Concluding Remarks

In several respects, migration differs markedly between the two seasons. In autumn, birds leave their breeding areas under decreasing daylengths as conditions are deteriorating. In moving toward lower latitudes, they generally meet more clement conditions (except where they have to cross a large area of sea or desert). In spring, by contrast, most birds leave their same-hemisphere wintering areas under increasing daylengths, when conditions are improving or at least benign. In moving towards higher latitudes, they generally encounter progressively worsening conditions; they are often held up by bad weather, and may even have to backtrack for part of the route. It is normally some time after arrival in nesting areas that conditions improve enough to allow them to breed. In fact, environmental conditions, and the appearance of food supplies at particular latitudes, may have much more influence on the progress of migration in spring than in autumn (Chapter 14).

In autumn, populations are large, and juveniles are making their first migration; but in spring, populations are smaller and all participants have experienced at least one previous journey. This could influence the time taken and the extent of deviations from the primary direction, young birds being more susceptible to wind drift than older ones (Chapter 10). But while in some species, the autumn migration season is more extended than the spring one, in others the reverse holds (especially in those species with a large non-breeding contingent, see above).

The birds themselves are also in different physiological states at the two seasons. In autumn, the migrants travel with regressed gonads, and in spring with growing gonads, which leads to seasonal differences in hormonal states, which may in turn influence aspects of the migration, including direction (see above). In some species, patterns of fat deposition, flight lengths and speeds of travel also differ in a consistent manner between the two seasons, associated with different travel routes and stopping sites. For these various reasons, therefore, the spring journey is not simply the reverse of the autumn one.

Differences in the spread of migration dates between autumn and spring are evident at many localities on migration routes (Enquist & Pettersson 1986, Lavée et al. 1991, Morris et al. 1994, Leshem & Yom-Tov 1996a). They could be due to: (1) greater spread in the start dates at one season than the other; (2) more variation in mean migration speeds at one season than the other; or (3) a combination of both these influences. Even captive birds behave differently at the two seasons: for example, caged White-crowned Sparrows Zonotrichia leucophrys showed more intense nocturnal restlessness in spring, but over a shorter period of days than in autumn (Ramenofsky et al. 2003). At least some of the differences in migratory behaviour and fattening patterns observed in many species between autumn and spring are evidently under endogenous influence, and presumably have an adaptive basis.

While the main ultimate effect of climatic seasonality on migration timing is through its effect on food supplies, some researchers have suggested a more direct influence, with migratory seasons evolving to coincide with those times that, on average, are favourable for travel, and to avoid those times that, on average, are unfavourable for travel. This view remains speculative, but the correlation between migration timing and ecological conditions is so strong, especially in spring, that average weather conditions are unlikely to have any more than minimal effect on the broad timing of migration seasons. If birds had greater freedom in migratory timing, they would presumably avoid the hurricane seasons, which span the main autumn migration period in several parts of the world. Weather does, however, influence the actual dates of flights within the migration season (Chapter 4), and possibly also the routes taken, which often differ between autumn and spring, in line with seasonal differences in prevailing winds or feeding conditions (Chapter 22). In addition, the immature non-breeders in some species have greater freedom in migratory timing than breeding adults, and tend to migrate later in spring and earlier in autumn, when conditions are better.

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