Ecological factors are known to influence the spatial distribution of individuals, which in turn governs social structure. However, social interactions can have important individual- and population-level consequences and may shape various ecological processes (Chapter 1). In this thesis, I examine the factors shaping social networks amongst wild birds (Paridae). I demonstrate that these social connections are biologically important, and influence numerous processes, including spatial structure itself. First, I demonstrate that both the spatial occurrence and dynamic social decisions of individuals underpin the non-random composition of foraging flocks (Chapter 2). Reciprocally, I show that the spatial arrangement of individuals' breeding locations is predicted by the previous winter's social network, as birds breed nearer to, and share territory boundaries with, those they held the strongest connections to (Chapter 3). I then use experiments to directly assess the consequences of social structure. I show that birds exhibit compensatory shifts in their social network position upon removal of their flock mates (Chapter 4). Following this, I introduce a novel experimental system that automatically controls which individuals could feed together. These externally-imposed foraging constraints cause social segregation at non-experimental food patches (Chapter 5), which also carries over into breeding site prospecting (Chapter 5). I show that social information transmission, regarding the location of novel food patches, also changes to correspond to the experimentally imposed social network structure (Chapter 6). Furthermore, social foraging associations are found to influence social learning, as birds appear to prioritise learning from those who provide relevant information on a regular basis. Finally, I demonstrate that birds choose to forgo maximising their own food intake to sustain their relationships (Chapter 7). Through prioritising their pair-bond over their own preferences, this also changes the social composition of foraging flocks, indicating how a single relationship can govern wider social network position. Additionally, I show how social relationships can influence individual behavioural plasticity and the learning of foraging strategies. This thesis thus contributes to elucidating the relationship between spatial structure and social networks within wild populations. It also demonstrates how social systems respond to external changes, and the direct causal implications of this for shaping spatial structure, information flow and individuals' social and foraging strategies. Together, these findings form a foundation for expanding our understanding of the ecological and evolutionary implications of social networks and the mechanisms underpinning this (Chapter 8).

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Throughout their lives, animals have to make trade-offs between current and future events in their annual cycle. Long-lived migratory species in particular have to balance the cost of reproduction with adult overwinter survival, which is heavily dependent on migration. Behavioural plasticity, perhaps through experience and learning, may play a role in shaping individual variations in life-history decisions. The mechanisms by which such variations develop, and their potential effects on life-history traits, are poorly understood. This thesis uses two species of long-lived migratory seabirds, the Atlantic puffin Fratercula arctica and the Manx shearwater Puffinus puffinus, to address these questions, combining spatial tracking data with fine-scale measures of individual behaviour during long-distance at-sea movements. At-sea behaviour (measured with estimates of daily activity budgets) varied amongst individuals of different sex, age, and colonies. Individual variations in non-breeding behaviour were affected by investment in the previous season, and such variations had important implications for individual fitness. Puffins which visited the Mediterranean Sea foraged more and had a higher breeding success than puffins which remained locally. In addition, females puffins which foraged more during the winter, regardless of their location, laid earlier (which is associated with higher fledging success) and had a higher breeding success. Shearwaters which invested more in reproduction and started fall migration later spent less time resting at the wintering grounds than in other years, laid later and had a lower breeding success the following season. Shearwaters which invested less in reproduction showed the opposite trend. These behavioural differences were reflected in the birds' energy expenditure. Higher energy expenditure often correlated with higher fitness. Finally, pairs of puffins which followed similar migratory routes laid earlier the following year, why this was the case remained unclear. Environmental conditions were likely drivers of individual variation in at-sea behaviour. Puffins from multiple colonies across their breeding range spent more time foraging when in colder and productive waters. Furthermore, puffins from colonies at higher latitudes foraged in colder waters, despite not necessarily remaining close to their colony; this suggests a local adaptation to temperature. In shearwaters, immature individuals foraged in less productive waters than breeding adults, which resulted in a lower foraging efficiency (mass gain per unit of time spent foraging). Spatial segregation occurred between individuals of different age, sex, colonies and potentially individuals of different quality, often accompanied by differences in activity budgets. Although intra-specific competition was a likely driver of the observed segregated at-sea distributions - for example between immature and breeding shearwaters - it was unlikely to be the only factor. Spatial segregation between pair members in puffins in the months leading to the breeding season, accompanied with higher female foraging effort and breeding success, suggests that segregation resulted from different energy or nutritional requirements, perhaps related to egg laying. Overall, this thesis highlights how the investigation of the behaviours underlying long-distance movements can be a powerful tool to study drivers of breeding and non-breeding distributions and migratory routes, and the important consequences that individual variation in behaviour may have on individual fitness, and ultimately on population dynamics and the evolution of life-histories.

