Morphology versus molecules: sexing red-winged blackbird nestlings

Past studies of offspring sex ratios in birds have often relied on sexually size dimorphic species where nestling sex could be determined based on weight at a given age. DNA‐based sexing techniques allow us to assess the accuracy of those techniques and to refine them for use when costs or convenience make DNA methods impractical. Using nestling Red‐winged Blackbirds (Agelaius phoeniceus) whose sex was determined using DNA, we compared sex ratios obtained using different morphological criteria. Conservative criteria from previous studies were completely accurate, but allowed sexing of few nestlings younger than 8 d old, and were more successful for sexing males than females. A new method was developed that allowed accurate sexing of nestlings beginning at day 6 posthatching and was less biased relative to known sex ratios. Using 11 years of data, the conservative method left an average of 55% of nestlings and 36% of fledglings unsexed, compared to 31% and 9% using the new method. Furthermore, the male bias in sex ratio estimates using the conservative method was greater, both absolutely and relative to estimates based on the new method, when the proportion of unsexed nestlings (because they were not weighed when older) was higher. Thus, estimates of population sex ratios will be more accurate as the number of nestlings measured on day 8 or older increases. However, if some nestlings that were not weighed past day 7 fledge, the new method allows more of those individuals to be sexed than the conservative method, and the population sex ratio estimate should be more reliable. Although our approach should apply to other sexually dimorphic species, the criteria used must be developed based on such species‐specific attributes as growth patterns and degree of hatching asynchrony.