In many species, movements in the non-breeding season are optional (facultative). They can take place at almost any time in the non-breeding season if feeding becomes difficult. In some northern species, the proportions of birds that leave the breeding range, and the distances they travel, vary greatly from year to year (Chapter 18). Most individuals stay in the north in years when food is plentiful there, wintering in, or just south of, their breeding areas, but moving further south in years when food is scarce. Such annual variation in migration is most pronounced in finches and other birds that depend on fluctuating tree-seed crops (such as Common Redpolls Carduelis flammea and Bohemian Waxwings Bombycilla garrulus), and in raptors and owls that depend on fluctuating (cyclic) prey species (such as rodent-eating Rough-legged Buzzards Buteo lagopus and Snowy Owls Nyctea scandiaca) (Chapters 18 and 19, Figure 16.4). Their so-called invasions or irruptions, in which every few years they appear in large numbers well outside their usual range, follow periodic widespread crop failures (finches) or crashes in prey populations (raptors). Irruptions therefore occur in response to annual, as well as to seasonal, reductions in food supplies. In some such years the birds can, in effect, be on passage for much of each winter, as they move from one area of temporary abundance to another, and reach the most distant parts of their wintering range only in the late winters of extreme years.
Although irruptive migrants provide extreme examples, year-to-year distributional changes dependent on flexible migration patterns occur in a wide range of birds, at least in the non-breeding season. Many bird species can migrate at any date in the non-breeding season, if stimulated to do so by reduced food supplies, and usually they move further along the regular migration route (Chapter 18). One species that shows this behaviour is the Yellow-rumped Warbler Dendroica coronata of the New World (Terrill & Ohmart 1984). In autumn the densities of this warbler at a number of sites in Arizona and northern Mexico were correlated
(a) Snowy Owl (b) Common Redpoll
Figure 16.4 North American winter ranges of (a) Snowy Owl Nyctea scandiaca and (b) Common Redpoll Carduelis flammea that winter mainly at high latitudes (shaded area), but in years of food shortage extend far to the south (dotted line).
with local food supplies. Subsequent changes in the abundance of both insects and warblers at particular sites were influenced by weather, with declines in both occurring during cold spells. Declines in warbler numbers at Arizona sites corresponded with increases at more southerly Mexican sites, suggesting movements, and the magnitude of changes were correlated with insect availability. These particular population shifts occurred in January. However, records of Yellow-rumped Warblers killed overnight at television towers showed that these birds were able to migrate at any date in winter, usually in association with cold snaps, the numbers varying greatly from year to year. Birds tested for directional preferences after dark showed southerly orientation (Terrill & Ohmart 1984).
Similar findings have emerged for many other species that migrate within the temperate zone (e.g. Pulliam & Parker 1979, Niles et al. 1969), as well as in many that reach the tropics (Chapter 24). For example, the numbers of Yellow Wagtails Motacilla flava at a roost in West Africa declined progressively from 16 000 in November to 2000-3000 in March, in line with a progressive decline in local food supply. A southward shift was revealed by ring recoveries (Wood 1978). The facultative nature of the movement was implied in the fact that the decline was substantially more marked in non-territorial birds that fed in flocks, than in dominant adult males that remained on feeding territories near the roost. Experiments on several species have shown that captive birds can develop migratory restlessness in response to food deprivation in winter, well beyond the normal migration period (Chapter 12).
In Africa many species move southwards in stages, pausing for weeks or months before proceeding (Chapter 24). In some species these movements are apparently 'obligate' and consistent in timing from year to year (reflected in the periods of migratory restlessness in captive warblers), but in others they are facultative, varying in timing and extent with prevailing conditions (Chapter 24; Lack 1983, Herremans 1993, 1998). In all such species, further movement in the usual direction of migration may give the birds the best chance of finding food.
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