FUNCTIONAL-RESPONSES AND LOAD-SIZE EFFECT IN CENTRAL PLACE FORAGERS -DATA FROM THE KESTREL AND SOME GENERAL-COMMENTS

In a single-prey loader. the load-size effect may select for transport ation of large prey to the nest and consumption of small prey at captu re sites. Sonerud suggested that analyses of prey items delivered by a vian predators to the nest may profoundly bias the shape of functional response (FR) curves. Therefore, FR may be underestimated, if alterna tive prey types are smaller than the primary prey type, whereas in the reverse situation FR may be overestimated. Data from Eurasian kestrel s Falco tinnunculus breeding in western Finland during 1989-92 showed that the proportion of voles of the genus Microtus and Clethrionomys ( the primary prey) increased to the same extent from low through interm ediate to high vole densities among both prey items captured and trans ported from capture sites to nests. Thirty-six male kestrels feeding t heir mates and nestlings and 18 females feeding their nestlings did no t carry small prey items less than large prey items. Therefore, we con clude that the bias due to the load-size effect in the FR curve is pro bably negligible and that the model apparently overestimated its impor tance in biasing the FR curve. The probable reason is that decisions b y central-place foraging avian predators are also influenced by factor s other than load-size effect: for example, prevention of extra-pair f ertilizations, defence of offspring and reduction of flight costs duri ng hunting.