Polygyny, Spacing and Sex Ratio among Hen Harriers Circus cyaneus in Orkney, Scotland

Abstract

Polygyny was studied in a marked population of hen harriers in Orkney. Data on 37-62 nests were collected annually from 1967-1974. Since about 1950 a high proportion of adult males were polygynous, but most 1st yr birds were monogamous. Hatching success of eggs and the mean number of young fledged per nest were higher in monogamous situations involving adult males than in polygynous ones. Males with 3 or more females gave rise to more young than those which were monogamous. The distribution of nests tended to be clumped with a mean distance to the nearest nest of 0.81 km. Adult males were fairly regularly spaced over moorland at a mean density of 0.38/km2, and the mean distance between them was 1.10 km. The mean distance between 1st yr males and the nearest adult male was significantly less. There was a significant overall excess of females fledged, and in the breeding population there were about twice the number of females as males. Polygyny apparently became frequent in Orkney when the breeding population exceeded about 50 females. Polygyny probably enables more females to breed than could do otherwise when the region''s holding capacity of males, as regulated by social factors, has been attained.