NESTLING SEX-RATIO VARIATION IN WESTERN BLUEBIRDS

We tested five hypotheses for facultative manipulation of sex ratios i n a population of Western Bluebirds (Sialia mexicana) studied for 13 y ears in California. Based on 2,187 nestlings from 549 broods, the over all mean (+/- SE) proportion of males was 51.9 +/- 1.1%, not significa ntly different from 50%. Sex ratios of nestlings were significantly bi ased in only 1 of 13 years, and we failed to detect any significant bi as related to brood reduction, breeder female age, presence of helper males, first-egg date, condition of the breeder female, or annual diff erences in environmental conditions. These results allow us to reject all five hypotheses, including: (1) nestling size dimorphism and local resource competition, both of which predict a female-biased sex ratio ; (2) local resource enhancement via the ''repayment model'' of Emlen et al. (1986), which predicts a more male-biased sex ratio than we obs erved; and (3) seasonal changes in either maternal condition or nestli ng condition, both of which predict shifts in sex ratios not observed in our data. These results prompt us to urge caution when reporting se x-ratio biases in natural populations and when interpreting published studies, many of which suffer from small sample sizes, post hoc analys es, and insufficiently conservative statistical tests. Sex-ratio biase s in birds are uncommon, and considerable data will be necessary to de termine which hypotheses (if any) are sufficiently robust to regularly select for avian sex ratios that deviate significantly from 50:50.