Extrapair Fertilizations and the Evolution of Colonial Breeding in Purple Martins

Abstract

We used DNA fingerprinting to show that, in Purple Martins (Progne subis), forced extrapair copulations (FEPC) result in age-biased extrapair fertilizations. Older males achieved 96% paternity of their broods and increased their fecundity at the expense of young males, which achieved only 29% paternity. Older males recruit young males and females to unused nesting cavities that they had previously defended against other older males. Each year, nearly half (45%) of the breeding martins were recruited young birds not born in the colony. Recruitments are individually timed and begin when each older male's mate has completed a nest. Adult males may have accrued an average of 3.6 fertilized eggs through forced extrapair copulations in addition to eggs produced by their mates (4.5 eggs) for an overall average of 8.1. Noncolonial males without the opportunity for FEPCs would suffer 44% lower lifetime fecundity. Thirty-six percent of the eggs in the nests of young males were the result of egg parasitism, the significance of which is unstudied. These findings support the hypothesis that colonial breeding evolved in Purple Martins to increase the opportunity for extrapair fertilizations. Martins may be an extreme example of a general trend in breeding systems where migration and temperate climate concentrate fertile females in time and space.