Body weights of Willow Warblers during autumn migration within Britain

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Abstract
The relationship between weights of Willow Warblers and the autumn migration strategy of the species was investigated by examination of weights of first‐autumn birds captured at two coastal and two inland sites in Britain over the period 16 July to 30 September. The migration ‘season’ was divided into two periods and the data from each compared. No account was taken of the sex of birds and only individuals which had completed moult were used. Capture times throughout the season were standardised and actual weights were used Mean overnight weight‐loss was found to be about 8% of total body weight and mean diurnal weight‐gain about the same over the first period of the season, slightly more over the latter period. Results implied that this order of increase was needed by birds to meet overnight metabolic requirements and, that they migrated at weights virtually equivalent to normal roosting levels. A migration strategy is proposed based on the findings of Baggott (1975, 1986) that approximately 13% of total wet weight of first‐autumn Willow Warblers comprises fat deposits. It is suggested that the majority of Willow Warblers use normal evening weight levels for successive short‐stage migratory flight (<300 km) through Britain, and that about 5% of body fats is held available in reserve for the funding of additional energy demands which might be imposed should adverse weather conditions be encountered. Estimates of the ‘fat’ loadings birds would require to traverse France and for trans‐Saharan flight from Iberia are discussed.