The diet of Epfesicus serotinus was investigated by faecal analysis and the validity of this technique as a means of obtaining reliable quantitative results was assessed. Knowledge of what the bats eat will allow predictions of which habitats contribute most prey. Three male serotines were kept in captivity during the months of July and August 1991 and fed known quantities of identified insects. With captive E. serotinus the remains of prey appeared within 33 min of first bite and continued to appear for up to 32 h after consumption. Scales from two Noctua pronuba moths consumed were contained in 59 subsequent droppings. Similarly, fragments of three beetles, Geotrupes stercorarius, were contained in 28 subsequent faeces.
Analysis of the faeces of free-living bats revealed insects from seven orders. Coleoptera were found to be present in 96.1% of the droppings examined; the next most frequently found order was Lepidoptera, occurring in 14.7%. The beetles were mostly associated with hay meadows (Aphodius spp. and Melolontha spp.), or grazed pasture (Aphodius spp. and Geotrupes spp.). Quantitative determination of prey eaten is not possible owing to the large number of the droppings shown to contain the remains of marker insects and the long period over which they are produced.