The sex and age groups of a population often differ in body size. This may affect their ability to withstand cold and food shortage, their dominance status and many other features that could in turn influence movement patterns. In most bird species, males are bigger than females, but in raptors, owls, some shorebirds and others, females are bigger than males. The differences in body size between the sexes are usually slight, with some overlap between them, but in some species (notably most raptors and some shorebirds) the differences are substantial, with no overlap between sexes. In most bird species, juveniles are also slightly smaller than older birds of the same sex.
In many species, as mentioned already, the different sex and age groups migrate different distances from one another, but with some overlap, giving rise to gradients in the sex and age ratios from high to low latitudes across the wintering range. Such patterns have been repeatedly shown from recoveries of birds ringed at the same breeding or staging locations (Figure 15.4).2 In most such species, females migrate further, on average, and winter at lower latitudes than males, and in some the sex difference is substantial. For example, in the Ruffs Philomachus pug-nax migrating through western Europe, most of the males remain within Europe, whereas most females winter in sub-Saharan Africa, where females (which are smaller) can outnumber males by more than 10 to 1 (Gill et al. 1995). In some
2For various finches and other seed-eaters see King et al. (1965), Ketterson & Nolan (1979), Prescott & Middleton (1990); for icterids and starlings see Dolbeer (1982); for Wood Duck Aix sponsa see Hepp & Hines (1991); for Canvasback Aythya valisineria see Nichols & Haramis (1980); for American Woodcock Scolopax minor see Diefenbach et al. (1990).
Figure 15.4 Mean distances between ringing and recovery sites for adult (N=961) and juvenile (N=84) Common Starlings Sturnus vulgaris. The birds were ringed in their breeding areas in eastern North America and recovered in winter (January-February) further south. Birds from further north moved longer distances, but juveniles moved further than adults. Modified from Dolbeer (1982).
albatross species, the sex differences in migration are also great. For example, in Wandering Albatrosses Diomedia exulans from the Crozet Islands, males were found to spend their non-breeding periods mainly in Subantarctic-Antarctic waters in latitudes south of the nesting colony, whereas females visited subtropical-tropical waters in latitudes north of the colony (Weimerskirch & Wilson 2000).
From ring recoveries, differential migration by age classes was found in 10 species of British seabirds, with immatures moving further, on average, than adults (Gannet Morus bassanus, Great Cormorant Phalacrocorax carbo, Shag P. aristotelis, Black-headed Gull Larus ridibundus, Lesser Black-backed Gull L. fus-cus, Herring Gull L. argentatus, Great Black-backed Gull L. marinus, Guillemot Uria aalge, Razorbill Alca torda and Puffin Fratercula arctica, G. Siriwardena & C. Wernham, in Wernham et al. 2002). The magnitude of these age-related differences varied between species, with median distances of 141 km and 84 km in immature and adult Black-headed Gulls, ranging up to 1380 km and 639 km in immature and adult Gannets. Guillemots were unusual in that juveniles were recovered significantly further north and east than adults. Similar differences have emerged in other studies of seabirds (e.g. Great Cormorant, Bregnballe et al. 1997, three large gull species, Kilpi & Saurola 1984), and in several species of terns the wintering areas of different age groups were almost completely segregated (for Common Terns Sterna hirundo in the West Atlantic flyway see Hays et al. 1997, for Roseate Terns S. dougallii in the eastern Asian-Australasian flyway see Minton 2003).
Other studies have examined sex and age ratios among birds from different wintering localities, either by observation or by examination of trapped samples or museum skins. They include a wide range of species,3 some of which winter in temperate areas and others in tropical areas (Komar et al. 2005). Such studies have
3For Dark-eyed Junco Junco hyemalis see Ketterson & Nolan (1976); for Evening Grosbeak Hesperiphona vespertina see Prescott (1991); for Yellow Wagtail Motacilla flava see Wood (1992); for Snow Bunting Plectrophenax nivalis see Smith et al. (1993); for White-throated Sparrow Zonotrichia albicollis see Jenkins & Cristol (2002); for Robin Erithacus rubecula see Catry et al. (2004); for various passerines see Komar et al. (2005), for American Kestrel Falco sparverius see Arnold (1991); for shorebirds see Myers (1981); for diving ducks see Alexander (1983), Nichols & Haramis (1980), Carbone & Owen (1995).
Figure 15.5 Sex ratios among wintering Pochard Aythya ferina in relation to latitude in western Europe. Redrawn from Carbone & Owen (1995).
also revealed geographical gradients in winter sex ratios, with a predominance of males nearest the breeding areas and of females furthest away. They are common in diving ducks (Figure 15.5; Bellrose et al. 1961, Nichols & Haramis 1980, Alexander 1983, Carbone & Owen 1995), but less pronounced in dabbling ducks, many of which form pairs in early winter (Owen & Dix 1986, Hepp & Hines 1991). The association between the sexes, and the protection given to paired females by their mates, may help to reduce any segregation through male dominance that might otherwise occur in these species (see below).
Geographical clines in age ratios of a different kind were found in Black-throated Blue Warblers Dendroica caerulescens on their breeding areas in eastern North America (Graves 1997). The proportion of yearling males increased as overall abundance decreased towards the margins of the breeding range. This was attributed to older males being sufficiently numerous to occupy almost all the habitat in the central (supposedly better) parts of the range, but not in the marginal parts, which therefore offered more opportunities for yearlings. The concentration of young breeders in poor habitat has been shown in many species on a local scale (see later), but not previously on a wide geographical scale, in the breeding season.
The supposed greater ability of large individuals to withstand cold has been proposed as an explanation of why, in so many bird species, males tend to winter at higher latitudes (nearer their breeding areas) than females, and why within the sexes, adults tend to winter at higher latitudes than juveniles (the so-called winter cold hypothesis, Ketterson & Nolan 1976, 1983). The idea is that, because large birds have a lesser surface area-to-volume ratio than smaller birds of similar shape, they experience a lower rate of heat loss from the body, giving a better energy return on foraging. Also, if energy stores are proportional to body mass, larger birds should have greater body reserves relative to their daily energy needs, enabling them to fast for longer periods (Ketterson & Nolan 1983). While this explanation of differential migration distances cannot be disproved, the laboratory evidence for differential energy management within species is at best equivocal (Cristol et al. 1999). In addition, the differences in body size between the sex and age groups of a population are often very small, possibly insufficient to have the effects attributed to them. Differential migration distances could also result from other pressures, such as dominance relationships as explained below, or from the need for males to obtain territories in order to breed (with males wintering nearer to nesting areas so as to return quickly as soon as conditions permit in spring, as mentioned above).
Moreover, in some species, sex differences in body size are associated with differences in bill size, which could influence feeding habits, leading the sexes to prefer different areas. Differences in bill lengths are especially marked in some shorebird species, such as the Ruff Philomachus pugnax mentioned above, in which the sexes show marked differences in their migration distances. It is an open question how much this distributional difference results from bill size, rather than from body size or other gender-based differences. For all we know, there may be latitudinal differences in the depth distribution of prey, which favour longer migration of one sex than the other (for Western Sandpiper Calidris mauri see Nebel et al. 2002). In situations like this, in which the sexes differ in several respects (migration distance, body size, bill size and feeding habits), it is practically impossible to say with confidence which aspects are causes and which are consequences. Moreover, where the sexes of a species occur in largely different regions, they could come under different selection pressures, leading to greater divergence in body size and other features than might otherwise occur.
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