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.

Technical Approach

Vocal recordings collected as part of another ongoing study of the Mexican spotted owl were used to demonstrate the effectiveness of acoustic techniques as an alternative noninvasive, efficient method for tracking owl movements, measuring population densities, and conducting long-term monitoring of individuals to obtain population parameters. Recordings were digitized, and each vocalization type was analyzed quantitatively using sophisticated automated techniques for individual, sex, and age-related differences. Several statistical clustering techniques were tested for use in identifying callers based on the acoustic properties of calls, especially in association with assessing the age-sex demographics of the population. Results from these tests were compared to the original data set of known call membership to provide a validation of the bioacoustic assessment algorithms.


This project has led to the development of an innovative set of bioacoustic assessment tools as an alternative, cost-effective, and noninvasive method for monitoring population densities and trends in species for which vocalizations play a primary role in social communication and territory maintenance. This research also has broad implications for studying the effects of military-related activities on threatened and endangered species distribution and behavior. The tools developed from this project will enable future investigations to address important conservation questions pertaining to military training and testing activities more comprehensively and efficiently. (Project Completed - 2006)