One of the main effects of body size differences within a population is that they influence the competitive relationships among individuals, with dominants gaining food at the expense of subordinates, which may therefore have to move elsewhere. In certain species, more individuals stay in their breeding areas in winters when food is plentiful than in winters when food is scarce, in mild winters than in cold ones, or in good habitats than in poor ones. Such findings imply that, in such species, competition for food and other resources can have an immediate influence on the proportions of birds that stay or leave each year, and the distances they travel (Chapters 12 and 18). The benefits of migration extend further up or down the social hierarchy, according to prevailing conditions.
Such competition can in turn influence the sex and age groups involved (the so-called dominance hypothesis, Gauthreaux 1978a, 1982a). For example, among Blue Tits Parus caeruleus in a breeding area in Sweden, the proportion of migrants increased from adult males (virtually none), through adult females, juvenile males and juvenile females (>40%). In addition, more late-hatched juvenile males than early-hatched juvenile males migrated (Smith & Nilsson 1987). These proportions were correlated with the dominance relations within the population, with adult males the most dominant and late-hatched juvenile females the least. Such migratory patterns are frequent among passerines, raptors, gulls and other seabirds (Gauthreaux 1982a, Dolbeer 1991, Kjellen 1994a, Catry et al. 2004), but the dominance relationships which supposedly produce them are often assumed (as a correlate of body size), rather than measured directly.
Where only part of the population leaves the breeding areas, the commonest pattern is for juveniles to migrate in greater proportion, to leave earlier and return later, and to winter further from the breeding areas than adults. Moreover, in several species, late-fledged young migrate further than earlier ones, and may return later to breeding areas the following spring (Jakober & Stauber 1983,
Bairlein 2001). The implication that the same individuals may move further in their first than subsequent years has been confirmed in some species by ring recoveries (Newton 1972, Schwabl 1983). During an unusually severe winter in southern France, 260 Little Egrets Egretta garzetta were found dead. Their carcasses revealed that young birds were affected first, then adults, with adult males succumbing last as the cold persisted. This provided independent indication that adult males could survive the usual cold periods better than adult females, and better still than juveniles. It fitted the facts that, in this species, adult males normally migrate in lower proportion than adult females, and adults in lower proportion than juveniles (Pineau 2000).
The role of competition in influencing migration distances emerged in a study of Grey Plovers Pluvialis squatarola on the Tees Estuary in northeast England (Townshend 1985). Some newly arrived juveniles established territories only to be soon displaced by larger juveniles, or after a few weeks by later arriving adults. Two of the displaced juveniles (which had been colour-marked) were subsequently seen on another estuary 900 km to the south. Because migration is costly in terms of energy needs and mortality risks, birds can be expected to minimise the distances moved, and settle in the first suitable site they reach within the wintering range (Greenberg 1980, Gauthreaux 1982a, Pienkowski & Evans 1985). Pressure from other birds pushes them further along the route, so that subordinate individuals are likely to move furthest.
In subsequent years, individuals may be assumed to gain other advantages by returning to a site with which they are familiar, and at which they overwintered safely as inexperienced juveniles, even if this site was not as near to the breeding areas as possible. On this scenario, therefore, winter distributions are partly a consequence of events in previous years, and if a population declined over several years, the site-fidelity of adults would ensure a lag in the withdrawal of birds from the furthest parts of a wintering range. This model of 'within-species' winter distribution is similar to the 'despotic model' used to explain the distribution of birds among different habitats, whereby birds occupy the best areas in preference and, as these become filled, other individuals spread to less good areas (Brown 1969, Newton 1998b).
