Effets de la prédation sur l’évolution des cris des jeunes oiseaux. I.

Abstract

Based on the premise that the call of a fledgling passerine is difficult to locate by ear, the authors have formulated the hypothesis that these vocalizations evolved to make perception and location difficult for the potential predator. To test this hypothesis various vocalizations (adult song and alarm call, fledgling call) of four different species of birds (english sparrow, lark, blackbird and corn bunting) were played back at the same volume to three carnivorous mammals : a dog, a Tayra and a Red Fox. In a preliminary series of experiments, a preconditioned dog was subjected to a multiple choice experiment in selecting one sound from the bird’s vocal repertoire emitted simultaneously through a loudspeaker. A second series of experiments was made with a pet Tayra let loose in a wood. This test attempted to establish the distance at which the Tayra shows, through its behavior, perception of the bird’s vocalizations, both adult and/or fledgling. A third series of experiments was held with a non-conditioned Red Fox which was spontaneously attracted by all the vocalizations. Finally, a fourth series was conducted in the presence of a sleeping dog and, separately, a Tayra engaged in play. The results of these experiments confirmed the validity of the initial hypothesis. When comparing the alarm call to the fledgling call, the latter elicits little reaction from the mammalian predator. In the choice experiment where the bird vocalizations were simultaneously emitted at the same volume, the dog almost invariably selected the adult calls. Even when the dog was asleep and the Tayra playing, both reacted to adult but not to fledgling calls. As surmised, the different bird calls had a widely different response elliciting power. The Lark’s whole repertoire had a low level of response for the predator tested, while the Blackbird and the Corn Bunting had a high one. Some experiments tend to demonstrate that the fledgling call may be perceived by the Carnivore, but does nos trigger a predatory response. This is obviously due to some cortical «filtering mechanism» rather than to an inability to perceive certain sound patterns. Further experiments will have to establish the parameters of the fledgling calls which are responsible for the low predator response level.