Sex ratios in some common British wintering ducks

Abstract

A preliminary survey of the sex ratios of Mallard Anas platyrhynchos, Wigeon A. penelope, Pochard Aythya ferina and Tufted Duck A. fuligula by field counts was made in the winter of 1983-4. Data were collected from 184 sites with a good geographical spread in Britain. There was no significant variation in the sex ratio for any species in any month from December to March. Only Pochard showed significant trend with latitude, but the effect was substantial. South of 51N there were 150 male Pochard per 100 females, compared with 840 north of 58N. There were significant variations among habitat types and size of flocks, and in both cases the variation was consistent with the hypothesis that there is competition between the sexes for favoured sites. The sex ratio in the shooting bag, as shown by national wing surveys, was similar to that in the field, so that there is no significant differential vulnerability between males and females. Data from the remainder of the wintering range indicate that the populations of each of the four species has a surplus of males. The estimates were 120-150 males per 100 females for Mallard, 132M/100F for Wigeon, 150-250M/100F for Pochard and in excess of 130M/100F for Tufted Duck. We support the hypothesis that inter-sexual competition rather than cold stress is the main factor leading to differential migration of the sexes. Females travel further and probably suffer greater mortality as a result. Inter-sexual competition is probably also responsible for increasing female mortality directly, through starvation caused by exclusion from wintering habitat.