Most species of birds must stop migrating in order to feed, the two activities being mutually exclusive. However, a minority of species, such as swallows and swifts that depend on aerial prey items, can pick up food while on migration, at least when flying over vegetation. Although they do not necessarily make the entire journey without pausing to refuel, an ability to catch and eat food on the wing could enable them to migrate with smaller body reserves and make faster progress than other birds of similar size. Swifts are normally on the wing day and night, except when they have nests to tend, so in this respect migration is probably little different from normal daily life. Some raptors and shrikes may also be able to get food at places offering little for other species, if only by catching and eating their fellow travellers.
Calculations of maximum possible theoretical migration speeds are informative, even though not all birds necessarily attempt to migrate as quickly as possible. Instead of being 'time minimisers', to use the jargon, they may be 'load minim-isers', migrating in short stages requiring no great fuel deposition, thus reducing their overall energy needs and predation risk (Lindstrom & Alerstam 1990; Chapter 5). In addition, few birds can feed at the maximum possible rate recorded for their species, poor food supplies or competition limiting their daily intakes. They may also at times be delayed by adverse weather long beyond the date at which they are physiologically prepared to depart. All these types of constraints tend to slow the overall migration period, and increase the variability within populations.
In addition, at least in autumn, it may be disadvantageous for some species to pass rapidly through areas of abundant food, if they cannot be sure of finding similar plentiful supplies further along the migration route. They would be better to stay and feed until the food was exhausted. This is a particular consideration in irruptive species, which depend on unpredictable food supplies, such as tree-seed crops (Chapter 18). Their migration timing therefore varies greatly from year to year, according to the seed crops encountered, and in most years migration speed is by no means close to the maximum possible (recall the slow rates of progress shown by irruptive species in Figure 8.3). For these various reasons, therefore, we can expect that most birds do not normally migrate at the theoretical speeds of which they are capable. It is perhaps in ideal conditions in spring, when birds are migrating to reach their breeding areas, that the maximum rates are most likely to be approached, but even then most birds cannot migrate long ahead of the appearance of suitable food supplies.
If a bird had no other events in its annual cycle, it would seem advantageous for daylength reasons to undertake both migrations as close to the summer solstice as possible, thereby gaining longer days for feeding and fuel deposition. This may be one reason why, in some large bird species, the immatures in their non-breeding years migrate from wintering to breeding areas much later than the breeding adults, and return much earlier, spending only a few weeks per year in breeding areas.
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