Laying dates, clutch size and breeding success in the Pallid Swift Apus pallidus in Sofia, Bulgaria

Antonov A., Atanasova D.

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Breeding biology of Pallid Swift Apus pallidus was studied between 1997 and 2000 in Sofia city, a place lying at the northern boundary of the species’ range on the Balkan Peninsula. It was found that first clutches were laid mainly during the second half of May and second clutches – the end of July and the first half of August. Breeding period in Sofia tended to be more compressed and synchronized compared to that in Piedmont (NW Italy). The proportion of pairs laying second clutches was 55 %, 90 % and 66 % in 1998, 1999 and 2000 respectively. Pallid Swifts laid on average 2.93 ± 0.54 eggs (2 -4) in the first clutches and 2.12 ± 0.48 eggs (1-3) in the second clutches. Comparisons with other colonies of this species in Piedmont (Italy), Gibraltar, Tunisia and Corse showed that clutch size of both first and second clutches was highest in Sofia. First breeding had an average overall success of 2.6 fledged young/pair and second breeding – 1.4 fledged young/pair. Values of the fledging rate proved greatest in Sofia though significantly different only in relation to Gibraltar.