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In species that form social foraging groups during all or part of their lifecycle, individuals are subject to costs in terms of a higher frequency of aggressive interactions and having to share resources with group members, which must be outweighed by the benefits of flocking for all individuals concerned if it is to remain an Evolutionary Stable Strategy. In many group living species, dominance hierarchies exist that reduce the need for repeated agonistic interactions between the same individuals, which use valuable energy and may result in injury. Dominance hierarchies allow the most dominant individuals priority of access to contested resources whilst subdominant individuals may suffer as a consequence. In this thesis, I examined how dominance hierarchies were structured in great tits, blue tits and coal tits and how dominance status affected an individual's foraging efficiency and body condition. The food handling times or vigilance levels of great tits and blue tits were not governed by dominance status but dominant coal tits were less vigilant than subdominant conspecifics thus probably allowing more time for foraging and other social activities i.e. mate finding and territory defence. Though subdominant great tits did not suffer costs in terms of food handling times, they were affected more by interference competition when foraging in mixed tit flocks compared to dominant great tits. Using ptilochronology as a measure of nutritional condition, the results from coal tits in this study contradicted those of previous studies in that dominant coal tits had slower rates of feather growth and produced feathers that were shorter and less dense than subdominant conspecifics, possibly a cost of their high dominance. Feather growth rate in great tits and blue tits was not related to dominance status. The immunocompetence of adult great tits or blue tits was not governed by dominance status nor did great tit or blue tit nestlings show any relationship between their place in the brood competitive hierarchy and immuocompetence.

Bi-parental care is necessary in a wide range of avian species to successfully raise offspring. I investigated a range of topics relating to sex-specific breeding ecology in common terns Sterna hirundo, a monogamous seabird with negligible sexual size dimorphism. Subtle size differences can be utilised to identify the sex of terns, enhanced by within-pair comparisons which increased the accuracy and simplified computational procedures. Under natural conditions, parental contributions were found to be flexible with respect to adult quality and body condition. Males were also found to provision more efficiently and to deliver more energy to offspring than females. Therefore there was no evidence for females investing more than males during a breeding attempt. Parental favouritism with respect to offspring sex was found, although why this should have occurred is uncertain. Experimentally increased egg production highlighted adult quality as an important factor in determining clutch size. Experimentally increasing male body mass did not result in lower provisioning rates or chick condition, suggesting that this species has a greater buffering capacity than previously thought. Environmental sensitivity of male and female offspring was examined under natural conditions. Mothers produced more female offspring at the end of the laying sequence, and male chicks from these eggs had higher mortality than females. This suggests that gender influences environmental sensitivity, even without sexual size differences.

The thesis describes the feeding distribution and feeding behaviour of two species of mussel feeding birds, eider <i>Somateria mollissima</i> and oystercatcher <i>Haematopus ostralegus</i> on intertidal mussel beds on the Ythan estuary, Aberdeenshire, in relation to the distribution of green macroalgae during different phases of growth. Two hypotheses were tested; (1) The presence of algae deters birds from feeding in a place. (2) The presence of algae negatively affects the feeding behaviour of birds. Five different growth phases of algae were identified; (1) Early re-establishment (Jan - Feb), (2) Late re-establishment (Mar - Apr), (3) Accelerated (May - Jun), (4) Peak (Jul - Aug) and (5) Declining (Sep - Oct). The greatest area of bare mud with scattered algal areas occurred during the early and late re-establishment phases and imposed minimal physical interference on the birds' feeding. A mosaic of patches of bare mud and algal-covered areas, which increased habitat heterogeneity, occurred during the accelerated and declining phases and imposed a substantial interference on the birds' feeding. A uniform layer of thick mat of about 0.5 cm and up to 2.6 kg FW.m^-2, covering more than 75% of the studied mussel beds, occurred during the peak phase and imposed maximal physical interference on the bird's feeding. The effects of algae on the feeding distribution and behaviour of mussel feeding birds were species specific. The first hypothesis was supported only during the re-establishment phase and was apparent only for eiders. In contrast, both the species appeared to be attracted by algae during the late phase of algal growth; eiders during the accelerated and declining phases and oystercatchers during the peak and declining phases. It was evident that the presence of algae over the mussel beds did not deter the mussel feeding birds absolutely from feeding in a place. The second hypothesis was supported only during the re-establishment phase and was apparent only for eiders.

