Sex differences in recovery pattern and migratory direction of Goldcrest Regulus regulus ringed in northern Europe during autumn migration

Abstract

Among Goldcrests ringed in Denmark, Sweden and Finland, and recovered later the same autumn in the British Isles, a remarkably high proportion (30:5) of males are found , compared with the assumption of equal sex ratio or a surplus (55%) of males in the migrating population during autumn. Sex ratio bias was not found in birds ringed in Norway. Provided that males and females can be assumed to have on average the same relative fat deposits and that the sexes are morphologically similar, both sexes should have the same flight range. Differences in flight capacity can thus not be a main explanation to the sexual bias in the short time recoveries in the British Isles. An alternative explanation may be a difference in migratory strategy between the sexes when facing long sea passages. If there is a choice, e.g. an alternative migratory route along the coast, the females, although for unknown reasons, fly out over the sea to a lesser extent than the males. Different geographical situations, compared to the rest of southern Scandinavia, is assumed to explain why Goldcrests ringed in southern Norway and later recovered in the British Isles do not show the same sexual bias.