directionality of their interactions, and show here that territory holding males engaged in far more reciprocated (bidirectional) partnerships with other territory holders than expected by chance. Reciprocated partnerships were also stronger (i.e., the partners interacted more frequently) than non-reciprocated partnerships, controlling for spatial proximity of the territories. An individual-level analysis revealed that a males social contribution to a given partner was predictive of the number of social interactions he received from that same partner. Finally, we show that reciprocation predicted the long-term stability of social partnerships one year later. Together, these results demonstrate that reciprocity is associated with stability in complex social networks where partner choice is flexible. LAY SUMMARYO_ST_ABSWhen animals are free to choose their partners, how does reciprocity influence social dynamics?C_ST_ABSIn the wire-tailed manakin, males form coalitions with other males to help attract mates. We tracked male social networks and found that some males form reciprocal partnerships where each male leaves its territory to display at the others territory. These bidirectional partnerships are particularly strong and stable, indicating that reciprocity is associated with stability in complex social networks.

Protogynous sex change, where individuals first function as females and later as males, is a key life-history strategy among polygynous reef fishes. In haremic systems, sex change is typically socially regulated, with dominants suppressing subordinates sex change through aggression. Females within a harem form a size-based hierarchy that can remain stable in most species through the threat of eviction. We studied a different situation in the cleaner wrasse Labroides dimidiatus, where larger females have incomplete control, as they spend most of their time alone at their own cleaning territory. We tracked over 400 individuals for 12 months, recording growth, behavior, social organization, and sex change. We confirmed earlier reports that both sexes direct aggression primarily at those ranked immediately below them. However, we observed 30 cases where smaller females outgrew larger ones, revealing hierarchy instability. Of 42 sex change events, 43% occurred in presence of the male, and half of these early sex changers were not the largest female, but individuals overlooked by the male. Fast growth relative to harem-mates and harem switching increased the likelihood of sex change. Local population densities also influenced growth and sex change, with individuals in high-density demes growing faster and changing sex at larger sizes. Our findings reveal flexible sex change dynamics in a system with incomplete social dominance. Such incomplete control and observations that becoming male confers both higher reproductive success and survival highlight the need to expand game-theoretical and life-history frameworks to encompass such strategic flexibility. Lay summaryDominant cleaner wrasse cannot fully control subordinates as individuals occupy distinct core areas. Tracking 400 fish for a year, we found that smaller females could outgrow initially larger ones, and early sex change despite a larger male. Fast growth and harem switching increased the chances of becoming male. Population density also shaped these strategies. Our findings reveal flexible sex change dynamics in a system where becoming male confers both higher reproductive success and survival.

Acoustic communication is fundamental to social interactions in many animal species, allowing individuals to transmit information about identity, reproductive status, and competitive ability. Because call production incurs inherent costs, individuals are expected to modify their vocalizations depending on the social context. However, while context-dependent call variation has been documented in several taxa, including anurans (frogs and toads), glass frogs (Centrolenidae) remain among those for which the acoustic repertoire across social contexts is poorly characterized. Here, we investigated context-dependent call modification in males of the Emerald glass frog, Espadarana prosoblepon, comparing calls produced across four social contexts: advertisement in isolation, advertisement in a group, courtship interactions with females, and agonistic interactions with other males. By integrating detailed behavioral field observations with a robust analytical framework, we present evidence that males modify multiple acoustic properties in response to the social context. Specifically, males produced longer, louder advertisement calls when calling in a group than when calling in isolation. Courtship calls contained more notes and were louder than other call types, whereas aggressive interactions were characterized by pulseless, low-frequency, soft calls. Our findings demonstrate that the distinct call types of E. prosoblepon are consistently associated with specific social contexts and can be reliably distinguished based on their acoustic structure, providing a framework for future studies investigating the functional significance of context-dependent acoustic signals in anurans.

Animals can exhibit remarkable reproductive plasticity in response to their social surroundings, with profound fitness consequences. The study of such plasticity in females, particularly in same-sex interactions, has been severely neglected. Here we measured the impact of variation in the pre-mating social environment on reproductive success in females and tested the underlying mechanisms involved. We used the Drosophila melanogaster model system to test the effect of varying female group size prior to mating and deployed physical and genetic methods to manipulate the perception of different social cues and sensory pathways. We found that socially isolated females were significantly more likely to retain unfertilised eggs before mating, but to show the opposite pattern and lay significantly more fertilised eggs in the 24h after mating, in comparison to grouped females. More than 48h of exposure to other females was necessary for this socially-induced plasticity to be expressed. Neither olfactory nor visual cues were involved in mediating these responses. Instead, we found that females detected other females through direct contact with the deposits they leave behind, even in the absence of eggs. The results demonstrate that females show striking reproductive plasticity in response to their social surroundings and that the nature of their plastic reproductive responses, and the cues they use, differ markedly from those of males. The results emphasise the stark contrasts in how each sex realises reproductive success.

