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

Publication Type:Journal Article
Year of Publication:1979
Authors:Balfour, E, Cadbury, CJ
Journal:Ornis Scandinavica
Date Published:1979
ISBN Number:00305693
Keywords:Accipitridae, Circus, Circus cyaneus
Abstract:The results of a study of polygyny in a marked population of Hen Harriers in Orkney are presented. Data on 37-62 nests were collected annually from 1967-74. Since about 1950 a high proportion of adult males were polygynous, but most first year 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 three or more females, however, 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 per km2 and the mean distance between them was 1.10 km. The mean distance between first year 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. It is suggested that polygyny 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.
Short Title:Ornis Scandinavica
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