Abstract
In this study we investigate for free-living insectivorous water pipits (Anthus spinoletta) whether prey is chosen according to biochemical quality as measured by protein, lipid, carbohydrate, energy and water contents and/or according to profitability as measured by density, size and catchability. Food preference - expressed in relation to availability - is estimated for 22 arthropod taxa (families and orders). Uni- and multivariate statistics detected no relations between food preference and nutrient contents, but revealed that larger prey items are fed to nestlings more than smaller ones, both for all prey taken together and within individual taxa. Also, slowly flying arthropods, which are easier to catch, were usually preferred over walking and fast flying ones. Combined with results from previous studies on the effects of vegetation, prey density and catchability on search times and energy intake, these findings suggest that water pipits select their prey primarily in order to maximize profitability, i.e. energy intake per unit time. Qualitative traits seem to be important only in specific taxa. For instance, toxins or poor digestibility may be responsible for avoiding heteropterans, beetles and ants and for feeding the nestlings fewer tipulids than expected at high tipulid densities.
Brodmann, P A