What mechanisms drive avian demographic and population responses to climatic change?

Our current understanding of the mechanisms underlying the impacts of climatic change on biodiversity is rudimentary, hindering the development of accurate predictions of these impacts, and appropriate adaptation and mitigation. The aim of this project is to determine the mechanism(s) through which the effects of climatic change on a temperate bird species are mediated. Demography determines population trends, and thus ultimately the range expansions and contractions that climate change is widely predicted to generate. Therefore, there is an urgent need to understand the mechanistic links between climatic change, demography and population trends. The research that has been conducted to date in this area has focused predominantly on the trophic mismatch hypothesis, which has most frequently been applied to birds. It suggests that species whose lay dates have not advanced at the same rate as spring temperatures are likely to breed after the peak in food availability, resulting in reduced reproductive success, and hence population declines. In contrast, species with marked advances in lay date are predicted to have high reproductive success and stable or increasing populations. The trophic mismatch hypothesis is frequently considered to provide a robust explanation for avian population trends in response to climatic change. Surprisingly, however, it receives support from studies of just two bird species. Moreover, alternative demographic mechanisms may explain associations between climatic change and population trends. Temperate bird species have tightly constrained breeding seasons, which may be lengthened by earlier reproduction, thus increasing the number of breeding attempts and the probability of breeding successfully, e.g. by reducing the impact of nest predation events. In resident bird species, winter mortality frequently regulates population size and is likely to be reduced by climatic amelioration thus generating population growth. This CASE studentship will provide the first rigorous test of these alternative demographic mechanisms linking climatic change with population trends. It uses the long-tailed tit Aegithalos caudatus as a model system. This species is ideal for this study as it exhibits the most marked phenological advance in lay date of any resident small passerine in the UK, its population is rapidly increasing, and it experiences high rates of nest predation and, in cold weather, winter mortality. The studentship combines analysis of national and local long-term datasets on breeding success, survival and recruitment, with intensive localised fieldwork to determine the relative effects of climate-induced changes in breeding phenology, predation resistance and winter survival on population size. Intensive study of a long-tailed tit population near Sheffield will provide 20 years of detailed data on population ecology and behaviour (17 years of data will be available when the studentship starts, with an additional 3 years of data collated during the studentship). The national schemes run by the British Trust for Ornithology (BTO), including its ringing programmes and nest record scheme, will provide data on a UK-wide scale. The BTO CASE partner is globally recognised as a world leader in assessing avian responses to environmental change, with regard to both population trends and the underlying causal demographic mechanisms. The training provided by the BTO in statistical modelling of demographic parameters and population trends thus adds real value to the studentship. Moreover, as an independent research organisation with close links to both the conservation and political communities the BTO CASE partner is ideally placed to facilitate knowledge transfer, thus maximising the impact of the results of this CASE studentship.