Population limitation in birds: the last 100 years

Over the past 100 years, much has been learnt about what limits the numbers of birds, and many field experiments have been conducted to check ideas. Such understanding is essential for the effective conservation or other management of bird populations. All the main factors likely to affect bird numbers, whether food or nest-sites, predators, pathogens or competitors, have been found to limit the numbers of one species or another. The same species may be limited by different factors in different areas or in different years. Some species may be limited by a combination of factors, which interact in their effects on population levels. Long-term trends and/or year-to-year fluctuations suggest that some migratory bird populations are limited primarily in their breeding areas, and others primarily in their wintering areas. Other aspects of population regulation are also discussed, including demographic factors (births, deaths and movements), and the role of territorial and other aggressive behaviour in the limiting process.