Flexibility in phenology and habitat use act as buffers to long-term population declines in UK passerines

Ecological responses to environmental change are wide-ranging, from alterations in the timing of life-history events to range and population changes. Explaining the variation across species in these responses is essential for identifying vulnerable species and for developing adequate conservation or mitigation strategies. Using population trend data from the UK Breeding Bird Survey, this study examined the association between long-term population trends (19942007) and phenological, life-history and resource-use traits of UK passerine species. Phenology, as well as productivity and resource use were significantly associated with long-term population trends. Average laying date and first clutch laying period were key predictors, with higher population growth rates associated with earlier laying dates and longer laying periods. This suggests that flexibility in the duration of reproductive periods buffers species against environmental changes. Average laying period was particularly important for migrant species. Flexibility in laying dates for these species is constrained by their arrival dates; mean change in arrival date over a twenty-five year period strongly predicted population trends amongst migrant species. Besides the key role phenological flexibility plays in buffering population declines, we also showed that more productive, generalist species were less likely to have declining populations than species with specialized habitat requirements, particularly those associated with farmland and urban areas and those reliant on highly seasonal food items (i.e. invertebrate eaters). These results underscore the need for a multi-faceted approach to understanding the mechanisms governing population trends. Additionally, species sensitivity to environmental change is likely to depend on interactions between species-specific phenology, habitat and resource-use traits.