Evidence that rural wintering bird populations supplement suburban breeding populations

ABSTRACT Capsule: Blue Tits Cyanistes caeruleus and Great Tits Parus major wintering in rural areas tended to move further to breed the following breeding season and more frequently cross the urban–rural boundary, than did those wintering in urban gardens. Aims: To explore and quantify the movements of birds from winter to breeding season locations across the urban–rural ecotone at the boundary of a large urban area. Methods: Birds were colour-ringed at rural and urban/suburban garden ringing sites during the winter. This was followed by breeding season surveys and resighting in the surrounding areas to quantify the movements of colour-ringed birds and changes in habitat selection between seasons. Results: Blue Tits and Great Tits ringed in winter in rural areas tended to move further and selected more urbanized breeding territories the following spring than did urban/suburban garden-ringed conspecifics. This shows that the populations were connected across the urban–rural ecotone by the seasonal movements of individuals, but that net movement was from rural wintering areas to urban breeding habitat. Conclusion: The results suggest that rural populations may not be subsidized by urban wintering birds and winter supplementary feeding, but that the resources in gardens within urban areas, perhaps spring/summer feeding and nestboxes, draw birds in to breed. Combined with independent evidence for lower breeding success in urban areas, this may show the operation of source-sink dynamics. The results suggest important implications for connections between populations, the conservation value of urban areas and potential for the spread of disease, but further work is required to extend evidence and inference to other regions and species. Nevertheless, the approach used here provides a scalable study model that can be applied in other contexts.