Current Ornithology

Volume 11 of the series Current Ornithology pp 143-198

Predation and Limitation of Bird Numbers

  • I. NewtonAffiliated withInstitute of Terrestrial Ecology, Monks Wood Experimental Station

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While food supply and other resources provide the ultimate check on bird numbers, some populations might be held by predators below the level that resources would permit (Lack, 1954; Newton, 1980; Martin, 1991). Although practically all birds are subject to predation, at least at the egg and chick stages, it is extremely difficult to assess the effects of this predation on population levels. Even where predators kill a large proportion of their prey each year, they do not necessarily reduce breeding numbers. The numbers of many bird species can more than double each year through breeding so that, if they are to remain stable from year to year, more than half the individuals present at the end of one breeding season must die before the next, if not from predation then from something else. For predators to reduce a breeding population below the level that would otherwise occur, at least part of the mortality they inflict must be “additive” to other mortality, and not simply “compensatory,” replacing other forms of death.