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In species with biparental care, the amount of care devoted to offspring is affected by the negotiation rules that the parents adopt. Theoretical models predict that biparental care can be evolutionarily stable if a decrease in parental investment by one parent is only partially compensated by its partner. However, empirical studies have found substantial variability in compensatory behaviour and have mainly used nesting provisioning as a single measure of parental effort. In this study, we investigated parental compensatory behaviour for two parental tasks, offspring provisioning and nest defence. These two tasks are likely characterized by different levels of risk as well as different cost and benefit functions for the parents, which may affect the expected level of compensatory responses. We experimentally widowed (by temporarily removing one parent) male or female Spanish sparrows, Passer hispaniolensis, and measured their compensatory responses to offspring provisioning and nest defence (after predator presentation of green whip snake, Hierophis viridiflavus, models). Parents differed in their compensatory responses in relation to parental task and sex: both sexes partially compensated for offspring provisioning, but females compensated by a larger degree than males. For predator defence, males instead decreased defence behaviour by increasing latency and reducing the number of attacks, while females did not change their behaviour when caring alone. This within-individual comparison indicates that parents adjust their compensatory behaviour according to parental task. We discuss how these differences could arise due to different costs and benefits of extra investments.

The amount of care parents provide to the offspring is complicated by an evolutionary conflict of interest (‘sexual conflict’) between the two parents. Recent theoretical models suggest that pair coordination of the provisioning may reduce this conflict and increase parent and offspring fitness. Despite empirical studies showing that pair coordination is common in avian species, it remains unclear how environmental and ecological conditions might promote or limit the ability of parents to coordinate care. We compared the level of pair coordination, measured as alternation and synchrony of the nest visits, of house wrens Troglodytes aedon pairs breeding in a rural (10 nests) and a suburban (9 nests) site and investigated how differences in parental behaviours were related to habitat composition, prey abundance and how they ultimately related to reproductive success. We found that parents alternated and synchronized their nest visits more in the rural site compared to the suburban one. The suburban site is characterized by a more fragmented habitat with more coniferous trees and less caterpillar availability. Offspring from the rural site were heavier at fledging than at the suburban site. Taken together, these results suggest that environmental conditions play an important role on the emergence of coordinated parental care and that considering environmental variables is pivotal to assess the fitness consequences of parental strategies.

For parents, rearing offspring together is far from a purely cooperative exercise, as a conflict of interest (‘sexual conflict’) exists over their optimum level of care. Recent theory emphasises that sexual conflict can be evolutionarily resolved, and complete parental cooperation can occur if parents directly respond (‘negotiate’) to each other and coordinate their level of care. Despite numerous experiments show that parents are responsive to each other, we still lack empirical evidence of the behavioural mechanisms by which this negotiation occurs. In this study, we investigated the spatio-temporal coordination of parental provisioning behaviour as a possible mechanism of negotiation over parental care. We deployed an automated radio-tracking technology to track the provisioning activity of wild great tit (Parus major) pairs during chick rearing. Our analyses represent the first detailed spatial and temporal description of foraging coordination in songbird parents in a natural context. We demonstrate that the foraging behaviour of the two parents is highly coordinated in space and time, with parents changing their foraging locations in conjunction with their partners’ movements. Therefore, foraging coordination could be a mechanism by which parents directly monitor and respond to each other’s level of investment.

Quantifying organismal capacity for compensatory mechanisms is essential to forecast responses to environmental change. Despite accumulating evidence for individual variation in physiological plasticity, the causes and consequences of this variation remain unclear. An outstanding question is whether individual reaction norms are consistent across different environmental challenges, i.e. whether an individual that is responsive to one environmental variable will be equally responsive to a different environmental variable. Additionally, are these reaction norms themselves consistent over time, i.e. repeatable? Here, we quantified individual baseline glucocorticoid responses in house sparrows, Passer domesticus, to sequential manipulations of temperature, wind speed and food unpredictability that were repeated in discrete blocks of sampling under both control and stressor-exposed conditions. Individuals significantly decreased their baseline corticosterone levels and increased their mass during treatment exposure. This response was consistent across environmental challenge types. There was high repeatability in the intercept and slope of the baseline corticosterone reaction norm between environmental challenges but broad credible intervals in the repeatability of the reaction norm slope, suggesting that although glucocorticoid levels during baseline conditions are repeatable, among-individual variation in the shape of the glucocorticoid response may be higher than within-individual variation. Within-subject variation in baseline corticosterone levels was mainly explained by within-individual variation in body mass during stressor exposure. Despite the high lability in physiological traits, endocrine plasticity is repeatable across environmental challenges and may be able to evolve as a result of genetic accommodation, in which selection acts on genetic variation of reaction norms.

