The Costs of Polygyny in the Tree Swallow (Tachycineta bicolor)

FERRETTI VALENTINA; WREGE, PETER H.; WINKLER, DAVID W.

Urbana-Champaign

Congreso; 121st Stated Meeting of the American Ornithologists Union; 2003

Several hypotheses have been proposed to explain high levels of extra-pair paternity (EPP) rates (see Griffith et al. 2002). Emlen & Oring (1977) proposed that monopolization of resources by members of one of the sexes would lead to increased variance in that sex’s reproductive success, resulting in polygamous breeding systems. This perspective suggests that female breeding synchrony (Stutchbury & Morton 1995) and nest density (Møller & Birkhead 1993), can explain variation in EPP rates. On the other hand, Birkhead & Møller (1992) proposed that the maintenance of a given EPP rate is driven by the costs and benefits it carries for males and females engaging in such tactics. Then, high EPP rates should be associated with little need for paternal care (because of the risk of male retaliation; Mulder et al.1994, Gowaty 1996), and to low adult survival (because of the high chance, for males, of not breeding otherwise, Mauck et al. 1999, Griffith et al. 2002). Tree swallows (Tachycineta bicolor) are socially monogamous birds that show one of the highest levels of EPP rates in Passerines (Conrad et al. 2001, Dunn & Robertson 1993, Lifjeld et al. 1993). Therefore, we used this species to test the paternal care hypothesis by looking at the rate of polygyny and the associated breeding costs. Over the past 14 years, we detected 52 (out of 1262 = 4 %) nests that were attended by polygynous males (N = 1262). After controlling for female age, the mates of polygynous males (FP) and females mated with non-polygynous males (FNP) did not differ in clutch size. However, FP females paid reproductive costs at both early and late stages of the nesting cycle. Initial brood sizes were lower because a higher proportion of eggs failed to hatch. It is possible that lower hatchability in clutches of FPs is due to sperm depletion caused by increased copulations of their mates or by a lower female quality within this group. After controlling for female age and initial brood size, fledging success was also lower in FP females, perhaps reflecting reduced paternal feeding rates of nestlings. Our results thus show that there is a cost in breeding success associated with polygyny. We believe that the high rates of extra-pair paternity observed in this species cannot be explained by a total emancipation from the need for male parental care.