Objective

Representations of automatic detection in ISHMAEL (left) and Praat 4.5 (right). Note the detection window below the spectrogram on the left; it provides a visual representation of levels exceeding a threshold within a specified frequency range. The line in yellow above the blue threshold line is a detection event. It matches each hoot in the spectrogram. On the right, the blue line shows the Praat 4.5 time-frequency profile of the fundamental frequency of a hoot (one in a series of four).

Birds that live within visually obscured environments, such as aquatic, arboreal, and nocturnal species, are dependent upon acoustic communication to find mates, care for young, hold territories, and defend themselves from predators. Such species are inherently hard to observe visually and thus monitor, but they may readily be studied acoustically. Although acoustic surveys have been conducted for many years to obtain estimates of bird density, long-term population trends and life-history parameters could not be collected because individual identities of calling birds were not known. For some species, however, individual recognition by voice is feasible. In these cases, individual vocal recognition could be an efficient and noninvasive tool for tracking population trends, analogous to photographic identification of large mammals. Quantitative bioacoustic techniques can be used to determine whether individuals’ vocalizations can be used to obtain individual signature information. These ‘acoustic fingerprints’ already have been found in several avian and mammalian species, and evidence has been documented for an acoustic basis of kin discrimination in some species. Acoustic fingerprints have not been applied as a long-term censusing tool in birds as yet, but recent advances in computing hardware and signal processing have made the approach feasible. A species that is well suited for long-term acoustic monitoring is the Mexican spotted owl (Strix occidentalis lucida), one of the Department of Defense's priority species.

The objective of this project was to develop quantitative bioacoustic techniques for use in monitoring population densities and trends, as well as assessing the effects of military training and other anthropogenic activities on the population dynamics of threatened and endangered birds, specifically the Mexican spotted owl.