In species with biparental care, the amount of care devoted to offspring is affected by the negotiation rules that the parents adopt. Recently, turn taking in provisioning visits has been proposed as a negotiation rule by which parents respond to their partner's behaviour, which results in a perfect alternation of the nest visits by the parents. Empirical evidence suggests that parents do not strictly alternate their visits, and, so far, this imperfect alternation has received no experimental investigation. In this study, we tested whether alternation of nest visits might be subject to time constraints affecting the ability of parents to strictly take turns. We manipulated the workload of 15 great tit, Parus major, pairs using a short-term brood size manipulation. Parental nest visits alternated more in reduced than control and enlarged broods. To understand whether this variability could be caused by changes in turn taking, we explored the rate and regularity of the parents' intervisit intervals. Treatment differences in alternation were still present when controlling for the rate and regularity of the visits by each of the two parents, suggesting that workload also affected alternation via the temporal sequence of the intervals (e.g. via turn taking). Our results show that alternation of nest visits varies in response to workload and is not merely a by-product of variation in visit rate or regularity.

Raising a family is not free of conflict for parents, as each parent benefits when its partner provides more care for the offspring. Resolving this conflict requires cooperation between the parents. One way to achieve such cooperation might be to coordinate parental provisioning by synchronizing (i.e. returning to the nest at the same time) or alternating (i.e. taking turns) offspring provisioning at the nest. Empirical studies in birds indicate that pair coordination of the nest visits is common; however, it is unknown whether this behaviour is directly related to different outcomes of sexual conflict, such as brood desertion. We used the rock sparrow, Petronia petronia, a species with high levels of sexual conflict, to explore whether alternation and synchrony of the nest visits were related to later brood desertion. Pairs with no desertion alternated and synchronized their nest visits more than pairs in which one sex deserted. This difference in coordination was not simply a by-product of differences in provisioning by the parents. Synchrony of the visits also increased with offspring age in the pairs with no desertion. We provide evidence, for the first time to our knowledge, that the degree of parental coordination is strongly associated with the ultimate consequence of sexual conflict, brood desertion, supporting the idea that coordination in parental behaviour might promote conflict resolution.

There is a renewed interest in investigating individual variation in hormone levels in relation to fitness metrics, as hormones act as mediators of life-history trade-offs. Hormone concentrations, however, are labile, responding to both internal and external stimuli, so the relationship between hormones and fitness can be non-consistent. One explanation of this inconsistent relationship is that a single hormone sample may not be representative of individual phenotypes in a free-living species. We addressed this issue by repeatedly sampling a free-living population of mountain white-crowned sparrows, Zonotrichia leucophrys oriantha, for baseline and stress-induced corticosterone (cort) and testosterone (T) across different stages of the breeding season. We measured (co)variation using three different methods, taking into account inter- and intra-individual variances, to determine whether hormone levels and the stress response are repeatable. We documented the temporal (over 3 months) and spatial (home-range) variation of individual hormone phenotypes and investigated how these components related to nesting success. At the population level, we found significant repeatability in male stress-induced cort concentrations but no repeatability in male or female baseline cort or male T concentrations. Using a new metric of intra-individual variance focusing on the stress response (profile repeatability), we found a wide range of variance scores, with most individuals showing high variation in their stress response. Similarly, we found a low level of repeatability of the reaction norm intercept and slope for the stress response across different life-history stages. Males with higher concentrations of stress-induced cort had more central home-ranges. Males with higher body condition had larger home-ranges; however, home-range size did not relate to male hormone concentrations or nesting success. We also did not find any significant relationship between variation in hormone levels and nesting success. We recommend that future studies combine both physiological and environmental components to better understand the relationship between hormones and fitness.

