Characteristics of Lesser Spotted Woodpeckers Dendrocopos minor on irruptive autumn movement along the coast of southern Norway

Abstract

The Lesser Spotted Woodpecker Dendrocopos minor is a facultative autumn migrant in Fennoscandia, but there is little published information about its migratory ecology. Analysing data from 26 years (1990–2015) of trapping at Lista Bird Observatory, southern Norway, we found that autumn migration varied in relation to sex and age in this species. Annual numbers of trapped birds fluctuated between 0 and 19 but there was no significant temporal trend in the numbers of birds trapped. However, the numbers were positively correlated with similar data from Falsterbo Bird Observatory in southern Sweden. Thus, the annual fluctuation of Lesser Spotted Woodpeckers at Lista probably reflects the situation across larger parts of the southern Scandinavian Peninsula. In total, 96% of the birds trapped at Lista (n = 136) were in their first calendar year, showing clear evidence for differential migration in the species. There was no sex bias in the material but the median arrival date was about 14 days earlier in males than in females. Wing length increased over the season in both sexes, although the explanatory value of this seasonal progression was low. Also, body mass was higher in males than in females and increased with date of trapping when body size (wing length) was controlled for. Our findings comply with the hypothesis that annual fluctuation in the number of migrating Lesser Spotted Woodpeckers is caused by varying breeding success in the source population(s), mediated by intra-specific competition (the competitive release hypothesis).