Extensive field data show that avian and mammalian predators take more Microtus epiroticus than coexisting Microtus agrestis voles. The different vulnerability of these vole species may arise from prey selection or susceptibility differences between the two vole species. We studied the prey selection and hunting behaviour of the kestrel Falco tinnunculus on two Microtus voles, in addition to how behaviour and group structure in voles affect their vulnerability. The main results were that 1) kestrels do not select either of these species, but 2) susceptibility of the vole species deviates significantly. Falcons were more successful in catching M. epiroticus than M. agrestis, indicating that the latter is better in escaping avian predators and thus may have a lower cost of predation. During the experiment M. epiroticus aggregated more than M. agrestis but group size did not significantly explain capture success. However, solitary M. epiroticus seemed to be predated faster than those in groups. Although our results suggest that the high density of conspecifics may provide protection for vulnerable M. epiroticus, the aggregative distribution in the field and the preference for an open uncovered habitat may expose them to patch-searching predators.