Abstract
A basic but rarely tested assumption in optimal foraging theory is that positive relationships exist between the foraging pattern of an animal, its short-term benefits in feeding, and its long-term fitness. We present evidence for these relationships for a central place foraging situation. We studied the foraging behavior of adult water pipits (Anthus sp. spinoletta) feeding nestlings in an Alpine habitat near Davos, Switzerland, with the following results: (1) searching effort decreases with increasing distance from the nest, (2) the amount of prey and the proportion of large items brought to the nest increases with increasing foraging distance, (3) water pipits do not forage according to habitat availability, but prefer vegetation types with the highest food density (mainly grass and herbs) and avoid those with the lowest, and (4) this selectivity is only expressed when the birds forage more than 50 m from the nest, i.e., usually outside the territory. Among the several potential interpretations of these results, the most parsimonious is that foraging decisions are based on profitability, i.e., on the net energy gain per time unit. Additionally, we found that food conditions translate into fitness: the number of fledglings per nest is related positively to the average prey biomass at the foraging place and negatively to the average distance between the foraging place and the nest. Maximum economic distances, which were predicted from this food-fitness relationship, agreed well with the actual foraging distances observed. This suggests a dose connection between foraging decisions and fitness. In addition to the theoretical issues, some conservation issues are also briefly discussed.
Frey-Roos, F