Concluding Remarks

The overriding role of food supplies in influencing the movement patterns of birds is evident from: (1) the link between migration and climatic seasonality, with increasing proportions of migrants in increasingly seasonal environments; (2) the relationship in different species between migration and diet; (3) within species, the regional variations in the proportions of birds that leave for the non-breeding season; (4) the locations of wintering areas; and (5) the precise seasonal timing of movements, such that birds are absent from breeding areas at a time when their particular foods are scarce there (Chapter 14). Some authors have stressed the role of climate and competition in influencing migration patterns, but both these factors are likely to act primarily through the food supply.

Migratory habits are so closely related to diet, and to the seasonal fluctuations in the food types involved, that it is hard to tell whether any other factors have any influence beyond those that act through food (such as climate and competition). The main problem is that diet is related to almost all aspects of the morphology, ecology and behaviour of birds, as well as to migration, so any factor could seem to be important to migration through association. For example, in a recent analysis of British breeding birds, Siriwardena & Wernham (2002) found no consistent association between migration and body size, but that territorial species tended to be more sedentary than colonial and semi-colonial ones, that hole-nesters tended to be less migratory than open-nesters, and that localised species tended to be more migratory than widespread ones. However, their analysis was based on a wide range of species from passerines to gulls, and many of their findings depended on the inclusion or exclusion of particular groups. They also found that migrants in general had higher survival and lower reproductive rates than residents, a finding expected on other grounds (Chapter 20). Their most robust finding, however, was a relationship between migration and diet, with insectivores and piscivores being more migratory than omnivores and herbivores.

In species that migrate between the northern and the southern hemispheres, the question arises why the same individuals do not breed twice in one year, in both summer and winter quarters. One reason in many species is that individuals moult while in winter quarters, a process that takes several weeks or months and could not be undertaken at the same time as breeding (the two processes being mutually exclusive in most birds, Chapter 11). Another reason is that many migratory species do not remain for long in the same area in winter, but periodically move to other areas in response to changes in food supplies (Chapter 24; Jones 1995). This exploitation of temporary abundances is one way in which migrants in the southern hemisphere could avoid competing with the local birds which, breeding at that time, are tied to fixed nesting areas. Neither explanation applies to all transequatorial migratory species, however, and there are still some that are sedentary while they are in both breeding and wintering areas, and would seem able to breed in both, six months apart, but do not.

A related question is why many birds, having had a winter break, do not breed more than once at different localities on their migration routes. Migrants that travel in spring northward through Europe and North America, and have short breeding cycles, would seem able to breed in the southern parts of these continents, before moving on to breed again further north. They could then raise more young per year than by exploiting only the short favourable season in the north. The fact that a few species are known to do this (see Chapter 16) makes it even harder to explain why most do not. However, many candidate populations also have competing conspecifics nesting at lower latitudes, where they can raise two or more broods in a season.

Long-distance migration is found not only in birds, but in various mammals and insects, as well as in various marine fish, turtles and invertebrates. Although food supplies and reproduction underlie the movements of all these animals, in some of them the pattern is different from birds. For instance, various whales migrate to high latitudes in summer, where they feed hard and accumulate body fat, before returning to winter in the tropics, where their young are born. However, the fact that they accumulate most of the food that permits breeding at high latitudes may make them less different from birds in this respect than appears at first sight. Various fish migrate only twice in their lives, from the breeding grounds as young, returning several years later as adults, when they spawn and die, or return repeatedly to spawn in successive years. Various butterflies breed at different points on the migration route, with successive generations making different parts of each journey. All this contrasts with the situation in birds, most of which migrate regularly back and forth every year between their higher latitude breeding areas and lower latitude wintering areas.

Appreciation of such large-scale, long-distance movements highlights the conservation problems that migratory animals present. Depending on a chain of areas, spanning a wide range of latitude, such species are vulnerable to human impact, not only in breeding and wintering areas, but also at crucial stopover sites en route. Nature Reserves established in one region can be expected to protect some species for no more than a few weeks each year, before they move on. It underlines the need for international collaboration in the conservation of migrants, a concept that has already led to concerted action for waterfowl and shorebirds in some parts of the world.

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