If body size-dominance relationships are important in differential migration, species with a greater degree of sexual size dimorphism should show a greater sex difference in migration distances than species that show little sexual size dimorphism. The winter distributions of two highly dimorphic icterid species (the Common Grackle Quiscalus quiscula and Red-winged Blackbird Agelaius phoeniceus) fit this prediction, whereas the sexes of the monomorphic Common Starling Sturnus vulgaris show no difference in winter distributions (as shown by ring recoveries of birds from the same breeding areas, Dolbeer 1982). However, female Brown-headed Cowbirds Molothus ater migrated the same average distance as males, even though they are considerably smaller. In addition, in some raptors in which females are bigger than males, males migrate further (e.g. Peregrine Falcon Falco peregrinus, Restani & Mattox 2000; Northern Goshawk Accipiter gentilis, Mueller et al. 1997; Eurasian Sparrowhawk A. nisus, Belopolsky 1971, Payevski 1990; Hen (Northern) Harrier Circus cyaneus, B. Etheridge, in Wernham et al. 2002; Rough-legged Buzzard Buteo lagopus, Kjellen 1994a; Snowy Owl Nyctea scandiaca, Kerlinger & Lein 1986; Northern Hawk Owl Surnia ulula, Byrkjedal & Langhelle 1986).
Among the Peregrines that breed in Greenland, the sex difference in migratory distance is extreme. Nearly 400 birds ringed in nesting areas have given 125 recoveries abroad. All females were found between the Gulf of Mexico (28°N) and the northernmost parts of South America (2°S), whereas all males were recovered in South America between 2°S and 26°S. On average, males were found 4000 km further south than females, and their migrations often exceeded 25 000 km annually (Lyngs 2003). Despite their longer migrations, males arrive back on nesting places no later than females. The findings on raptors are thus consistent with both the dominance and winter cold hypotheses, but not with the territorial defence hypothesis (that males winter near breeding areas in order to get back quickly in spring). Such differences are not apparent in all raptors, however, and in Common Kestrels Falco tinnunculus, females migrate in greater proportion, further and earlier than males (Wallin et al. 1987, Village 1990, Kjellen 1992, 1994). In various diurnal raptors, the juveniles move furthest, the sequence (nearest-furthest) being adult females, adult males, juvenile females, and juvenile males (Table 15.1).
As well as latitudinal gradients in sex and age ratios, some species show alti-tudinal gradients. For example, among Snow Buntings Plectrophenax nivalis wintering in Britain, the proportion of males decreased from north to south within Britain, and also from mountain to coastal sites, reflecting the tendency of males to winter nearest the breeding areas, and for females to occur in climatically milder places (Smith et al. 1993). In Dark-eyed Juncos of the race Junco hyema-lis carolinensis, females moved further downslope than males from their ridge-top breeding areas, a tendency more marked in severe winters than mild ones (Rabenold & Rabenold 1985). Likewise, among Willow Ptarmigan Lagopus lagopus and Rock Ptarmigan L. mutus on Alaskan mountains, most males remained in winter on the alpine tundra where they breed, while most females moved downslope into the forest zone (Weeden 1964). These findings were again consistent with the hypothesis that social dominance in competition for food or feeding places affects winter distribution patterns. However, not all montane birds follow these patterns: the Blue Grouse Dendrogapus obscurus is unusual in that both sexes often move upslope for the winter, vacating fairly open breeding areas for dense forest, where they eat conifer needles (Cade & Hoffman 1993). However, the males moved furthest, so that, like the other montane species just mentioned, they wintered at higher elevations than the females.
Among the species examined by Cristol et al. (1999), females migrated further than males in at least 77% of 53 species, and young migrated further than adults in at least 38% of 53 species. The individuals migrating further were usually members of the class whose body size was smaller (71% of 69 size comparisons between population classes), socially subordinate (82% of 44 comparisons), and later arriving in breeding areas (74% of 58 comparisons). However, there was a great deal of deviation from these patterns. In most species, the smaller sex migrated further, but in others less far. There can be no doubt, therefore, that the patterns described above are widespread among birds, but not universal.
Little evidence of sex differences in migration distances was found in the American Woodcock Scolopax minor, Sanderling Calidris alba and Grey Phalarope Phalaropus fulicarius, despite females being larger than males (Myers 1981, Diefenbach et al. 1990). Nor were such differences found in Eurasian Siskins Carduelis spinus and Savannah Sparrows Passerculus sandwichensis in which the sexes are about the same size (Payevsky 1998, Rising 1988). On the other hand, among Indigo Buntings Passerina cyanea migrating from North to Central America, the usual sequence was reversed, as females predominated in the northern part of the wintering range and males in the southern part (Komar et al. 2005).
