Provisioning behaviour in relation to food availability and nestling food demand in the water pipit Anthus spinoletta

Insufficient food provisioning by parents can reduce growth rate of altrici al nestlings, their size and weight at fledging, and also affect survival. Therefore at low food availability parents should increase their food provi sioning effort. In insectivorous Water Pipits Anthus spinoletta prey biomas s differed up to 50 times among home ranges; nevertheless, number, size and weight of fledglings did not differ between home ranges with low and high prey biomass. Based on predictions from the literature, we investigated whe ther and how parents adjust their provisioning behaviour to food availabili ty, measured by arthropod biomass, and to nestling demands, measured by bro od size, nestling age and ambient temperature. Foraging distances did not v ary significantly with any of the four variables. Feeding rates were also n ot related to food availability, but increased with brood size in both sexe s. In addition, females showed a tendency to increase feeding rate with dec reasing air temperature. Our results suggest that, at least in some years, arthropod food is so abundant in the birds' environment that it does not li mit reproductive success.