Scale and seasonal sex‐ratio trends in northern goshawk Accipiter gentilis broods

• Published in: Journal of Avian Biology
• Main Authors: Byholm, Patrik, Brommer, Jon E., Saurola, Pertti
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2002
• Subjects: Accipiter gentilis; Northern Goshawk
• Online Access: http://dx.doi.org/10.1034/j.1600-048x.2002.02952.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1034%2Fj.1600-048X.2002.02952.x; https://onlinelibrary.wiley.com/doi/pdf/10.1034/j.1600-048X.2002.02952.x

In many populations of size dimorphic birds, brood sex ratios change with advancing laying date. The slopes of these trends, however, vary in time and space, both between and within species. We studied brood sex ratios (proportion of males) of northern goshawks Accipiter gentilis in Finland in relation to laying date using ringing data from 1989 to 1998. At the nationwide scale, i.e. the whole of Finland, between‐year variation in offspring sex ratio was moderate, and the sex ratio did not change with later laying date. At a regional scale, the sex ratio was seasonally constant in one region but decreased in another, although the laying‐date/brood‐size relationship was identical. Hence, the size and sex composition of goshawk broods are locally two uncoupled facets of reproduction. Both the national and regional patterns differ drastically from the pattern of a Dutch population, where sex ratio increased seasonally. We suggest that spatial variation in inter‐annual seasonal sex‐ratio trends might be indicative of the scale at which sex‐ratio feedback functions. The sex ratio of breeders is a factor that could add to the understanding of the observed geographical differences in seasonal sex‐ratio patterns.