Loop-migration and non-breeding locations of British breeding Wood Warbler Phylloscopus sibilatrix

UK-breeding Wood Warblers have seen their numbers crash by two-thirds since 1995, and the reasons for their steep decline have so far remained unclear. Assessing the threats these migrant birds face throughout the year – not just during their time in the UK – is key to ensuring their continued survival. Until now, Wood Warbler migration routes, stopover sites and non-breeding distribution have been poorly known. This study set out to address this, using tiny tracking devices called geolocators, which can be fitted to individual birds to record day length and solar noon, and which allow researchers to reconstruct a bird’s global position through time.

In this study, a collaboration between RSPB and BTO, the authors fitted geolocators to Wood Warblers breeding in Dartmoor, Devon and the New Forest, Hampshire. The resulting tracking data, received from four males, showed that the tagged Wood Warblers followed a clockwise loop migration. After leaving their breeding sites at the end of July, birds made their way to an initial stopover location in south-central Europe, especially Italy, where they spent much of August. They then resumed their journey and crossed the Sahara in a single, non-stop flight, stopping off briefly before continuing westward to a further, major stopover in the Sahel, where they remained  for 7-11 weeks. In late October to late November, birds made their way to their final destination in western Africa (Guinea, Sierra Leone and Liberia), where they are likely to have undertaken their annual moult. The tagged Wood Warblers began their spring migration in April, heading north across the Sahara in another non-stop flight, to arrive back at their breeding grounds at the start of May.

Although this study's sample size is small, it provides valuable new information on the habitat requirements of Wood Warblers throughout the annual cycle. We now know that Wood Warbler conservation necessitates suitable habitats being available in four regions (their breeding and wintering grounds, and the two stopover locations in southern Europe and the Sahel). These results allow researchers to understand how and where best to focus their efforts when attempting to determine causes of Wood Warbler decline. Future research should therefore prioritise improving our understanding of Wood Warbler ecology at these non-breeding locations, providing a better basis on which effective decisions can be made to reverse the fortunes of this declining species.