Local Distribution Patterns

In addition to geographical gradients in the age and sex ratios of wintering migrants, extreme sex and age ratios can be seen at smaller scales, and have again mostly been attributed to dominance relationships (e.g. Marra et al. 1993, Catry et al. 2004). Differences between the sexes in habitat during the non-breeding season have been described in a wide range of species, from passerines to raptors and ducks (Nisbet & Medway 1972, Nichols & Haramis 1980, Ardia & Bildstein 1997). Among passerines, for example, such differences were studied in the Great Reed Warbler Acrocephalus orientalis (Nisbet & Medway 1972), American Redstart Setophaga ruticilla (Sherry & Holmes 1989), Hooded Warbler Wilsonia citrina (Lynch et al. 1985), Black-throated Blue Warbler Dendroica caerulescens (Wunderle 1995), Red-backed Shrike Lanius collurio (Herremans 1997), Cape May Warbler Dendroica tigrina (Latta & Faaborg 2002), Eurasian Robin Erithacus rubecula (Catry et al. 2004), and several other species (Lopez & Greenberg 1990). Habitat differences might arise from male dominance forcing females into different places (through current competition) or from innate differences between the sexes in habitat selection (evolved through competition in the past).

An attempt to distinguish between these explanations was made with Hooded Warblers Wilsonia citrina in Mexico. Morton et al. (1987) removed male warblers from their winter territories in primary forest, to find whether they would be replaced by females from the adjacent scrub. In fact, the males were not replaced, and females remained in scrub, despite the presence of vacancies in better habitat nearby. The authors concluded that, in this species, the habitat difference between males and females resulted from inherent preference, rather than current male dominance. This conclusion was later supported by experimental work in which the habitat preferences of each sex were tested in isolation (Morton 1990). In the American Redstart Setophaga ruticilla, in contrast, removed males were replaced mainly by females, implying that in this species males had directly excluded females from mutually acceptable wintering habitat (Marra et al. 1993).

In species that show sex differences in habitat, it is the larger sex (usually male) that occupies the most food-rich habitats, is usually in best body condition and shows the strongest site fidelity (Wunderle 1995, Latta & Faaborg 2002, Catry et al. 2004). These findings suggest that the difference results from competition, whether here and now (leading to a dominance-enforced difference) or in the past (leading to an evolved difference). However, another explanation is possible in some species, namely that, being of different sizes, the sexes are adapted to different habitats. Differences in size may have evolved through sexual selection, but then, as an evolutionary by-product, influence the habitat preferences of males and females. This is apparent in those shorebirds in which one sex has a longer bill than the other, and can probe for food to greater depth. It is then not just a case of subordinates being 'driven' from certain areas, but of their preferring other areas because they can feed more efficiently there. Whatever the origin of the habitat difference, it means that the sexes could have different food supplies, reducing competition between them, but affecting local or overall sex ratios.

At the local level, even among flocking species, good competitors tend to accumulate in the richest feeding sites, and gain the highest rates of food intake, while weak competitors end up in poor sites, with lower rates of intake. This is not necessarily because poor competitors are driven from the richer sites; it may be because, if they attempted to feed there, they would suffer more interference and have lower rates of intake, and for this reason they prefer to feed elsewhere (for Herring Gull Larus argentatus see Monaghan 1980, for White-winged Crossbill Loxia leucoptera see Benkman 1997). In some species, dominance relationships seem to influence local movements, dominants tending to remain at local food sources and subordinates moving on. To bird-ringers operating at a single site, winter populations in some species can seem to consist of residents and transients. The two categories differ in sex and age ratios and sometimes also in average weights and fat levels (for Siskin Carduelis spinus see Senar et al. 1990, 1992).

In addition to foraging areas, birds compete over roost sites, which vary in the amount of protection and shelter they provide. Among species that roost communally, the distribution of individuals within a roost can be markedly non-random with respect to age, sex and body condition. Statistically significant segregation of age and sex groups within winter roosts has been noted in many bird species, including Brambling Fringilla montifringilla (Jenni 1993), Red-winged Blackbird Agelaius phoeniceus (Weatherhead & Hoysak 1984), Starling Sturnus vulgaris (Feare et al. 1995) and Dunlin Calidris alpina (Ruiz et al. 1989). In each species, adults predominated in the safer and more sheltered sites and the juveniles in the more exposed sites. Such local observations give some inkling of how larger-scale distributional differences between sex and age groups might arise, leading to gradients in sex and age ratios across a wintering range.

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