In sexually dimorphic species, the larger sex is often assumed to exhibit greater vulnerability during the period of parental care, due to their assumed greater nutritional requirements. However, results in the literature are mixed regarding sex-biased environmental sensitivity, and it is uncertain to what extent these inconsistencies are due to the shortage of experimental studies in this area, or to flawed assumptions regarding the factors influencing the development of male and female offspring. In this thesis, long term data, along with experimental work were used to test whether habitat quality had sex-specific effects on nestling growth, survival to fledging, immune development, overwinter survival, and recruitment in a population of the great tit. I also investigated whether these sex-specific selective patterns relate to any observed bias in sex ratios. Consistent patterns were observed for greater female sensitivity to poor rearing conditions (relative to males) with regards to their growth; however, males showed greater vulnerability in poor conditions (relative to females) with respect to their post-fledging survival as well as their recruitment success. Investigation of sex allocation suggests that sex ratios become more male biased with improved habitat quality, which appear to correspond to the patterns of selection (e.g., survival and recruitment); however, the overall results suggest that some other factor was likely causing the mismatch observed between growth and survival. Previous work suggests that the development of immunity may influence short and long term fitness, and that males and females may show different priorities in how they allocate resources during development when exposed to harsh conditions (e.g., growth versus immunity). Therefore, I also explored whether rearing conditions had sex-specific effects on the development of immunity, and whether these differences correlate with the survival and recruitment of offspring. Though I was unable to detect any affect of sex, habitat, or their interaction on immune response, I did find that the survival of male and female nestlings varied depending on the habitat in which they were reared, and that those individuals with greater immune responses survived better: female nestlings survived relatively better than males in poorer quality habitats, whereas males survived better than females in good quality habitats, and the survival of male and female nestlings was positively associated with their immune response in those habitats in which they showed overall greater survival. Rearing environment had an opposite effect on the cell mediated immunity (CM I) of male and female nestlings, although these patterns were only evident among nestlings that survived overwinter. Among surviving females, CMI increased with declining rearing conditions, while having the opposite effect among surviving males. Since I found CMI to be important for the survival of nestlings, and found that male and female nestlings showed opposite effects of rearing environment on CMI, it seems plausible that differences in immune function may be at the root of the observed mismatch between results for growth, survival and recruitment. The results from these studies illustrate how sex-specific patterns of vulnerability may be more complex than is commonly assumed. Thus, finally, I examined the support in the literature for three different explanations for sex-specific vulnerability to poor rearing conditions using met a-regression. My results demonstrated that there is no support in the literature for hypotheses based on size or sex alone. However, met aregression revealed a joint influence of sexual size dimorphism and clutch size in explaining patterns of vulnerability. Overall, the results from this thesis suggest that there are many factors which can have sex-specific effects on offspring performance, and that predicting the effects within particular species may be very difficult.

Social interactions influence how individuals handle environmental challenges. Jackdaws (Corvus monedula) are gregarious corvids that vary in how they respond to environmental challenges, but the reason for this variation is not understood. In other avian species, individual variation relates to social network structure, dominance hierarchies, and pairbonding. Jackdaw group structure is hierarchical, containing both long-term and transient relationships, and fitness is linked to social interactions. Group dynamics are therefore expected to play a role in individual variation in response to environmental challenges. I studied two groups of captive jackdaws from fledging until their second breeding season. I quantified the* ontogeny of social network structure, dominance relations, and active affiliative relationships. I found evidence that individuals maintain affiliative relationships outside of the pairbond. I assessed individual variation in a naturally occurring challenge, parasites, through the presence of faecal parasite eggs and oocysts over eight months. More aggressive, dominant individuals had higher parasite burdens, suggesting that dominance is a social stressor in the species. I further assessed how individuals responded to the challenge of novel resources. I firstly measured wariness of novelty, neophobia, in a group setting by presenting novel objects alongside food. Dominant birds were less neophobic than lower ranking group members. I then examined neophobia, exploration, and problem solving by presenting a novel foraging apparatus. Dominant birds were less neophobic and more exploratory. More central group members were less neophobic and habituated faster to the apparatus, perhaps due to increased access to social information. In this thesis, I show that social interactions are related to health in jackdaws. I also show that individuals vary by network position in how they take advantage of changes in their environment. My thesis shows that understanding group structure is crucial for understanding both variation in behaviour and the fitness implications of such variation.

