Female mate choice of extra-pair males: breeding synchrony is important

Abstract

Recently, Stutchbury and Morton (1995) proposed that breeding synchrony among females favors the evolution of extra-pair mating strategies in birds, and reported that extra-pair fertilization (EPF) frequency is positively correlated with breeding synchrony in 21 genera of passerines. They then showed that tropical passerines, which typically have low breeding synchrony, have small relative testis size compared with temperate-zone species suggesting a low level of sperm competition and EPFs in tropical species. Weatherhead (1997) stated that there is reason to be skeptical of the breeding synchrony hypothesis because "the logic underlying the hypothesis may be faulty." Here, I respond to his assertion that the breeding synchrony hypothesis can be dismissed because (1) the correlation between species is spurious and (2) the hypothesis is based on faulty logic. First, does breeding synchrony really affect EPF frequency in a comparison among passerines? The empirical evidence indicates the answer is yes (Fig. 1). Data on synchrony are shown for 13 newly studied species in addition to those available to Stutchbury and Morton (1995). Among 34 species from 30 genera and 12 families, the correlation remains highly significant (r0.51, P = 0.002). A pairwise test is a simple way to control for phylogenetic effects, by comparing closely related species that differ in breeding synchrony. For nine pairs of related species, in eachl case EPF frequency was higher for the species with the higher breeding synchrony (Table 1; sign test P = 0.004). Weatherhead's study is among several that have found no correlation between synchrony and EPFs within a species. Based on this, he suggests that the correlation between species may be spurious, resulting from an additional variable, like breeding density. But in a comparative study using the linear contrasts method,