A mathematical but mechanistic model is presented that is based upon the simplified physiology and feeding ecology of an individual dive foraging animal. In previous theoretical studies of dive foraging behaviour, models assumed dive foraging animals dive optimally based on oxygen as the sole state variable, thus ignoring other likely important physiological factors that are related to the instantaneous energetic requirements of the diving animal. The physiological mechanistic dive foraging model presented includes the additional state variables, namely food (in the gut) and the body mass of the diving animal, for which the oxygen is ultimately required to burn for energy. The physiological model was parameterised for the Common Guillemot <i>Uria aalge,</i> although it may also be applied to other dive foraging animals. Various output parameters are possible from the model, and these were generated from computer simulations to investigate the merits of optimal diving behaviour. The physiological dive foraging model was also used to estimate possible feeding rates of Guillemots from fisheries data made available, which were recorded during winter 1997, in the Moray Firth. These were then used in further computer simulations of the dive foraging model and the various possible output parameters obtained from the model were subsequently used as habitat suitability indices in a novel application of the Ideal Free Distribution, to generate predicted maps of Guillemot distribution, which were tested against the actual concurrently recorded distribution of Guillemots in the Moray Firth. The potential value of the physiological dive foraging model, its limitations and where it, or how its predictions, may further improve, are also discussed. It is hoped that the model will eventually provide an invaluable tool for industrial fisheries, and even wider marine ecosystem management.

The northern gannet Morus bassanus population has been increasing in size over the last century. Larger gannet colonies in the British Isles are growing more slowly per capita than the smaller ones. For the first time, this study provides empirical evidence consistent with the hypothesis that seabird populations are regulated through density dependent competition for food (a theory proposed four decades ago). On average, gannets from larger colonies have to travel further to forage than birds from smaller colonies, leading to reduced provisioning rates at larger colonies. Differences were found in the foraging behaviour of male and female gannets. Females dived deeper and spent more time resting on the sea surface than males. Females also tended to forage in a specific location whereas males were much more variable. This was the first demonstration of sex-specific foraging behaviour in a monomorphic seabird. Foraging location and depth may be driven by sex-specific dietary needs, differences in foraging efficiency or competitive ability. State dependent foraging decisions were examined. Typically, gannets alternate foraging at sea with periods attending the chick at the nest. However, sometimes the attending parent will leave the chick alone. This decision is influenced by the prolonged absence of the partner and a low arrival mass of the adult, prior to its attendance shift. Unattended foraging trips were shorter in duration and therefore closer to the colony than attended trips. Since leaving the chick unattended is risky, there is a trade-off between offspring predation and short-term food requirements. When aspects of the foraging behaviour of gannets were compared between a large and small colony, both striking contrasts and similarities were evident. At the larger colony there were no sex differences in trip duration or the propensity to leave the chick alone whilst at the smaller colony males made significantly shorter trips than females and left the chick alone much more than females. However at both colonies unattended trips were shorter than attended trips. These results suggest interactions between density dependent competition and sex-specific individual behaviour. Identifying important areas for foraging is a major challenge for marine conservation. This study has demonstrated that colony size, sex and condition all affect the foraging decisions of the northern gannet.

Bird ringing is a key ecological research technique that involves the capture and handling of birds. It is used extensively to obtain information on population dynamics of wild birds, and many aspects of avian behavior, physiology and life-history, which would otherwise be unfeasible to obtain. Despite millions of birds ringed every year, little is known about the short- or long-term impacts on birds, and whether there are negative welfare, conservation and scientific consequences, which can ultimately bias the interpretation of data from wild bird studies. In this thesis I study the type of intrusion that capture and handling causes to the bird, by analyzing their interlinked physiological and behavioral responses to capture stress, including hormonal and immunological responses, energy regulation decisions (feeding behavior and thermal regulation) and breeding effort. I further study the immediate effects that capture and handling has on birds through analyzing types of injuries and the rates at which injuries and mortality occur. I have focused mainly on mist-netting, which is the most widely used capture technique, and captures of passerine birds, which is the most frequently sampled taxon. However, I also demonstrate how the impacts of capture and handling can be studied in marine birds and applied to other capture methods. These studies reveal the range of short-term impacts that capture and handling may have on wild birds, and highlight aspects of methodology that have a strong effect on these impacts. The longer term consequences for lifetime fitness and demographic change require further study. This thesis demonstrated the importance for researchers to be aware of the potential effects of their activities on their study subjects, particularly for susceptible species and situations, and to continuously reasses their methods for effective improvement. I propose several guidelines, which aim to promote the birds’ welfare in regards to data collection.