Some bird populations might be limited by conditions encountered on migration. This could occur at stopover sites where competition for restricted food supplies can reduce fuelling rates, migration speed, and subsequent survival or breeding success.

When preparing for migration, birds must normally obtain more food per day than usual, in order to accumulate the body reserves that fuel their flights. Birds often concentrate in large numbers at particular stopover sites, where food can become scarce, thus affecting migratory performance. Rates of weight gain, departure weights and stopover durations often correlate with food supplies at stopover sites, sometimes influencing the subsequent survival and reproductive success of individuals, which can in turn affect their breeding numbers. Many studies have provided evidence for food depletion and interference competition at stopover sites, but relatively few for migration conditions influencing the subsequent breeding or survival of individuals, and even fewer for effects on subsequent breeding numbers. The scarcity of studies showing effects on breeding numbers is probably due mainly to the difficulty of study, rather than the rarity of the phenomenon.

In addition, disturbance caused by natural predators or people at stopover sites can lower the food intake of birds, and in some populations of geese has proved sufficient to reduce subsequent breeding rates. In addition to disturbance and other food-related effects, birds may be exposed to greater rates of predation at stopover sites, and greater risks of picking up parasites and pathogens. Several mass-mortality events attributed to disease organisms or botulism have been described in waterbirds concentrated at stopover sites. Mortality on migration is hard to estimate as a separate component of the overall annual mortality in birds. However, in one small warbler species, 85% of the total annual mortality was estimated to occur during the two migration periods, and among arctic-nesting geese major losses of juveniles have been recorded over the first autumn migration.

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