Populations of many species of Afro-Palearctic migrant birds, including the Whinchat (Saxicola rubetra), have shown severe declines over the last few decades. Habitat change on the breeding grounds, especially agricultural intensification, is thought to be the main driver of the decline in Whinchats. However, recent evidence also suggests that the decline may have a common source, such as low over-winter survival, which affects the whole UK population. To better understand the declines, this study investigated the key demographic parameters driving the breeding Whinchat population change on Salisbury Plain, UK, which is an area of agriculturally unimproved grassland where Whinchats are still relatively common. Territory settlement and nesting attempts of colour-ringed individual Whinchats were monitored intensively during 2012-2014. Pairs were significantly more likely to breed in sheltered valleys with long, grassy, structurally diverse vegetation and a high density of tussocks. Territories with an abundance of perches, for use in foraging, were also preferred. The first occupied territories, by returning birds in spring, tended to have higher invertebrate availability, and the order of territory occupancy was positively correlated between years, which suggests that territory quality was consistently perceived. Nestling starvation was rare because food availability did not limit reproductive output. Neither did the availability of suitable breeding habitat apparently limit the population, but nest productivity was lower than expected, mainly because of a high rate of nest failure due to nocturnal predators. Adult apparent survival was high relative to other open-nesting passerine migrants; however, the survival and recruitment of Whinchats in their first breeding year was low. This low apparent survival could partly be explained by natal dispersal, which was greater than breeding dispersal. From the reproductive output, survival and recruitment quantified in this study, it is apparent that the Salisbury Plain population is not currently self-sustaining.

Globally, migratory behaviour is changing in response to climatic and anthropogenic change. In recent decades, previously wholly migratory species started forming resident populations in the breeding area. Partially migratory species with resident and migratory individuals in the same population provide an opportunity to understand the causes and consequences of changing migratory behaviour. This study focuses on the influence of climate and food availability in determining movement and behaviour patterns of birds in populations that have recently become resident. The white stork Ciconia ciconia recently established a resident population in Iberia, likely facilitated by the availability of abundant anthropogenic food resources including landfill and the invasive red swamp crayfish Procambarus clarkii. Movement data from individual white storks fitted with GPS data loggers showed that year-round nest use by resident individuals dictates many aspects of foraging behaviour, including frequency of landfill use and foraging range. Storks visited landfill from nests further away than previously expected (~48 km). High productivity near landfills has likely influenced the rapid population increase observed in recent decades, however breeding success in colonies far from landfill, particularly those located near rice fields, is low. This suggest that the imminent closure of landfills, due to EU directives, will have significant impact on white stork numbers. Many species are still too small to be tracked with GPS tags, so stable isotopes can be used to identify breeding and wintering quarters of migratory birds. Stable isotopes were used to separate residents from migrants in a partially migratory population of lesser kestrels Falco naumanni. Carbon isotopic composition of feather samples indicated that birds completing their moult in Africa could be identified. However, resident birds could not be separated from birds that moulted in Iberia prior to migration. This emphasises the need to understand moult timing and sequence to correctly interpret stable isotope data.

In this thesis, I investigate whether differences in behaviour among individuals, termed personality, are genetically determined and/or shaped by state and whether personality influences reproductive success and strategy. Using the Cousin Island population of Seychelles warbler (Acrocephalus sechellensis), I measured five personality traits: exploration of a novel object, exploration of a novel environment, obstinacy, stress response and escape response. I estimated the repeatability of each trait, the heritability of the repeatable traits and tested for correlations among repeatable traits. I found that exploration of a novel environment and object were repeatable and correlated, and novel environment exploration was heritable, suggesting that it may be under selection in the population. I then used a candidate gene approach to further investigate the genetic variation associated with personality, specifically targeting SERT and DRD4. I found no genetic variation in DRD4, but identified four polymorphisms in SERT that did not correlate with the novel environment or novel object exploration. These results suggest there was no association between these behaviours and variation in the candidate genes tested in this population, and that a genome-wide study might be beneficial to detect the relevant genes underlying personality. I then looked at how personality is potentially generated and maintained in a social living species by investigating whether personality is social state-dependent or reproductive state-dependent. I found that the novel environment and novel object exploration were not correlated with social status and behavioural consistency was unaffected by social status. Novel object exploration was instead associated with the interaction between insect abundance at year of birth and age (a proxy for reproductive state). Lastly, I investigated the fitness consequences of personality, particularly looking at its influence on reproductive behaviour. I found that disassortative pairs for novel environment exploration were more likely to have females engage in extra pair parentage, and that novel environment and novel object exploration were not associated with the number of offspring sired or the ratio of within to extra group young. Overall my results show that there are consistent among individual differences in behaviour in wild cooperative breeders that may be generated by future fitness potential and are associated with reproductive behaviour within the social pair.

