It must not be supposed that these journeys are free from dangers. Far from it; the perils are many and varied. (William Eagle Clark 1912.)

While the numbers of some migratory bird species are apparently influenced primarily by conditions in breeding or wintering areas (Chapter 26), the numbers of others could be influenced by conditions experienced on migration, mainly at stopover sites, but also during the flights themselves. It would indeed be surprising if bird breeding numbers were unaffected by conditions on migration. The process can in some species occupy several weeks or months each year (including stopovers), outward and return journeys together taking more than six months in extreme cases. Substantial mortality is therefore likely to occur during this period, and annual variations in this mortality could be reflected in annual variations in subsequent breeding numbers. In addition, in order to accumulate the body fat and other reserves necessary to fuel migration, and sustain themselves over the flight periods involved, birds need to obtain more food per day than usual. Although they have various ways to increase their energy consumption, such as change of diet, gut structure or daily foraging routine, their fattening rates are still constrained by intake rates or digestive capacities (Chapter 5). Moreover, because the same stopping sites can be used by large numbers of birds at a time, both solitary and flocking species, densities are often high and competition intense, resulting in severe depletion of food supplies (see later). The problems are magnified at those staging sites where birds must accumulate the extra large reserves necessary to cross an 'ecological barrier', such as a long stretch of water or desert. Hence, obtaining sufficient food in the limited time available could be a major constraint on the timing and success of migration.

The potential for population limitation on staging areas is perhaps especially acute in shorebirds and waterfowl, which in many regions have only a limited number of possible refuelling sites, often at widely spaced intervals along the migration route, but each holding very large numbers of birds. This situation contrasts with many landbird species, which migrate through mainly favourable habitat, and would seem to have feeding opportunities at many places along their route. However, the quality of any stopover site depends not just on the available food supplies, and levels of competition, but also on the security the site offers against predation, disturbance and other threats (for trade-offs between feeding rate and predation danger see Lindstrom 1990, Cresswell 1994, Ydenberg et al. 2002).

Events at stopover sites may thus affect not only the migratory performance of birds, but also their subsequent reproduction or survival, with potential consequences on population levels. In this chapter, therefore, I examine whether conditions at stopover sites can influence: (a) refuelling rates and migration speeds; (b) subsequent reproduction or survival; and (c) eventual population level or trend. Together, these various aspects encapsulate the subject area of 'stopover ecology' (for previous reviews of particular aspects of this subject see Moore & Simons 1992, Moore et al. 1995, Drent et al. 2003, Jenni & Schaub 2003, Newton 2005, 2006a). Consideration of the effects of storms and other extreme weather events that are sometimes experienced by migrants is left for Chapter 28.

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