Ardea
Official journal of the Netherlands Ornithologists' Union

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Newton I. (1980) The role of food in limiting bird numbers. ARDEA 68 (1-4): 11-30
1. Food-shortage can be considered limiting if it prevents a population from increasing. However, social behaviour and other mortality agents may also be involved in the limiting process. In any study, it is important to define the time period, because populations may be limited by different factors at different seasons, or in different years. It is also important to define the area, for food may limit local densities, but not total numbers if birds excluded from one area find food in another. 2. In seasonal environments, food is likely to be limiting only at certain times of year. It may limit numbers in a density-dependent or a density-independent manner, and need not necessarily entail any direct deaths from starvation. This is especial1y so (a) when social behaviour regulates local density in relation to food, and excluded birds succumb to other mortality, such as from disease or predation; or (b) when food shortage lowers the breeding and recruitment rates, and thus the population density, without causing deaths of full-grown birds. 3. Indisputable proof that food limits a population is unattainable, and so we have to decide what will constitute acceptable evidence. Existing evidence might be graded as follows, in order of soundness: (a) Circumstantial evidence, followed by experiment, in which a deliberate change in food-supply promotes a corresponding change in numbers, compared with trends in a control area, where food is not manipulated. (b) Circumstantial evidence alone, useful for framing hypotheses which are open to experimental testing; of two main types: (I) where a drop in numbers coincides with one or more of the following: starvation, low weights, low feeding rates, long feeding periods, increased fighting, or depletion of food stocks; (II) correlations between bird numbers and food-supplies in different areas, or in different years in the same area. (c) Indirect evidence, not open to experimental testing, such as the correlation between body weight and territory size among birds in general, or the existence of diet differences between species. 4. Data on mortality, weights, feeding rates, feeding periods, fighting and food-stocks are sometimes helpful in indicating likely periods of food-shortage, but on their own they provide no evidence on population limitation, and may be misleading. 5. Whenever social behaviour has been studied, it has been shown or suspected to be important in regulating density in relation to food, through territorialism or dominance hierarchies, both of which may result in mortality falling most heavily on the subordinate (often young) members of a population.


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