Variation in weight of migratory DippersCinclus cinclusin their Finnish winter quarters

Abstract

Weight variation in the Dipper was studied during 9 winters in southwest Finland, where the species is a winter visitor. Males were heavier than females and adults heavier than first-year birds, while weight was independently correlated with wing length. Weights increased during the day, varied during the course of each winter, and differed between winters. The extra fat deposits apparently available at dawn in mid-winter were sufficient for 4–6 hour's survival, assuming normal diurnal activity, as is typical of passerines living in predictable environments in the temperate zone. These reserves would be sufficient for the search of a new wintering site within a range of c. 200 km if the place where the bird was living became unsuitable through freezing. In late February and early March expected survival times are at their shortest, especially in first-winter birds. Prior to spring migration, Dippers gain weight considerably. All sexes and ages, but particularly adult males, are potentially able to fly non-stop for 700 km, which would allow them to reach their breeding areas. The timing of pre-migratory fattening and spring migration is very early in relation to the start of breeding in central Norway.