As governments pledge to combat climate change, wind turbines are becoming a common feature of terrestrial and marine environments. Although wind power is a renewable energy source and a means of reducing carbon emissions, there is a need to ensure that the wind farms themselves do not damage the environment. There is particular concern over the impacts of wind farms on bird populations. In this thesis I have explored how to assess the cumulative impacts of wind farms on birds. Cumulative impact assessment is a legislative requirement of environmental impact assessment but too frequently it has been tacked on to the end of assessments as an afterthought. Reasons for this are numerous but a recurring theme is the lack of clear definitions and guidance on how to perform cumulative assessments. Therefore I developed a conceptual framework to promote transparency. The core concept is that explicit definitions of impacts, actions and scales of assessment are required to reduce uncertainty in the process of assessment and improve communication between stakeholders. Only when it is clear what has been included within a cumulative assessment, is it possible to start to make comparisons between developments. I also recommend a more strategic approach to cumulative impact assessment. If birds avoid wind farms then the structures act as barriers to movement and birds must fly further to reach their destination. If the additional distance has an associated energetic cost then this will impact an individual. With data collected using surveillance radar, I investigated the impact of the Nysted offshore wind farm on a population of common eider Somateria mollissima migrating from Finland to the Wadden Sea. The impacts of the wind farm appeared trivial and it required 100 equivalent wind farms before a significant impact was detected. Using the same radar data I also constructed a model to quantify the movement process of birds in response to wind turbines and therefore provide wind farm developers with a useful tool to predict the impacts of different wind farm designs. The impacts of wind farms may be greater for birds that interact with the turbines on a daily basis than for migrating individuals. Using an energetic modelling approach I explored the impact for a suite of breeding seabirds commuting past a wind farm between their breeding and feeding areas. Impacts of flying increasing distances associated with increasing numbers of wind turbines were species-specific and costs were highest for species with high wing loadings and high daily frequency of foraging flights. However, costs of extra flight to avoid a wind farm appear much less than those imposed by low food abundance or adverse weather conditions. Finally, a spatially-explicit individual-based model was developed to assess cumulative impacts of wind turbines through collision mortality and direct and indirect habitat loss, on a population of hen harriers Circus cyaneus on Orkney. Increasing numbers of wind turbines caused declines in the hen harrier population but the population response varied according to where turbines were located. Therefore, although wind turbines impact hen harriers, it may be possible to reduce the effects by considering hen harrier ecology during the planning procedure and/or implementing mitigating measures such as rough grassland restoration in strategic locations away from turbines.

In der vorliegenden Broschüre werden Untersuchungen zur Wirkung von landwirtschaftlichen Vogelschutzmaßnahmen auf Agrarflächen vorgestellt. Auf den Flächen wurde dazu das Vorkommen von Vegetationsstrukturen und ausgewählten Insektenarten erfasst und hinsichtlich ihrer Verfügbarkeit für Vögel geprüft. Die unterschiedlichen Ansaatmischungen sowie standort- und pflegebedingte Einflüsse führten auf den verschiedenen Blühbrachen im zeitlichen Verlauf zu einer Vielfalt an Pflanzenarten und Habitatstrukturen. Damit verbunden war eine stark variierende Zusammensetzung der Insektenfauna. Im Ergebnis ist mit landwirtschaftlichen Vogelschutzmaßnahmen eine Förderung der Vogel- und Insektenfauna in relativ strukturarmen Agrarlandschaften möglich. Zielgruppe dieser Publikation sind Landwirte, Berater, Behördenvertreter, Naturschutz- und Berufsverbände sowie die interessierte Öffentlichkeit.

Oxidative stress has recently been highlighted as a potential physiological mechanism underpinning life-history trade-offs in animals. While the role of oxidative stress in mediating such trade-offs is receiving increasing attention, its importance in wild populations remains poorly understood. In this thesis, I use a wild population of cooperatively breeding white-browed sparrow weavers (Plocepasser mahali) to investigate the role that oxidative stress plays in mediating the costs of reproduction and immune defence. Cooperative animal societies offer a unique opportunity to investigate the costs of reproduction, because dominants frequently monopolise breeding opportunities (exhibiting higher reproductive effort than subordinates), and subordinate cooperative contributions frequently lighten reproductive workloads. My findings reveal, first, that dominants’ reproductive monopolies do not arise because they exhibit superior oxidative balance, as no such rank-related differences in oxidative state exist prior to breeding (Chapter 2). However, the higher reproductive effort of dominant females may underpin their differential declines in antioxidant protection after the breeding season (Chapter 2). Second, experimental manipulation of reproductive effort reveals marked oxidative damage and body mass costs incurred during reproduction. However, these costs are entirely mitigated in large social groups, suggesting that the cooperative contributions of helpers may offset the costs of reproduction for all group members (Chapter 3). While this represents rare evidence of an oxidative stress cost of reproduction in the wild, longitudinal data suggests that these costs do not endure after the breeding season (Chapter 4), highlighting that circulating markers of oxidative balance are unlikely to mediate long-term costs of reproduction. Finally, an immune activation experiment reveals that, while mounting an immune response causes no net change in oxidative balance, the scale of the response can be adjusted according to baseline antioxidant protection in an oxidative-condition-dependent manner (Chapter 5). Together my results provide support for the role of oxidative stress in shaping life histories in the wild. Furthermore, evidence of rank-related disparities in oxidative balance and the avoidance of reproductive costs in large social groups may have important implications for our understanding of both the evolution of cooperative breeding and the patterns of health and ageing in societies.