As urban areas continue to expand globally, understanding how and why species respond to novel habitats becomes increasingly important. Knowledge of the mechanisms behind observed phenotypic changes in urban animals will enable us to better evaluate the impact of urbanization on current and future generations of wildlife. Physiological changes, such as those involved in the endocrine stress response, may allow individuals to inhabit and thrive in urbanized areas, but it is currently unknown how these changes arise in natural populations. In this study, we performed a four-way cross-foster experiment in free-living house wren chicks, Troglodytes aedon , to disentangle whether differences in baseline corticosterone between urban and rural individuals are a result of genetic and/or plastic mechanisms during development. We found that urban chicks already had higher corticosterone levels than their rural counterparts on the day they hatched, which suggests a possible genetic component to the corticosterone phenotype. However, rural offspring that were moved to an urban environment significantly increased their corticosterone levels, mimicking those of urban offspring. Our findings suggest that, although differences in baseline corticosterone concentrations between urban and rural individuals may have a genetic component, plasticity plays a pivotal role and can modify the corticosterone phenotype in response to the environment experienced in the first two weeks of life.

The aim of this thesis is to investigate the behavioural mechanisms underlying pair coordination over parental care. For more than twenty years, theoretical and empirical work suggested that parents and offspring would inevitably suffer as a result of a sexual conflict between parents over the individual levels of parental investment. However, a recent theory proposed that a form of parental coordination, turn-taking during offspring provisioning, could lead to conflict resolution. Using a bi-parental songbird and an automatic radio-tracking system, I investigated the behavioural mechanisms generating parental provisioning patterns. The main finding of this thesis is that the provisioning pattern of nest visits is a flexible behaviour and is mainly created by coordination between the parents during both foraging movements and nest visits. These results provide support for a novel perspective in which sexual conflict can be resolved through cooperation rather than conflict.

Social foraging is thought to provide the possibility of information transmission between individuals, but this advantage has been proved only in a handful of species and contexts. We investigated how social connections in captive flocks of house sparrows (Passer domesticus) affected the discovery of (i.e. feeding for the first time from) two hidden food patches in the presence of informed flock-mates. At the first-discovered and most-exploited food patch social connections between birds affected the order of discovery and presumably contributed to a greater exploitation of this patch. However, social connections did not affect discovery at the second food patch despite its close spatial proximity. Males discovered the food sources sooner than females, while feeding activity was negatively related to patch discovery. Age had no effect on the order of discovery. Birds that first discovered and fed at the food patches were characterized by higher level of social indifference, i.e. followed others less frequently than other birds in an independent context. Our findings provide experimental evidence for the importance of variable social connections during social foraging in house sparrow flocks, and suggest that social attraction can contribute differently to the exploitation of different patches when multiple food sources are present.

Several studies have investigated how individuals’ social status or behavioural and physiological traits affect social organisation in group-living species, but the potential role of ornamental traits has been rarely studied. Here, we analysed initiator–follower interactions in relation to experimentally manipulated sexually selected ornaments in captive flocks of male bearded reedlings (Panurus biarmicus). We manipulated beard length or tail length on the most and least frequently followed individuals in unisex flocks and examined how trait manipulation affected social prestige (i.e. frequency of being followed) and centrality (i.e. frequency of following others) of these individuals. We found that prestige/centrality after manipulation was significantly positively associated with their before-manipulation measure, whereas body condition did not affect social position within flocks. Similarly, the manipulation of the ornamental traits had no significant effect on the social positions of the manipulated i...

Forming foraging groups may help to improve protection from predators, foraging efficiency and exchange of information, but to benefit from these advantages, animals need to maintain group cohesion. Several studies have investigated how social status or behavioural and physiological traits promote leadership and followership in various species, but the potential role of ecological factors in modulating social cohesion of foraging groups has received less attention. We used social network analysis to investigate how a temporary shortage of food affects within-group following behaviour in house sparrow, Passer domesticus, flocks and which individual characteristics influence birds' positions in the following networks. To test this, we observed nine captive house sparrow flocks, composed of 12 individuals each, in three consecutive sessions, in which the food availability was altered. We found that food reduction did not modify the following network structure and that individuals had consistent positions within the flocks, as males and adults were followed more often than females and juveniles, respectively. However, short-term food reduction affected individuals' following behaviour according to their age: adults increased their following rates during food scarcity, whereas the opposite trend was found in juveniles. This pattern also remained after the re-establishment of initial conditions, suggesting that ecological changes can also induce individual-level lasting effects. Our study provides empirical evidence that food reduction does not affect the global social structure of house sparrow flocks but nevertheless individuals respond differently to ecological changes according to their age.