Abstract
There is an urgent need for tools to predict the effects of habitat change on wildlife. After earlier abandoning carrying capacity as a useful concept, scientists now agree that in the case of migratory birds the carrying capacity of a non-breeding site can be meaningfully defined as the number of bird-days a site can support. Current models of carrying capacity assume a single giving-up food density (GUD), often equal to the food density at which a bird can just meet its daily energy requirement (i.e., the critical density). More refined work however showed that there is considerable variation in GUDs within a site. In theory, the lowest observed GUD as well as the average of the GUDs may differ from the critical density. This means that the actual carrying capacity may greatly deviate from the one calculated by a single GUD model. The challenge is to develop more advanced and more accurate methods to calculate carrying capacity that incorporate spatial or behavioural aspects. The proposed research will focus on within-site differences in GUDs of Bewick's swans feeding on pondweed tubers in the Lauwersmeer (Netherlands). This study system is a very suitable one for a test of carrying capacity models for many reasons. The research will consist of a combination of field work, experiments and modelling. The observed number of bird-days will be compared with the ones predicted by models incorporating one or more of the factors that hypothetically affect the carrying capacity, namely the spatial variation in food density, food availability, and metabolic costs, individual differences, the additional food loss due to kleptoparasitic ducks and the quality of the preceding site (Estonia). As the proposed research can build on laborious and time-consuming measurements and model developments, this provides a unique opportunity to test carrying capacity models of varying degrees of complexity.
Netherlands