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This thesis investigates nesting success, foraging behaviour, chick diet and growth of three ecologically contrasting species (skylark Alauda arvensis, yellowhammer Emberiza citrinella and song thrush Turdus philomelos) on farmland, in relation to microhabitat and the availability of invertebrates. Fieldwork was conducted on a lowland mixed arable farm in Leicestershire over the breeding seasons 2000 - 2002. Telemetry was used to monitor adult song thrushes and fledgling skylarks. Skylark nest density was highest in set-aside and lowest in crops, yellowhammer nest density was greatest in herbaceous vegetation and field margins whilst song thrush nest density was greatest in hedgerows and woodland. Birds did not utilise the available habitat at random for nesting. Provisioning skylarks and yellowhammers utilised managed set-aside strips more than would be expected, although both foraged in cereals. Song thrushes foraged primarily in woodland and boundary habitats, avoiding cereal crops, permanent pasture and set-aside. The largest cause of nest failure for each study species was predation. Skylark nest survival improved with increasing vegetation density at the nest site. Yellowhammer nest survival was affected by temperature and rainfall. Song thrush nest survival was affected by predator control, nest site location on the farm and habitat. Skylarks selected sparse vegetation and bare ground when foraging in winter cereals. Each species selected some invertebrate taxa over others, independently of chick age. Skylarks and yellowhammers did not forage in sites with greater invertebrate abundance than others, but song thrushes tended to forage in habitats with higher invertebrate biomass. Skylark chick growth rates were positively influenced by set-aside and negatively by spiders in the diet and the proportion of 'other habitats' around the nest. Skylark chicks with higher growth rates were more likely to fledge. No habitat effects or environmental factors were found to affect song thrush chick growth rates. Tagged skylark chicks exhibited a low post-fledgling survival rate, with most predated whilst flightless. This thesis identified actual and potential conservation benefits for nationally declining skylark, yellowhammer and song thrush populations arising from the management of cropped and non-cropped habitats.

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This thesis presents two studies where natural and sexual selection have interacted to evolve sexual behaviours. The thesis uses mathematical modelling to understand how these forces have caused each behaviour to evolve. This is useful because the results allow for reflection on the potential role of sexual selection in adaptation of these species to a changing environment. The first study is of early male arrival to spring breeding grounds in migratory avian species, this is termed protandry. The study explores the main hypotheses for avian protandry and then tests the susceptibility of each hypothesis to changing environment. The second study is of convenience polyandry in species where there is conflict over mating rate. Females have multiple strategies to avoid harassive males but strategies vary in cost and success rate; she must balance her strategy use to minimise her fitness depreciation. The study identifies the main factors that cause convenience polyandry to evolve and paves the way for future studies to investigate if sexual selection over resistance strategy provides these species a future advantage in adaptation to a changing environment.

Social behaviour is shaped by complex relationships between evolutionary and ecological processes interacting at different scales. Benefits gained from social associations can range from predator dilution to collective sensing, but little is known about how these can be influenced by social structure and phenotypic composition. In this thesis, I investigated how individual decision-making affects phenotypic social structure, and how this mediates social behaviour through emergent properties of collective group behaviour. First, using mixed-species flocks as a model system, I showed individual tits (Paridae, chapter 2) and thornbills (Acanthizae, chapter 3) varied significantly in their social positions. Within-species variation in network position was as large as between-species variation, sug- gesting that prescribing functional roles at the species level may not sufficiently account for potential differences in fitness operating at the individual level. Rather, this suggested that structure may be driven by phenotypic traits, underpinning network structure (chapter 4). Next, I used an extensive data set of foraging records to explore factors determining the composition, of flocks of great tits (Parus major, chapter 5). For example, assortment by dispersal phenotype (immigration status) was the result of spatial disaggregation, and I showed that this may facilitate social selection for breeding territories (chapter 6). Finally, I investigated how decision-making shaped mixed-species social structure. I found that tits used a common strategy for managing pressures of predation and starvation by shifting from exploration to exploitation at different times of the day (chapter 7). I then found that a very simple interaction rule successfully replicated mixed-species group structure (chapter 8). Strikingly, the same rule was applied to both conspecifics and het- erospecifics, potentially playing an important role in the maintenance of flock structure. Through experimental manipulation of ecological conditions, I found that heightened per- ceived predation resulted in stronger social attraction overall, whereas increased competition led to a reduction in attraction to conspecifics (chapter 9). Simulations suggested this could be one potential mechanism underpinning fission-fusion dynamics in these species. Together, the results in this thesis form a framework linking social behaviour to individ- ual fitness where natural selection is shaped by the social environment. This approach may prove useful for testing whether following common social rules reduces variance in benefits accrued by individuals, and how within-species variation in social behaviour can impact emergent properties of groups.