A feeding experiment on migratory fuelling in whitethroats, Sylvia communis

The influence of food abundance on whitethroats' behaviour when preparing for autumn migration was investigated in a feeding experiment on the island of Gotland in the Baltic. Whitethroats attracted to a feeding site during post-juvenile moult attained a stable body mass 1.4 g higher than birds in natural conditions. This fuel load is well below the maximum that whitethroats could deposit. After maintaining stable body mass during moult, the whitethroats started to gain mass and thereafter soon departed on migration. This shift was due to the birds increasing their gross food intake by about 70%. Both the average rate of fuel deposition (about 7% of lean body mass per day) and the average departure fuel load (over 50% of lean body mass) were much higher for birds at the feeding site than for birds in natural conditions. No correlation between rate of fuel deposition and departure fuel load was found at the feeding place. Larger departure fuel loads in late individuals suggest that departure decisions are affected by temporal cues. Comparisons with models of time-minimization during migration show that the lack of correlation between rate of fuel load and departure load contradicts the predictions in one model. In another model, where a finite distance is included, the relationship between rate of fuel deposition and departure fuel load is predicted to be step-wise. This means that within certain ranges of rate of fuel deposition the departure fuel load should not change, which is consistent with the result found in this study. (C) 1998 The Association for the Study of Animal Behaviour.