Mate choice is a wide-spread phenomenon with important effects on ecological and evolutionary dynamics of successive generations. Increasing evidence shows that males can choose females if females vary in quality and these mating choices can strongly impact fitness. In the herbivorous spider mite Tetranychus urticae males engage in precopulatory mate guarding of quiescent females, and it is known that females vary in their time to sexual maturity and fecundity. However, our understanding of how males maximize their reproductive success and which female phenotypic traits are important cues for their mating decisions are still limited. In many arthropod species, female body size and pheromones are well known proxies for fecundity. These traits--and thus possibly male mating decisions--are however sensitive to environmental (dietary) stress. By allowing males to freely choose amongst many (synchronized) females in a controlled semi natural environment, we found that guarded females have a higher fecundity and are closer to sexual maturity than non-guarded females. Despite the fact that female body size was positively correlated with fecundity and significantly influenced by the environment, males did not discriminate on body size nor did we find evidence that they used other cues like cuticular pheromones or copying behavior (social cues). In conclusion we were able to show male mate preference for females that are closer to sexual maturity and have higher fecundity, but we were unable to identify the female traits that signal this information

Behavioural defences are a key adaptation across animals, but confirming decay under relaxed selection is challenging. If expression involves decision-making, then the defence may only be expressed when the right environmental cues are available (cryptic plasticity). Even if defences wane, they could be quickly restored once selection resumes as animals can learn by observing others (social memory). Here we tested cryptic plasticity and social memory under relaxed selection in a cuckoo host, the Common Reed Warbler, by manipulating perceived parasitism risk with social information in geographically-distinct areas differing in allopatry (100 to 1000 years). Despite predicting social information of a cuckoo vs. a control would elicit otherwise cryptic behavioural defences, only birds that mobbed model cuckoos prior to manipulation upregulated this defence, and only in the more recently allopatric population. Social information also had no effect on low-to-absent rates of foreign egg rejection, a second line of defence common to cuckoo host species. These results contrast with parasitised locations where similar methods provoke mobbing and increase egg rejection ten-fold. Our study therefore demonstrates that, even when manipulating information to account for reaction norms, behavioural defences may degrade rapidly under relaxed selection, and thus facilitate a geographic mosaic of coevolution.

In animal displays, motor performance can provide key information to recipients associated with whole-organism condition. It has been argued that biomechanical processes constrain motor displays and performing near the phenotypic boundary can be an honest indicator of quality. Using data from 140 blue tits Cyanistes caeruleus, including male and female songs, we found a new phenotypic boundary defined by a trade-off between song length and note length. Male "song length competence" - a composite metric considering the trade-off with note length - peaked seasonally during the female fertile period and was positively associated with reproductive success. These findings support song length competence as a fitness indicator in blue tits. Recovery competence, a well-known composite metric assessing the efficacy to produce consecutive notes, showed similar seasonal and reproductive variation but with a much smaller impact on reproductive success. We further confirmed previously known trade-offs between 2) frequency modulation vs. note length and 3) frequency jumps vs. inter-note gaps between notes, but these metrics seemed biologically irrelevant with no significant variation in relation to season or individual fitness. We encourage further research on birdsong and other animal displays to shed light on the universality of these principles in the assessment of motor performance.

In bi-parental species, reproduction is not only a crucial life-history stage where individuals must take fitness-relevant decisions, but these decisions also need to be adjusted to the behavioural strategies of a partner. Hence, communication is required, which could be facilitated by condition-dependent signals of parental quality. Yet, these traits have (co-)evolved in multiple contexts within the family, as during reproduction different family members may coincide and interact at the site of breeding. In this study we explore whether a condition-dependent trait acts a quality signal and regulates intra-family interactions in a bird species, the blue tit (Cyanistes caeruleus). As a family is a complex network where signals could be perceived by multiple receivers, we expected that experimentally blocking the reflectance of an adults UV/yellow colouration of breast feathers may affect the behavioural strategies of all family members. We found an increase of parental investment in nests with an UV-blocked adult, as the partner compensated for the perceived lower rearing capacity. As the UV-blocked adult did not change its provisioning behaviour, as was to be expected, their partner must have responded to the (manipulated) signal but not to a behavioural change. However, offspring did not co-adjust their begging intensity to a signal of parental quality. Opposite to adults, we propose that offspring respond to the behaviour but not to the parental signal. Overall, our results show experimentally at the first time that UV/yellow colouration of blue tits acts as a quality signal revealing the rearing capacity to mates.

