Did Tree Swallows Cross a Polygyny Threshold?

Abstract

Recently, Quinney (1983) demonstrated that female Swallows (Tachycineta bicolor) involved in bigamous relationships had poorer reproductive success than monogamous females. Despite this, he entitled his paper Tree Swallows cross a threshold. Because Quinney did not assess the reproductive success of polygynous females (those females choosing already-mated males), a comparison between concurrently settling females was not made, as is usual in assessments of threshold models (Garson et al. 1981). Quinney's conclusions therefore are premature. Quinney couched his arguments in terms of the Verner-Willson-Orians (VWO) model for the evolution of (Verner 1964, Verner and Willson 1966, Orians 1969). This model assumes that is adaptive and will evolve if a female mating with an already-mated male has a reproductive success equal to or greater than that of a female mating with a bachelor male. However, tests based on this premise often are invalid due to the potential depressing effects of beta females on alpha females (Altmann et al. 1977) or the effects of discontinuous habitat or male quality on female success (Simmons MS). Using the VWO model as his criterion, Quinney showed that monogamous females were significantly more successful in fledging young than were alpha and beta females (4.7 vs. 2.1, P < 0.01) and that monogamous females raised slightly heavier young. Assuming that alpha plus beta (= polygynous, Quinney 1983) productivity is equivalent to the (more correct) beta female productivity, then according to the VWO model appears maladaptive for polygynous females. However, Quinney states that polygyny is advantageous to secondary [beta] females at the Sewage Lagoon if these females could not breed otherwise. First, it is possible that the beta females could have bred at Backus Field (or other nearby sites), since not all of the nest boxes were occupied there. Second, if a lack of males was preventing nesting at alternate sites, Quinney is invoking the desperation hypothesis (Simmons 1983), which assumes that arises from a forced choice among females who cannot breed without choosing a mated male. This should not be confused