Weather Conditions and Fat Accumulation Dynamics in Pre-Migratory Roosting Barn Swallows Hirundo rustica

Abstract

The relation between fat accumulation and weather conditions was studied at a Barn Swallow roost in northern Italy during a 5-year period. Data were collected from the end of the breeding season, in early July, to the beginning of October, when the roost is abandoned. The post-breeding period (4 July-20 August) was characterised by a stationary or decreasing mean body mass. The pre-migratory period (1 September-3 October) was characterised by a rapid mass increase. During the post-breeding period, fat reserves were higher during worse than average weather conditions (wind and low temperatures), an indication that in this period fat stores function as a reserve to be used in case of food scarcity. By contrast, during the pre-migratory phase, fat reserves were lower during worse than average weather conditions (low air humidity and low temperature). This is expected if swallows build up the energy stores for migration as rapidly as they can, and fattening rate is sub-optimal in unfavourable weather conditions. Juveniles, inexperienced foragers compared to adults, should run a higher risk of starving during adverse weather and consequently were expected to carry more fat during the post-breeding phase. Conversely, their maximum fattening rate should be lower than that of adults. Thus, they were expected to be less fat during the pre-migratory period. Variation in fat reserves in the two groups was in agreement with these predictions, juveniles being significantly fatter than adults in the first part of the season, and leaner before migration.