Breeding synchrony best explains variation in extra-pair mating system among avian species

Abstract

Stutchbury (1998) used new data and a comparative pairwise test to confirm that there is a strong correlation between breeding synchrony and extra-pair fertilization (EPF) frequency in passerines, as first reported by Stutchbury and Morton (1995). Although a pairwise test is a legitimate and powerful comparative method, Weatherhead and Yezerinac (1998) call for "better analyses to assess the correlation between synchrony and EPFs." There is a strong correlation between breeding synchrony and EPF frequency regardless of the comparative method used (Stutchbury and Morton 1995; Stutchbury 1998). Here I present a linear contrasts analysis on the 34 species given in Stutchbury (1998) using the phylogeny shown in Westneat and Sherman (1997) with other species added using the phylogeny in Sibley and Ahlquist (1990). I used version 2.0 of the Comparative Analysis by Independent Contrasts (CAIC) program (Purvis and Rambaut 1995), and assumed equal branch lengths. As expected, there was a strong positive correlation between contrasts in arcsinetransformed EPF frequency (proportion of broods with EPFs) and the synchrony index (r = 0.52, n = 33 contrasts, P < 0.002). The results for breeding density were similar to Westneat and Sherman (1997), with no correlation between contrasts of arcsine-transformed EPF frequency and nearest-neighbor distance (r -0.10, n 28 contrasts, P = 0.60). The relationship between breeding synchrony and EPFs across species demands explanation; either synchrony causally affects the evolution of extra-pair mating systems as argued by Stutchbury and Morton (1995), or both synchrony and EPF frequency are correlated with some unidentified causal factor. Weatherhead and Yezerinac (1998) favor the explanation that "both breeding synchrony and extra-pair mating [are] corre-