Chapter 26 - Population limitation — breeding and wintering areas

Abstract

This chapter mainly describes the changes in the numbers of migratory birds, and the relative importance of events in breeding and wintering areas in influencing these changes. The population sizes of migratory birds may be influenced by conditions in more than one part of the world: in areas that are used for breeding, as well as in areas that are visited at other times of the year. Conditions in a particular region can be considered limiting if they slow a population's increase or cause its decline. When areas of habitat are lost or added through human action, bird numbers often change accordingly. This chapter explains species affected by events in breeding or wintering areas with examples. Sometimes migratory bird populations are influenced throughout their range by widescale climatic fluctuations. The same trends in weather do not necessarily have the same effects on both breeding and wintering areas. Carry-over effects of winter conditions on breeding success have been detected not only in large birds, which carry substantial body reserves to breeding areas, but also in small birds, which carry relatively smaller reserves. In general, therefore, it seems that different periods in the annual cycles of migratory birds are inextricably linked, and that events occurring during one period can affect performance in a later period. Range size and population limitation, range segregation, and sex ratios also affect migratory birds. Changes in the numbers of migratory birds, either long-term or year to year, may be caused by changes in conditions in the breeding or wintering areas or both.