Primary moult in non-breeding second-calendar-year SwiftsApus apusduring summer in Europe

Little is known about the extent of primary moult in non-breeding second-calendar-year Swifts during summer in Europe. A sample of 352 Swifts of this age group was captured during May–July in a village in the Netherlands. Primary moult was almost completely restricted to July. During the first and second halves of July, respectively, 20% and 60% of the second-calendar-year Swifts were moulting. Moult was restricted to the first four inner primaries, but 62% of individuals moulted the innermost primary only. Surprisingly, in 63% of moulting birds, primary moult was asymmetric.