In a migrant species, reproduction and the main mortality may occur in regions several hundred miles apart. This greatly complicates the study of the factors influencing numbers. (David Lack 1954.)
In the limitation of their populations, migratory birds differ from residents in at least one important respect. Their population sizes 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 year. For many migratory species, the breeding and wintering ranges are widely separated geographically, and might differ greatly in the numbers of birds they can support. Hence, factors operating in the migration or wintering range might limit the numbers that can occur in the breeding range, or vice versa. This is most clearly apparent where changes in the numbers of a species over a period of years are associated with changes in conditions in one area, but not in the other. In this and subsequent chapters, I address some of the issues involved in the population limitation of migrants, using as examples species whose long-term or year-to-year numerical changes can be clearly linked to events in breeding or non-breeding areas. These are increasingly important issues, because many migratory species are declining, and it is important to know where to direct any conservation measures.
Conditions in a particular region can be considered limiting if they slow a population's increase or cause its decline. Such conditions can seldom account entirely for a given population level, however, because among migrants both mortality and reproduction are also influenced by conditions in other regions at other times of year. Moreover, whether in breeding or wintering areas, bird numbers may be limited in a density-dependent or largely density-independent manner. Density-dependent regulation implies competition (perhaps for food or predator avoidance), as a result of which the percentage of birds affected is greater when numbers are high than when they are low. In density-independent limitation, competition is unimportant, and the proportion of birds affected bears no consistent relationship to population density. Probably few situations occur in which there is no competitive (density-dependent) element in reproduction or mortality, although this may often be slight or hard to discern.
In understanding population changes, it is helpful to separate long-term trends from year-to-year fluctuations about the trend, because different factors might be involved (Newton 1998b, 2004a). For example, some species may undergo long-term change in numbers, resulting from progressive habitat change, but may also continue to fluctuate from year to year in response to different factors, such as annual variations in rainfall and associated food supplies. Moreover, within species, population trends and limiting factors may vary across the range, and also at different time periods, so that local research findings cannot necessarily be extrapolated over wider areas or longer periods. Because of the problems of studying birds which routinely occupy two or more different areas each year, it has seldom been possible to follow the same individuals throughout the year, and most of the evidence on population limitation in migrants is indirect.
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