Forced copulation results in few extrapair fertilizations in Ross's and lesser snow geese

Extrapair paternity varies from 0 to over 70% of young among various popula tions of birds. Comparative studies have suggested that this variation is r elated to nesting density, breeding synchrony and the proportion of extrapa ir copulations. We used minisatellite DNA fingerprinting to examine levels of extrapair paternity in Ross's geese, Chen rossi, and lesser snow geese, C. caerulescens c. (hereafter snow geese) nesting in the largest known goos e colony in the world. These geese have one of the highest known percentage s of extrapair copulation (46-56% of all attempted copulations), and all of these appeared to be forced. Among all successful copulations, 33 and 38% were extrapair in Ross's and snow geese, respectively. Despite the high per centage of extrapair copulations, extrapair paternity was low in both Ross' s and snow geese (2-5% of young). Extrapair paternity was not related to ne st density in either species. However, in snow geese, extrapair paternity w as more likely to occur in nests of females that nested asynchronously, eit her early or late in the season. This is one of a few reported examples of a negative relationship between extrapair paternity and breeding synchrony. Extrapair young also tended to come from eggs laid later in the clutch. Al though forced extrapair copulations appear to be a relatively inefficient r eproductive tactic for males, they may provide a reproductive advantage for some males. (C) 1999 The Association for the Study of Animal Behaviour.