Conspecific alarm calls were more threatening than heterospecfic alarm calls to the larger great black-backed gull, but the smaller herring gull weighed both equally. Vocalizations of two potential predators, the human voice and bald eagle call, differed in their threat level compared to each other and to alarm calls. Gulls were able to discriminate among a diverse set of threat indicators and respond in a graded manner commensurate with the level of threat. We experimentally exposed a single population of two sympatric gull species to auditory stimuli representing a range of potential threats in order to compare the relative threat of heterospecific alarm calls, conspecific alarms calls, predator vocalizations, and novel auditory cues.

Threat sensitivity has undergone limited testing in the auditory modality, and the relative threat level of auditory cues from different sources is difficult to infer across populations when variables such as background risk and experience are not properly controlled. Doing so allows potential prey to balance the costs and benefits of anti-predator behaviors. The threat sensitivity hypothesis predicts that organisms will evaluate the relative danger of and respond differentially to varying degrees of predation threat.

Predator sound effects