Breeding Biology of Polygynous Hen Harriers Circus c. cyaneus in Orkney

A marked population of Hen Harriers in which polygyny was widespread was studied on West Mainland, Orkney from 1975-1981. Yearling males were usually monogamous, but a male's harem size tended to increase as he aged. This trend was temporarily halted or reversed for most males in a cold, late spring. As harem size increased, the distance between nests decreased. Individual females tended to lay the same size clutch from year to year, and those which laid most eggs reared most young. Successful harem females reared on average as many young as successful monogamists, but as harem size increased, progressively less females reared young. In bigamous groups, females which laid first appeared to be the most favoured by the male and were the most likely to rear young. The only clear advantage of polygyny for females in Orkney was the opportunity to breed in a population where there was a shortage of males. Polygynous males, however, were more likely to rear young than monogamous males, and, to a lesser extent, they tended to rear more young. Fiftynine per cent of males and 66% of females moved less than 5 km from their natal site to their first breeding site in Orkney, but there were only four examples of related birds in polygynous groups.