Nor is it the case that juveniles invariably migrate further than adults. Among Chaffinches Fringilla coelebs, Bramblings F. montifringilla and Eurasian Siskins Carduelis spinus that were ringed on autumn migration on the southern Baltic coast, adults were recovered at significantly greater distances than juveniles (Payevsky 1998). The same was true in the males of various finches in North America, including the Dark-eyed Junco Junco hyemalis, White-crowned Sparrow Zonotrichia leucophrys and American Goldfinch Carduelis tristis (King et al. 1965, Ketterson & Nolan 1982, 1983, Morton 1984, Prescott & Middleton 1990). In Dark-eyed Juncos, the most abundant classes from highest to lowest latitudes in winter were juvenile males, adult males, juvenile females and adult females (Ketterson & Nolan 1982, 1983).
In experiments, Rogers et al. (1989) found no evidence that social dominance was responsible for variance in migration distances in juncos. In one experiment, juncos caught in winter in Michigan were each matched in captivity against another junco of the same sex and age class caught in Indiana (further from the breeding range). Michigan birds were dominant in only half the experimental dyads (21 out of 41 dyads), not in the majority as expected on the dominance model. In a second experiment, young males wintering in Michigan were tested against adult males from Indiana. In 19 out of 25 dyads, the more southern-wintering old males were dominant, which is also counter to the prediction of the dominance hypothesis. The authors concluded, therefore, that social dominance did not play an important role in influencing the wintering latitude of Dark-eyed Juncos (see Chapter 12 for other experiments on juncos).
Among waders, the Red Knot Calidris canutus provides another example of an unexpected age difference in migration. Most juveniles of the race rogersi spend their first year in eastern and southeastern Australia, and only make the additional journey to more distant non-breeding areas in New Zealand at the beginning of their second year. Thus, the proportion of first-year birds in eastern Australia is particularly high (up to 70% in years of good breeding success), while in New Zealand only a small proportion of first-year birds is ever recorded (Minton 2003). These various species thus seem not to fit either the dominance or the territorial-ism hypotheses, and the only explanation offered is that juveniles are physiologically less capable of migrating long distances than adults (Prescott & Middleton 1990). They cannot therefore move on to places where competition might be less.
No age-related differences in wintering latitude were apparent from ring recoveries of the Evening Grosbeak Hesperiphona vespertina (Prescott 1991), Cedar Waxwing Bombicilla cedrorum (Brugger et al. 1994), Barn Swallow Hirundo rustica (C. Mead in Wernham et al. 2002), Osprey Pandion haliaetus (Poole & Agler 1987), Mallard Anas platyrhynchos (Nichols & Hines 1987), American Black Duck Anas rubripes (Diefenbach et al. 1988) and American Woodcock Scolopax minor (Diefenbach et al. 1990). In fact, age-related differences in migration distances and winter distributions do not appear to be nearly as frequent as gender-based differences. Whether this is because age-related differences are smaller, and therefore harder to detect, or because they are genuinely less frequent, remains to be seen.
While dominance relationships appear to be general in bird populations, they are likely to result in differential survival and migration chiefly in competitive situations, where food or some other resource is limiting. In fights over food, dominants usually win over subordinates, and in the same area dominants may survive the winter in greater proportions (Kikkawa 1980). Among Mallards Anas platyrhynchos in much of northern Europe, males predominate in wintering populations but, in areas where birds are artificially fed, the sex ratio is more nearly equal (Nilsson 1976). Among non-migratory Song Sparrows Melospiza melodia, the survival rate of subordinates in the presence of dominants was increased when food supplies were experimentally supplemented (Smith et al. 1980). Among European Robins Erithacus rubecula, apparent overwinter survival was greater among individuals that remained near their breeding areas than among those from the same locality that migrated to distant wintering areas (Adriaensen & Dhondt 1990). The assumptions of the dominance hypothesis thus gain some support from field studies.
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