The fat contents and flight ranges of four warbler species on migration in North Wales

Abstract

The fat contents of four warbler species killed during a migratory flight in N. Wales are reported for both spring and autumn. From the fat content and the wingspan the potential still‐air flight range and flight times are calculated for individual birds by the formulae of Pennycuick (1975) : the inaccuracies of these predictions are discussed in relation to attainable range. For three of the warbler species examined (Willow Warbler, Sedge Warbler and Whitethroat) the ranges predicted are consistent with the known migratory destinations of these species in autumn. The ranges predicted for the fourth species, the Grasshopper Warbler, indicate that in autumn some birds are capable of reaching S. France in a single migratory flight. In spring, potential flight ranges of the Willow Warbler, Grasshopper Warbler and Whitethroat suggest that these birds could reach destinations only within the British Isles. However, some Sedge Warblers had sufficient fat to reach Norway in a single flight in still air. In both spring and autumn the times required to fly the predicted still‐air ranges are mostly less than 24 hours. The Willow Warbler, Sedge Warbler and Whitethroat are species that scale in an isometric manner; the Grasshopper Warbler is not similarly proportioned as it has a short wingspan for its body weight. A method based on isometric scaling is developed for the prediction of relative flight ranges. All species had markedly larger wingspans in the spring compared with the autumn.