Chapter 27 - Population limitation — conditions on stopover

Abstract

This chapter illustrates different types of evidence indicating that events at stopover sites can influence the migratory bird populations. The chapter views the limitation of breeding populations through migration events mainly as a three-step process, involving food availability at stopover sites, carry-over effects of migration performance on subsequent survival or breeding success, and reducing food availability through depletion or interference. Events at stopover sites may affect not only the migratory performance of birds, but also their subsequent reproduction or survival, with potential consequences on the population levels. The chapter explains weather conditions at stopover sites can influence refueling rates and migration speeds, subsequent reproduction or survival and eventual population level or trend. Dominance relationships influence the outcome of competitive interactions at stopover areas, just as in breeding or wintering areas, even though the individuals concerned may be present for only short periods. In conclusion, increasing bird densities at stopover sites have been shown to intensify the competition, reducing food availability through depletion or interference. Many birds have been trapped, ringed, and weighed at migration sites enabling their subsequent survival rates to be assessed from later surveys of the population. Disturbance at stopover sites caused by natural predators or people can have marked effects on the rates and extent of weight gain by migrants, and hence on subsequent survival or breeding success. These various observations imply that conditions at stopover sites, including competition and predation/disturbance can influence the migration speeds, reproduction and survival chances of individual migrants and in extreme cases can affect their breeding numbers.