Until very recently, ecologists have been dependent on ringing to provide an insight into bird movements between breeding and wintering sites. Ring recoveries, however, can produce a biased picture owing to spatial and temporal differences in ringing effort and recovery rate. Relatively novel techniques involving the use of intrinsic markers provide an alternative method of assessing the breeding origins of birds on winter sites.
Stable-isotope analysis is one such technique. It relies on the fact that chemical elements occur in two or more forms that differ slightly in their molecular weight. These variants are known as isotopes. For hydrogen, a constituent of water, ratios of the different isotopes exhibit predictable gradients across continents. Because all animals assimilate these isotopes in their tissues through their food, hydrogen isotope ratios in animal tissues provide a crude geographic marker. For birds like the woodcock, which have a single, annual post-breeding moult, hydrogen isotope ratios in feathers sampled on wintering sites provide an indication of the bird’s breeding ground.
We first assessed the accuracy of the method using woodcock feathers collected from 26 known breeding locations across Europe. We found that hydrogen isotope ratios in feathers showed good correspondence with hydrogen isotope ratios in rainwater at the same locations. We then analysed over 1,100 feathers from woodcock shot during December and January in six wintering areas: southern Scotland, west Wales, western Ireland, Norfolk, Hampshire and Cornwall.
Figure 1: Woodcock migration routes (map © Google)
Overall, we estimated that approximately 51% of the woodcock wintering in Britain and Ireland come from northwestern Russia and the Baltic States, 39% from Scandinavia and Finland, with only 10% from central Europe, Britain and Ireland.
There was a high degree of mixing among woodcock originating from Scandinavia, Finland, Russia and the Baltic States across all six of the wintering areas sampled. Nevertheless, there was an indication of broadly parallel links between the core breeding regions associated with each wintering area. A higher proportion of the woodcock wintering in Scotland come from central and northern Scandinavia, whilst in southern England a higher proportion of birds originate from southern Sweden and the Baltic States.
We are grateful to the Countryside Alliance Foundation, Shooting Times Woodcock Club, Natural Environment Research Council and contributors to the Woodcock Appeal for funding.
We would also like to thank everyone who has contributed feathers for stable-isotope analysis and those counting woodcock each spring.
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