Antonov A. & Atanasova D. (2001) Second clutches in the Alpine Swift Apus melba. ARDEA 89 (3): 543-544

During a 2-year study in Sofia, Bulgaria, second clutches were recorded in Alpine Swift Apus melba, a species stated to raise only one brood per year. Each of the second clutches was laid after a successful first breeding attempt, in one case proved by marked birds. The frequency of second clutches was low - in each of the two years only two out of 35 pairs