Using stable isotopes to link breeding population trends to winter ecology in Willow Warblers, Phylloscopus trochilus

Capsule: Stable isotope composition of feathers grown in Africa indicates that Willow Warblers, Phylloscopus trochilus, breeding in areas of Britain experiencing different population trends may vary in their distribution, timing of moult and/or use of prey resources during the non-breeding season. Aims: To compare stable isotope ratios of feathers of Willow Warblers breeding in parts of Britain with differing rates of population change. Methods: Feathers were collected from three regions with differing recent population trends; northern Scotland (population increased), western Wales (declined slightly) and eastern England (declined rapidly). Collection occurred at ringing sites run by British Trust for Ornithology (BTO) volunteers as part of the Constant Effort Sites (CES) scheme between April and July in 2008 and 2009. Results: Willow Warblers from northern Scotland had significantly higher δ 15N and δ 13C values than those from eastern England, although the regional mean values did not differ greatly. There was no significant difference in δ 15N and δ 13C values between the sexes, but δ 15N values were significantly greater in individuals caught later in the breeding season. Conclusions: During the premigratory moult period in Africa, Willow Warblers from different parts of Britain may vary in their distribution, timing of moult and/or use of prey resources.