Animal communication systems and methods

Animal Communication: The Science of Songbirds

Marler (1955) identified some important acoustic differences between alarm calls and mobbing calls in song birds. What were these differences, and how did Marler link these properties to the different functions that these calls serve?

Mechanism (How does it work?)

Scientists note that birds are using the same or similar sensory apparatus humans use to process and transmit sounds, and use different sounds for different functional reasons (Brody, 1991:1). Many bird species mob by giving loud calls combined with wing and tail flitting which appear to attract additional birds of many different species, unlike species specific alarm calls designed to minimize detection. At first, the benefits of mobbing behavior among small birds were not clear (Cully, 1986: 103). Now it is believed that by attracting more aggressive species to mob, small birds may be able to drive predators away from their territories and derive the same benefits as the more aggressive species use of mobbing (Cully, 1986: 203).

In contrast, Marler's sound spectrographic analyses of hawk's alarm calls revealed that the bird's calls minimized the cues available for localization, disseminating alarm while reducing the risk of attracting attention" (Marler, 1957).

Function (What is it do for?)

While the hawk's alarm call called for species-specific dispersion and minimization of detection, the mobbing calls of small birds identical signals recognizable to other specifies to encourage birds to mob together (Cully, 1986).

Ontogeny (How does it developed?)

Contrary to popular belief, bird song is not entirely instinctive, although most birds show an innate propensity to learn the song of their species. Song birds raised in isolation in the laboratory without having heard their species sing develop an incomplete and abnormal version of the song...they produce a song that is nearly correct if allowed to hear a tape of it during the so-called sensitive period for song learning, which varies from species to species. Deaf birds, on the other hand, never come close to singing the right song. Even if they heard it before becoming deaf, they sing with serious distortions, apparently because they cannot hear and correct their own performances" (Brody, 1991:1). Evolution may select certain birds that are able to produce mimicking mobbing calls and allow birds able to produce more effective alarm calls to survive, but there is also a neurological learning component from other birds. Also, "unlike children, who can learn any language they are exposed to, the musical language of most birds is somewhat constrained by their genetic heritage. Given a choice of two songs -- their own and that of another, even a closely related species -- they will learn their own. but, if exposed only to the song of another species, they will learn a version of it" (Brody, 1991:1). Thus, birds can develop certain species specific calls, like alarms, but also can within some constraint learn calls that sound the same and are recognizable to other species. Functionality is also an issue, as songs for mating, as discussed by Brody (1991) only have use in terms of reproduction, unlike mob calls.