Bird migration, in its more highly developed forms, is both too regular in its performance and too provident in its anticipation of events to be conceivable as being created anew each year by the mere pressure of external forces. . . . Migration must, then, be a recurrent manifestation of a mode of behaviour which has become inherent in the nature of various species. This implies that the behaviour serves useful ends which give it a survival value; that it became implanted in the inheritance of the species by some originating cause; that it recurs annually in the life of the individual in response to immediate stimuli; and that its exhibition brings into play still other factors which determine the actual path and goal of the movement performed. (A. Landsborough Thomson 1936.)
Migration might be expected to occur wherever individuals benefit more, in terms of survival or reproduction, if they move seasonally between different areas than if they remain in the same area year-round (Lack 1954). The usual reason why breeding areas become unsuitable during part of the year is lack of food. Such food shortages occur for many birds because plant growth stops for part of the year, and many kinds of invertebrates die or hibernate or become inaccessible under snow and ice. At high latitudes, daylengths also shorten in winter to such an extent that many diurnal birds would have too little time to get enough food, even if it were available. Hence, the purpose of the autumn exodus from high latitudes is fairly obvious.
The reason why birds leave their wintering areas to return in spring is less obvious, because many wintering areas seem able to support the birds during the rest of the year. But if no birds migrated to higher latitudes in spring, these latitudes would remain almost empty of many species, and a large seasonal surplus of food would go largely unexploited. Under these circumstances, any individuals that moved to higher latitudes, with increasing food and long days, might raise more young than if they stayed at lower latitudes and competed with the birds resident there. So whereas the advantage of autumn migration can be seen as improved winter survival, dependent on better food supplies in winter quarters, the main advantage of spring migration can be seen as improved breeding success, dependent on better food supplies in summer quarters. Compared to survival, reproduction also has more stringent requirements in terms of specific food needs and predation avoidance.
In effect, migration reduces the seasonal fluctuations in food supplies to which a breeding population could otherwise be exposed. Species that breed in one hemisphere and 'winter' at an equivalent latitude in the opposite hemisphere, where the seasons are reversed, would seem to get the best of both worlds. Further, some habitats offer excellent conditions for survival but cannot be used for reproduction -tidal mudflats used by wintering shorebirds providing an example. Thus, as well as lessening the exposure of individuals to seasonal variation in resource levels, migration facilitates the exploitation of different and widely separated habitats for survival and reproduction (and sometimes also moulting, Chapter 16).
While seasonal change in food supply is clearly important for bird migration, this does not rule out an influence of other factors, such as reduced predation (Fretwell 1980, Pienkowski 1984), parasitism (Piersma 1997), or competition (for food or other resources, Cox 1968, von Haartman 1968). All these pressures may decline with increasing latitude because of the general latitudinal decline in the total numbers of animal species, whether these animals act as predators, parasites or competitors. At one time or another, all these factors have been proposed as contributing to the evolution of migration, but on scant evidence (for more details, see above references). Moreover, it does not necessarily follow that any species suffers fewer losses through having only one predator (or parasite) species to contend with, rather than many. Much depends on the kinds of predators (or parasites).
Whatever the main selective forces, therefore, the migratory habit ensures in the long term that species in seasonal environments adopt and maintain movement patterns that allow individuals to survive and breed better than if they remained in the same area year-round. The main cost of migration is seen as the increased mortality associated with the journey itself. Most recorded mortality incidents relate to storms and unseasonable weather, which can kill thousands of birds at a time, sometimes causing widespread reductions in breeding numbers (Chapter 28). Other risks include exposure to a greater range of pathogens, predators and competitors, as mentioned above, all of which could have additional fitness costs. For the migratory habit to persist, therefore, despite the risks involved in long journeys, the net fitness benefits to individuals of moving both ways must outweigh the costs. Conversely, year-round residence presumably persists in populations where the net benefits of staying in one area outweigh the costs of seasonal movement.
At the population level, one consequence of migration, it may be assumed, is greater overall numbers. Because the breeding area (and sometimes also the wintering area) could not support that number of birds year-round, the overall population is larger as a result of seasonal movement. For many species, the geographical range is also larger, because birds can breed in areas where they could not remain year-round, nesting at higher latitudes only by virtue of migration. Species that are entirely migratory, with separate breeding and wintering ranges, exist today only through their seasonal movements. Other animals, that are less mobile than birds, cope with seasonal shortages in other ways, notably by hibernation - a metabolic shutdown involving torpor. Hibernation occurs in at most a handful of bird species, the best known participant being the Common Poorwill Phalaenoptilus nuttallii, though hummingbirds and others show shorter periods of torpidity (Chapter 1).
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