There is an urgent need to understand better how recent climatic changes and shifting marine environments are affecting the population dynamics and foraging behaviour of marine central-place foragers. I use both single and multi-colony, and single and multi-species approaches to investigate how different aspects of colonial breeding in seabirds impact on parental foraging behaviour under these changing environmental conditions. I combine historical and recent colony counts and trip duration data to consider population-level interactions and relationships for northern gannets Morus bassanus. I also consider the role of intra-specific competition in limiting neighbouring colony growth for three additional North Atlantic seabirds, the Atlantic puffin Fratercula arctica, European shag Phalacracorax aristotelis and black-legged kittiwake Rissa tridactyla, with differing population trends over the past three decades. I then proceed to focus on finer-scale effects on individual foraging behaviour and parental care of northern gannets at one colony over a period of 14 years, using direct observations and a variety of bird-borne logging devices. My findings support models of foraging based on competition for prey around seabird colonies. I also found that annual variation in foraging trip durations was particularly marked at large colonies, making them especially vulnerable to adverse effects of low prey availability at sea. Furthermore, as foraging ranges altered with colony sizes, the scale of apparent interactions between conspecifics at neighbouring colonies also altered, providing novel support for the limiting effect of neighbouring conspecific density on population growth. Gannets also showed annual flexibility in diet, habitat use and finer-scale search strategies, although other aspects of their foraging, such as the mean scale of Area Restricted Search behaviour, appeared less flexible. Further study is now required to understand if gannets have the capacity to alter the scale of their search behaviour under more extreme conditions. Gannets were also flexible within breeding seasons in the foraging and parental effort they expended, suggesting that the chick’s requirements also contribute to adults’ foraging behaviour. Flexibility in foraging and parental behaviour may buffer the potential adverse impacts of variable environments on provisioning and productivity. These findings have implications for the population dynamics and potential resilience of a wide range of seabird species and other central-place foragers.

Global environmental changes are predicted to have severe consequences for biodiversity and the provisioning of ecosystem services and functions. Historic biodiversity losses have been principally attributed to habitat loss and degradation, and human overexploitation. Today, however, biodiversity is increasingly threatened by anthropogenic climate change. Establishing the relative importance of climate and land use in determining species abundance is important if we are to fully understand the potential impacts of future environmental change. Designating species of conservation concern relies principally on measures of population change, and is inevitably backward- rather than forward-looking. Yet, with projections of substantial future climate change, knowing species that will become imperilled in future is also important for conservation planning. To date, studies of the impacts of future climate change have focussed on projecting range shifts of species, but rarely on projecting species’ abundances, which limits their utility for conservation. In this thesis, I investigate the relative importance of climate and land use in determining the recent abundance of breeding birds across Europe, and I assess the potential impacts of future climate change. I use species abundance models, applying novel approaches, to improve the understanding of species-environment relationships. From these, I demonstrate that climate is generally more important than land use in determining recent species abundances at a European scale. Importantly, however, the importance of abiotic factors for determining species abundance varies across Europe, with climate being most important in the north, and land-use in the south. This suggests that northerly distributed species will be particularly susceptible to climate change; unfortunate, given that this is exactly where climate change is projected to be most pronounced. I further demonstrate, for the first time, that the population trends of migratory birds are more closely related to climate on their breeding grounds than climate on their non-breeding grounds. My species’ abundance models, using climate and habitat data, estimate national abundances of species well, even when projected into novel parameter space. I then use these models to project the abundance of species under climate change, and demonstrate that substantial changes in both the abundance and distribution of species are highly likely. Furthermore, species responses will be individualistic, leading to significant changes in the distribution of avian communities. In this thesis I have, in part, been able to address some fundamental questions in ecology, including: ‘What limits the abundance of migratory species?’ and ‘Is climate or habitat the primary determinant of population size in species?’. The work presented here advances our understanding of the potential future shape of biodiversity, and should inform forward-thinking conservation.

Migration is one of the most spectacular wonders of the natural world but is increasingly threatened by large-scale environmental change. Migrants are key components of biological systems at higher latitudes, where the magnitude and velocity of climate change is most pronounced. In this thesis, I consider how the distribution of migratory species may change in the future, across the globe, in response to environmental change. Using global data on avian species distributions, the environmental and biological factors that affect migratory species richness were investigated. The cost of migration distance between breeding and non-breeding areas, and resident species richness were two important determinants of migrant species richness in a given location. Species distribution models (SDMs) were used to relate migratory species occurrence in breeding and non-breeding seasons to contemporary climatic variables. The importance of climatic variables for predicting migrant species’ occupancy depended on season (breeding or non-breeding). Whilst most SDMs focus on individual species breeding ranges, and model within the occupied realm, this can be problematic for migratory species that frequently use multiple realms, especially if trying to project newly suitable areas in future. Here, I developed a novel method to assess climatic suitability for migratory species globally, within species migration flyways. Selecting absence data to condition SDMs for species occurring across multiple realms can be problematic, as regions distant from the range may be suitable but unoccupied. To minimise this issue, I developed a modelling approach that selected absences preferentially from closer to a species distribution, using a distance weighting function; compared to other methods, this improved model performance on withheld test data. Using this approach, I projected the potential future breeding and non-breeding ranges of all global terrestrial long-distance migrants under climate change. Migrant breeding ranges were projected to shift poleward, but non-breeding range shift projections are less consistent in direction. To date, predictions of migratory changes under future climate scenarios have usually estimated a single distance between breeding and non-breeding range centroids. However, this approach ignores the variation in migratory movements within species. I developed a method to estimate the range of potential migration distances for individual populations, both now and in the future, which agreed well with recovery data for a well-recorded European trans-Saharan migrant. This approach projected longer median migratory distances in the future for many species. This thesis highlights the importance of climate for migratory species, and suggests that the observed general decline in many long-distance migratory species may be exacerbated by ongoing climate change.

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