Patterns of migratory fuelling in Whitethroats Sylvia communis in relation to departure

Whitethroats at a coastal autumn stopover site were supplied with mealworms arl libitum and their fuel accumulation rates were recorded by repeated weighings by means of a remote-controlled balance. This study focuses on two questions: (1) How does the daily fuelling rate change in relation to departure? (2) Is the decision to depart affected by weather conditions'? The fuelling rates were not constant during the fuelling period. A decreasing fuelling rate close to the departure was evident in both first-year and adult birds and as many as a fourth of the birds did not gain any body mass on the last day before departure. Among possible explanations for the observed pattern are mass-dependent costs, adverse weather conditions as well as physiological changes in preparation for the Eight. The decision to depart was not affected by local weather conditions (wind and cloudiness) and some individuals departed under head wind and overcast conditions. The frequency of strong head winds was, however, low during the period of departure. Weather data do not support the hypothesis that individuals that did not gain any body mass on the last day did so as a result of their departures being delayed as a consequence of adverse weather.