Lesser Whitethroats under time-constraint moult more rapidly and grow shorter wing feathers

Migrating passerines moulting in the breeding quarters before autumn migration sometimes end up with less time than needed for a normal moult, To deal with this the birds could for example suspend moult or moult faster. In this paper we investigate the effect of an induced time-constraint on the moult of Lesser Whitethroats Sylvia curruca. The time-constraint was induced through a shift in light regime large enough to transfer the birds to a date when, under normal conditions, they already should have started moulting. Time-constrained birds moulted faster and also grew shorter wing feathers, resulting in a shorter wing, compared to control birds. Only one individual responded by interrupting moult and retained a number of inner primaries unmoulted. The observed adjustments of moult, and the higher fuel loads towards the end of moult, are consistent with the ideas that time is an important factor in bird migration, affecting not only migration but also the events preceding it.