not in Young The diets of the sibling species, which occur sympatrically in the study area, were investigated by fecal analysis. Prey composition was not differing significantly at the level of (sub)orders. In both species small or even very small specimens of Diptera (midges and flies), followed by Hymenoptera (ichneumonid wasps), Homoptera (aphids and cicadas), and Planipennia (lacewings) were mostly prefered. Hence calculated trophic niche overlap attained a high degree. The value for standardized niche width stands for a high degree of specialization in both species. In fact this value is biased due to size selection by the hunting bats. According to the broad spectrum of prey items and the bats' foraging behavior both pipistrelle species may rather forage mainly opportunistically making use of distinct feeding places within the study area. Our results are compared with habitat preferences and related differences in the composition of diets of the species under study. BR Nutrition. Diet. Prey. Feeding behaviour. Ecology. Predators. Habitat utilization. Land zones. Palaearctic region. Eurasia. Europe. Insecta: Prey; Pipistrellus pipistrellus (Vespertilionidae): Predator; Pipistrellus pygmaeus (Vespertilionidae): Predator. Insecta: Mammalian predators, Pipistrellus, Prey selection and distribution relationships, Germany, Heidelberg Forest; Pipistrellus pipistrellus, Pipistrellus pygmaeus: Insect prey, Prey selection and habitat preference relationships, Food preferences, Prey selection, Distribution within habitat, Habitat preference, Germany, Heidelberg Forest. ST Insecta; Mammalia, Chiroptera, Vespertilionidae. Arthropods; Bats; Chordates; Insects; Invertebrates; Mammals; Vertebrates. none2003