O_LIAnimal personality differences, i.e. consistent among-individual behavioural differences within populations, are prevalent across the animal kingdom. However, we are just beginning to understand the adaptive significance of the observed behavioural variation. We are particularly in need of empirical studies testing hypotheses of proposed theoretical frameworks aiming to understand the existence animal personality differences. In this study, we investigated a hypothesis derived from a framework suggesting that sexual selection may generate and maintain personality variation. The authors of this framework propose parental care as a mechanism linking animal personality and reproductive fitness. C_LIO_LIWe repeatedly measured individual boldness in male and female rainbow kribs, Pelvicachromis pulcher, a West African cichlid known for territorial cave breeding and shared parental care. We then formed 54 breeding pairs with varying behavioural contrasts in boldness. For pairs that produced offspring (N = 20), we repeatedly recorded parental care behaviour (parental boldness and brood guarding) of both parents over one month. Reproductive success was measured as the likelihood to reproduce, the number of offspring, and offspring size at the end of breeding. C_LIO_LIIn both sexes, we found consistent among-individual differences in boldness before breeding and in parental care behaviours. Bolder males were less likely to reproduce but, when breeding was successful, produced bigger broods compared to less bold males. Female parental boldness positively correlated with the number of offspring produced. However, individual boldness before breeding did not predict parental care behaviour in either sex and we found no effects of the pairs contrast in prebreeding boldness on parental care coordination or reproductive success. C_LIO_LIThe outcomes of our study may suggest that among-individual variation in male boldness is maintained by males with different behavioural types playing different reproductive strategies of equal average fitness. Future research should delve into understanding the intricate link between male boldness and reproductive success, exploring for instance underlying genetic mechanisms and interactions with environmental conditions. C_LI

O_LIUnderstanding the maintenance of genetic variation in reproductive strategies and polymorphisms in the wild requires a comprehensive examination of the complex interactions between genetic basis, behavior, and environmental factors. C_LIO_LIWe tested the association between three color genotypes and variation in female pheromone signaling and male antennal morphology in the wood tiger moth (Arctia plantaginis). These moths have genetically determined white (WW, Wy) and yellow (yy) hindwings that are linked to mating success and fitness, with heterozygotes (Wy) having an advantage. We hypothesized that attractiveness and reproductive success are correlated, with Wy females being more attractive than the other two genotypes which could contribute to maintaining the polymorphism. C_LIO_LIFemale attractiveness was tested by baiting traps with females of the three color genotypes both in low-(i.e., field setup) and in high-population density (i.e., large enclosure setup). Males ability to reach females was correlated to their own color genotype and antennal morphology (length, area, and lamellae count). C_LIO_LIContrary to our prediction, morph-related reproductive success and attractiveness were not correlated. Heavier Wy females attracted a lower proportion of males compared to WW and yy females. Specifically, an increase in weight corresponded to a decreased Wy but increased yy female attractiveness. yy females were generally more attractive than others likely due to earlier pheromone release. In males, lamellae count and genetic color morph were linked to the males ability to locate females. Furthermore, male traits affected their ability to reach females in a context-specific way. Males with denser antennae (i.e., higher lamellae count) and white males reached the females faster than yellows in the enclosure, while yellow males located females faster than whites in the field. C_LIO_LIOur results indicate that higher yy female attractiveness was likely affected by the combined effect of early pheromone release, female weight and higher population density. Males searching success was affected by morph-specific behavioral strategies and local population density. Ultimately, the combined effect of genotype-related pheromone signaling strategies of females together with environment-dependent male behavior affect male response and potentially contribute to maintaining variation in fitness-related traits. C_LI

Life history theory provides a framework for understanding how trade-offs generate negative trait associations. Among nestling birds, developmental rate, risk of predation, and lifespan covary, but some associations are only found within species while others are only observed between species. A recent comparative study suggests that allocation trade-offs may be alleviated by disinvestment in ephemeral traits, such as nest-grown feathers, that are quickly replaced. However, direct resource allocation trade-offs cannot be inferred from inter-specific trait-associations without complementary intra-specific studies. Here, we asked whether there is evidence for a within-species allocation trade-off between feather quality and developmental speed in tree swallows (Tachycineta bicolor). Consistent with the idea that ephemeral traits are deprioritized, nest-grown feathers had lower barb density than adult feathers. However, despite substantial variation in fledging age among nestlings, there was no evidence for a negative association between developmental pace and feather quality. Furthermore, accounting for differences in resource availability by considering provisioning rate and a nest predation treatment did not reveal a trade-off that was masked by variation in resources. Our results are most consistent with the idea that the inter-specific association between development and feather quality arises from adaptive specialization, rather than from a direct allocation trade-off.