Carrying capacity in overwintering birds: when are spatial models needed?

Abstract

We sometimes need to predict the maximum number of bird-days that can be supported by the food supply in a site used by migratory birds outside the breeding season. So defined, carrying capacity is often estimated using the daily ration model (DRM). In this, the total biomass of accessible food, aggregated across all patches of differing food density, is divided by an individual's daily requirement. Carrying capacity can also be estimated using spatial depletion models (SDM), in which patches of differing food density are treated separately. We identify here some of the features of the food supply that enable patches to be amalgamated so that the very simple DRM can be used instead of a more complex SDM. 2. We show by theoretical modelling that the predictions of the DRM and SDM are often the same even though initial food density varies between patches. The wide range of conditions over which this is so are specified.