Consideration of population limitation in migrants is further complicated by the fact that, in the non-breeding season, the sexes of a species may occupy partly different areas or habitats from one another (Chapter 15). These differences are seldom absolute, but rather statistical tendencies, with the females in many species tending to migrate further from the breeding areas than males, or to occupy less food-rich habitats. Both differences have been widely attributed to male dominance and inter-sex competition, the assumption being that, unless females separated from males, their mortality, and the resulting sex ratio distortion, might be even greater than they are. However, not all species show such differences, and other explanations have been offered of such segregation in particular cases (Chapter 15).
Whatever the underlying cause, the existence of range and habitat segregation means that the two sexes may be exposed to somewhat different pressures, and that the numbers of each sex could become limited at different levels. Distorted sex ratios also occur in resident species, and for various reasons; but the geographical segregation adds another potential causal factor in migrants. Some of the most distorted adult sex ratios recorded among adult birds on their breeding areas were in highly migratory species of ducks and passerines, the numbers of breeding pairs being limited by the scarcest sex (e.g. Stewart & Aldrich 1951, Bellrose et al. 1961, Owen & Dix 1986). In some such species, males exceeded females by more than two to one.
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