Nestling sex ratio in a critically endangered dimorphic raptor, ridgway's hawk (buteo ridgwayi)

Abstract

Variation in nestling sex ratio is an important concept in population ecology, and has particular implications for the conservation status of small populations. Research on species exhibiting Reversed Sexual Size Dimorphism (RSSD), in which females are larger than males, have shown that significant biases in nestling sex ratios can result from demographic and environmental conditions experienced by parents during the breeding episode. We collected morphometric measurements from the critically endangered Ridgway's Hawk (Buteo ridgwayi) over a 4-yr period, verifying that the species exhibits RSSD. Females weighed 25% more than males and were significantly larger for 7 of 12 body measurements. Nestling sex-ratios were determined by PCR amplification of the CHD1 gene. The results revealed a weak but consistent trend toward female-biased broods for the small remaining population. Parents may potentially be producing more females, the more costly sex, due to an ample food supply and adaptive allocation of parental care. A female-bias sex ratio may also signal management concern for the species if it is caused by inbreeding; however, currently the bias is not significant enough to warrant immediate intense nest intervention or nest management for this critically endangered species. © The Raptor Research Foundation, Inc.