Body mass dynamics of wintering Tufted Duck Aythya fuligula and Pochard A. ferina in Switzerland

Abstract

Using body mass as an indicator of body condition, data from 2973 wintering Tufted Duck and 1335 Pochard trapped at Lake Sempach or shot at Lake Constance were compared and analyzed with respect to sex, age, year, seasonal trends, temperature and sites. Throughout the season, males were heavier than females in both adult and first-year age classes, and adults were heavier than first-winter birds of the same sex. Age-related differences were retained throughout winter. Only minor differences were found between years. Body mass of Pochard at Lake Sempach and of female Tufted Ducks at Lake Constance decreased with decreasing ambient temperature. Seasonal patterns differed markedly between the two wintering sites according to feeding conditions and food availability. Diving ducks were able to keep their body mass constant until January or February at Lake Constance, while at Lake Sempach population means of body mass peaked in December when feeding conditions were favorable, and clearly dropped during mid and late winter when feeding conditions were poor. Here, body mass changes of individual ducks suggested a quick adaptation to low site-specific values. This rapid weight loss might reduce survival probability or might force ducks to change wintering sites. Body mass dynamics reflecting habitat quality provide strong evidence that diving ducks attempt to keep body weights as high as possible throughout the winter, and that energy reserves are controlled mainly by proximate factors superimposed on endogenous regulation.