Extreme fattening by sedge warblers, Acrocephalus schoenobaenus, is not triggered by food availability alone

The strategies adopted by birds during migration are expected to be strongly influenced by resource availability and the efficiency with which these resources are used. The migratory strategy of the sedge warbler in northwest Europe has been linked to superabundances of plum- reed aphids Hyalopterus pruni that enable birds to accumulate extensive fat reserves and make nonstop flights to sub- Saharan Africa. Food availability was therefore expected to be the main determinant of whether sedge warblers accumulated extensive reserves or not. In this study, birds were provided with an unlimited supplementary food source, but only 10 out of 24 birds accumulated large reserves. In view of this, temporal and geographical cues reflecting seasonal variations in aphid abundance are considered, in addition to food availability, to determine whether extensive fuelling occurs. Optimality models of resource use by migrants predict two modes of fuelling behaviour, as seen here, if strategies have evolved to divide a journey into an optimal number of stages or if the perception of resources at future sites varies between birds or in time/ space. If sedge warbler perception of future resources reflects the temporal and geographical variation in aphid abundance in northwest Europe, then the distribution of fuelling behaviours observed here is expected. Despite some birds accumulating large fuel reserves, they did not show a high degree of sensitivity to wind conditions when initiating migratory. flights. (C) 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.