The auditory perception of eight species of raptors was examined to test the hypothesis of Marler (1955) that these avian predators are unable to locate certain songbird alarm calls. In particular, Marler proposed that mobbing calls have characteristics that enhance their locatability and that these characteristics are absent in the high-frequency ‘seet’ calls given by individual songbirds. To test this hypothesis, the behavioral responses of four species of owls and four species of hawks, housed at two different raptor rehabilitation sites, to tape recorded alarm calls were examined. Each raptor was exposed to a random order of 10 trials of mobbing calls and 10 trials of a seet call. Responses were scored based upon head angle orientations. Hawks and owls responded more often and more accurately to mobbing calls than to seet calls. In general, owls responded to significantly more calls than hawks. The results are consistent with Marler’s hypothesis that raptors have difficulty locating passerine seet calls. Nevertheless, future studies should test mobbing calls that vary in their frequency and duration (Ficken & Popp 1996) to determine whether some mobbing calls are more difficult to locate than others.