Replacement of body feathers is associated with low premigratory energy stores in a long-distance migratory bird, the barn swallow (Hirundo rustica)

Moult and pre-migratory fuelling are energy-demanding processes in birds, usually showing a limited degree of temporal overlap. The barn swallow Hirundo rustica, a strictly aerial insectivorous, long-distance passerine migrant (15-20 g), accumulates conspicuous energy stores (up to 30-40% of lean body mass) before embarking on the autumn migratory flight between western European breeding areas and African sub-Saharan winter quarters. During the pre-migratory fattening period (September October), both adult and juvenile birds replace body (contour) feathers. The seasonal overlap between moult and energy storing was studied in barn swallows in Italy. Data from 23 000 birds, measured during eight study seasons, were analysed to investigate whether the simultaneous replacement of body feathers and fuelling was associated with reduced energy stores. Among all age and sex-classes, non-moulting individuals showed a relatively greater level of energy stores compared to moulting birds. This was particularly evident during the last phase of the pre-migratory fuelling period, when birds were building up energy stores at maximum rate. The proportion of moulting birds was greater among adults as compared to juveniles, and adult males moulted faster/earlier than females. The frequency of individuals with growing feathers among the innermost primary feathers was greater among birds replacing body feathers. The presence of moulting wing feathers in adult birds only marginally affected energy stores, as compared to the replacement of body feathers. Hence, our data show a negative association between replacement of contour feathers and energy stores in a long-distance migratory bird, suggesting that a physiological trade-off between these two energy-demanding processes may occur, and that a late moult may ultimately have a negative effect on the successful termination of the migratory journey.