Zoological Studies

Parity in Fledging Sex Ratios in a Dimorphic Raptor, Montagu's Harrier Circuspy gargus

Meritxell Genovart, Martín Surroca, Alejandro Martínez-Abraín, and Juan Jiménez (2008) We analyzed sex ratios of 809 fledglings from 285 broods of Montagu's Harrier (Circus pygargus) in a population located in Castellon (on the northeastern Iberian Peninsula) in the period 1996-2005.  Montagu's Harrier females are about 15% larger than males, and according to Fisher's theory, a bias in the secondary sex ratio towards the cheaper (smaller) sex should be expected.  This was previously found in another population of the same species.  However, overall brood sex ratios in the present population did not depart from parity during the study. Additionally we analyzed offspring sex ratio variations among different years and 4 breeding areas, and although the sex ratios did not show the same trends, differences were not statistically significant.  Results suggest 2 non-mutually exclusive explanations: 1) the growth of females (the larger sex) might not necessary imply greater energy requirements and 2) environmental (e.g., food availability) and demographic variables (e.g., age of recruitment) may vary among populations and differentially affect offspring sex ratios.  Nevertheless, although the sample size for the overall sex ratio was large enough in our study, it was reduced when analyzing this parameter in different years and breeding areas, and such power constraints may have precluded detecting sex ratio deviations from parity in space and time.