Declines in British-breeding populations of Afro-Palaearctic migrant birds are linked to bioclimatic wintering zone in Africa, possibly via constraints on arrival time advancement

Capsule The bioclimatic zone and habitat type within which birds winter are the most important determinants of population trends. Aims To investigate whether regional factors on wintering grounds, phenological mismatch, or habitat on breeding or wintering grounds show relationships with population changes of Afro-Palaearctic migrant birds. Methods We modelled breeding bird survey trends of 26 species of Afro-Palaearctic migrant birds that breed in Britain, and assessed the most important variables. We also investigated spatial variation in population trends within the UK. Results Bioclimatic zone and wintering habitat type were the most important determinants of population trend. After accounting for the effects of the other variables, species that spend the winter in southern Africa or the humid tropics of central and western Africa showed more negative population trends than those that winter in the arid zone. Species occupying open and woodland habitats declined and generalist species increased, on average. We also suggest that the effect of bioclimatic zone on population trends may be through constraints on the timing of migration. Conclusion Correlations between population change and both wintering area and winter habitat suggest regional changes in climate or land-use in the humid tropics are driving declines in many long-distance migrant species, possibly partly through migratory constraints.