THE RELATIVE IMPORTANCE OF HABITAT STRUCTURE AND OF PREY CHARACTERISTICS FOR THE FORAGING SUCCESS OF WATER PIPITS (ANTHUS-SPINOLETTA)

While many studies on foraging have related energy gain to the density and the size of prey, only few have investigated whether and hour hab itat structure modifies the gain through affecting foraging success. I n this study, the influences of habitat structure and prey characteris tics on the foraging success of water pipits, Anthus spinoletta, were investigated experimentally. The birds take longer to find prey in tal l than in short vegetation. The effects of vegetation on searching tim es differ between prey types. These differences are probably caused by variation in prey behaviour and in cryptic colouration, but not by pr ey size. Searching times increase with decreasing density for mealworm s and tipulids, but not for caterpillars. Handing large prey items req uires more time than handling smaller prey. Tipulids and caterpillars, which were offered alive, are handled for a longer time than dead mea lworms of corresponding size. The success of attacks on flying insects is probably influenced by the prey's flight speed: fast houseflies ar e missed more often than slow tipulids. Overall, the results show that the time costs of foraging water pipits are influenced to a comparabl e degree by vegetation structure, by prey density and by other specifi c prey characteristics such as camouflage, hiding behaviour or agility . The amount of food gathered per unit time is determined primarily by factors that affect searching times, and less by handling and handlin g times. Insertion of our data into an optimal diet model leads to the prediction that water pipits should be generalist foragers, which agr ees with the observed behaviour.