Seasonal sex allocation by Common Grackles? Comment

Abstract

or differential mortality by sex in 2004-2006, unlike the ones I studied in 1976 and 1977 in Michigan (Howe 1976, 1977). In theory, heterogametic female birds can control primary sex of offspring through nonrandom meiosis, as well as that of older nestlings through manipulation of parental care. The Michigan study reported contingent female control of sex ratio during the season, presumably through nonrandom meiosis, and male-biased late broods likely reflecting differential mortality of males as part of an adaptive brood reduction strategy when food was scarce. Maddox and Weatherhead (2009) broaden understanding of sex-ratio adjustment, and its absence, in this bird. The larger issues of contingency in ecology in general and sex-ratio adjustment in particular that have come to light in recent decades deserve more emphasis than their report offers. The data from the Michigan study are what they are (Howe 1976, 1977). Sexual dimorphism by fledging led to a prediction that sex ratios near independence should be skewed in favor of the smaller females, assuming males were more costly to rear than females ?80% of male living mass (sensu Fisher 1930). Skew in favor of female fledglings was suggestive in 1976 and 1977 in clutches of five, and the data for both years pooled differed from 50:50 with a two-tailed test (P = 0.024; Table 2 of Howe 1977). A seasonal skew of sex ratio of 12-day embryos in large clutches, with nests in early and mid April mostly female and those in late April and early May mostly male, was entirely unexpected (Fig. 1 of Howe 1977). Two-tailed Pearson correlations using the angular transformation were just or almost statisti