Fledgling sex ratio variation and future reproduction probability in Montagu's harrier, Circus pygargus

I studied patterns of sex ratio variation in 543 fledglings from 192 broods of Montagu's harriers Circus pygargus (1992-1998) in Madrid, central Spain, to determine whether offspring sex ratio was adjusted according to variables influencing future reproduction. Fledgling sex ratio variation was related to estimates of food availability during the pre-laying period, with more females produced in years of higher food availability. This variation was related to the probability that females, but not males, were recruited to the breeding population and were recruited at younger ages, if fledged in good food years. Furthermore, the relationship between food and sex ratio was only significant for the first two nestlings in each brood, and only nestlings from first and second ranks benefited from good breeding conditions to advance their age of first breeding. Results presented here differ from other published data for the same species. Geographical variation in observed sex ratio might be explained by differences between populations in factors influencing age of first breeding or other demographic variables.