Seasonal clutch size variation of multi‐brooded bird species: comparisons between breeding season and latitudes

Abstract

Single‐ and multi‐brooded species of birds differ in their seasonal patterns of clutch size. Single‐brooded species start with a maximum clutch size that declines continuously as the season progresses, whereas the clutch sizes of multi‐brooded species usually increase to a mid‐season maximum peak and then decrease progressively until the end of the breeding season. Previous studies have shown that multi‐brooded migrant species present seasonal patterns that are similar to single‐brooded species at high latitudes but similar to multi‐brooded non‐migratory species at lower latitudes. We studied the Greenfinch Carduelis chloris and Goldfinch C. carduelis populations in eastern Spain (Sagunto, Valencia) between 1975 and 2002 to compare seasonal variations in clutch sizes between years with early and late starts to the breeding season. The period over which clutch sizes increase was longer when the breeding season started earlier. The Goldfinch population showed no pattern of initial increase in clutch size when there was a late start to the breeding season: a late start shortens the season giving them less time to breed, and may also coincide with maximum food availability. Thus, the pattern of single‐brooded species was observed. In the Greenfinch population, a trend toward the seasonal pattern of single‐brooded species was also observed when the following indices were compared: clutch size increase, modal timing, initial slope and timing of maximum clutch size. We have also compared the seasonal patterns of clutch size of both species in eastern Spain with the patterns observed in Britain. Our results show that for both Goldfinches and Greenfinches, the non‐migrant southern populations of Sagunto in eastern Spain do not tend towards a more multi‐brooded seasonal pattern of clutch size than the migrant Goldfinches of Britain.