Nestling provisioning in water pipits (Anthus spinoletta):: do parents go for specific nutrients or profitable prey?

In this study, we investigated whether free-living insectivorous water pipits (Anthus spinoletta) choose prey 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 relationships between food preference and nutrient contents, but revealed that more larger prey items are fed to nestlings than smaller ones, both for all prey taken together and within individual taxa. Furthermore, slow-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 to maximize profitability, i.e. energy intake per unit time. Qualitative trails seem to be important only for specific laxa. For example, toxins or poor digestibility may be responsible for the avoidance of heteropterans, beetles and ants and for feeding the nestlings fewer tipulids than expected at high tipulid densities.