Bird Vocalizations

There are 914 species of birds in North America, each with one or more distinct vocalizations. Many birds use different vocalizations at different times of year, depending on whether they are mating, migrating, brooding or just trying to survive another day. As a result, it takes years of study to learn to reliably identify even a regional subset of these species.

The case of the Empidonax genus of flycatchers is of particular interest to this project. These species are very closely related and physiologically similar enough that they can only be positively identified by their vocalizations. Due to the difficulty in learning to identify birds by song, the tool this project is attempting to develop would be invaluable for birders and researchers alike in identifying these particular birds.

Because most birds vocalizations are learned, regional dialects develop. This means that birds of identical species that were born in regions several kilometers apart can have slightly different songs. As the distance increases, so does the variation in vocalization. Experienced birders can readily distinguish between species from the east and west coasts by these variations. The following figure shows a the frequency spectra of vocalizations produced by four different House Finches. It is clear that the dominant frequencies of these sample vary by more that a thousand Hertz. These recordings were obtained from a variety of sources and illustrate the range found within the mating call of this particular species.

To make matters more difficult, closely related birds often also have closely related songs. The figure below shows the frequency spectra of a House and Purple Finch. Both of these birds also exhibit a variety of dialects, as described above, and as such, significant overlap in the primary frequency bands of the vocalizations will occur.

Possibly the most difficult aspect of bird vocalization recognition is the presents of mimics. The same learning process that produces variation in dialects as mentioned above allows some birds to learn the songs of unrelated species. Starlings are known as fantastic mimics. One common group of calls for Starlings to learn are those of birds of prey. It is thought that they use these calls to frighten other small birds from their territory. The call of a Bald Eagle, and that of the European Starlings own call are shown in the figure below. Also shown is the frequency spectrum of a European Starling mimicking a Bald Eagle.