EXPERIMENTAL ANALYSIS OF THE AUDITORY DETECTION PROCESS ON AVIAN POINT COUNTS

ABSTRACT. -

We have developed a system for simulating the conditions of avian surveys in which birds are identified by sound. The system uses a laptop computer to control a set of amplified MP3 players placed at known locations around a survey point. The system can realistically simulate a known population of songbirds under a range of factors that affect detection probabilities. The goals of our research are to describe the sources and range of variability affecting point-count estimates and to find applications of sampling theory and methodologies that produce practical improvements in the quality of bird-census data. Initial experiments in an open field showed that, on average, observers tend to undercount birds on unlimited-radius counts, though the proportion of birds counted by individual observers ranged from 81% to 132% of the actual total. In contrast to the unlimited-radius counts, when data were truncated at a 50-m radius around the point, observers overestimated the total population by 17% to 122%. Results also illustrate how detection distances decline and identification errors increase with increasing levels of ambient noise. Overall, the proportion of birds heard by observers decreased by 28 ± 4.7% under breezy conditions, 41 ± 5.2% with the presence of additional background birds, and 42 ± 3.4% with the addition of 10 dB of white noise. These findings illustrate some of the inherent difficulties in interpreting avian abundance estimates based on auditory detections, and why estimates that do not account for variations in detection probability will not withstand critical scrutiny. Received 8 February 2006, accepted 30 August 2006.

Key words: ambient noise, detection probability, measurement error, point counts.

Análisis Experimentales del Proceso de Detección Auditiva en Puntos de Conteo de Aves

RESUMEN. - Desarrollamos un sistema para simular las condiciones de los muéstreos de aves en donde los individuos son identificados por sonido. El sistema emplea una computadora portátil para controlar un juego de reproductores MP3 amplificados ubicados en sitios conocidos alrededor de un punto de conteo. El sistema puede simular de modo realista una población conocida de aves canoras dentro de un rango de factores que afectan las probabilidades de detección. Los objetivos de nuestra investigación son describir las fuentes y el rango de variabilidad que afectan las estimaciones en los puntos de conteo, y encontrar aplicaciones de la teoría y las metodologías de muestreo que produzcan mejoras prácticas en la calidad de los datos de censos de aves. Los experimentos iniciales en un campo abierto mostraron que, en promedio, los observadores tienden a subestimar el número de aves en conteos de radio ilimitado, aunque la proporción de aves contadas por los observadores individuales varió entre 81% y 132% del total real. A diferencia de los conteos con radio ilimitado, cuando los datos fueron truncados a un radio de 50 m alrededor del punto, los observadores sobreestimaron la población total entre 17% y 122%. Los resultados también enseñan cómo las distancias de detección disminuyen y cómo los errores de identificación aumentan con mayores niveles de ruido ambiental. En general, la proporción de aves escuchadas por los observadores disminuyó un 28 ± 4.7% bajo condiciones de brisa, 41 + 5.2% con la presencia adicional de aves de fondo y 42 ± 3.4% con la adición de 10 dB de ruido blanco. Estos hallazgos ilustran algunas de las dificultades inherentes para interpretar las estimaciones de abundancia de aves basadas en detecciones auditivas, y por qué las estimaciones que no tienen en cuenta las variaciones en las probabilidades de detección no soportarán un escrutinio crítico.

POINT COUNTS ARE used extensively to moni tor spatial and temporal patterns of bird abundance, to assess species-habitat relationships, to evaluate the response of populations to environmental change or management, and to estimate species diversity. Surveys of breeding birds rely heavily on auditory detections, which can make up 70% of observations in suburban landscapes (Sauer et al. 1994), 81% in tropical forests (Scott et al. 1981), and 94% in closed-canopy deciduous forests (Dejong and Emlen 1985). Hundreds of thousands of point counts are conducted annually in North America across a spectrum of scales, from short-term site-specific studies (Simons et al. 2006) to long-term continental-scale surveys such as the Breeding Bird Survey (BBS; Sauer et al. 2005). Although the BBS, which comprises -150,000 unlimited-radius counts conducted along some 3,700 roadside routes surveyed annually in the United States and Canada, is the largest and best-known survey, many additional point-count surveys are conducted each year by state, federal, and private land-management agencies. Bart (2005) estimated that between 1,000 and 2,000 independent programs currently gather long-term data on bird abundance in the United States and Canada. Many individual state natural-resource programs now conduct hundreds to thousands of point counts annually through their participation in the Partners in Flight program (Rich et al. 2004, Bart 2005). On federal lands, point-count surveys are important components of long-term natural-resource monitoringprograms conducted by the following departments of the U.S. government: Department of Interior, National Park Service (Briggs et al. 1996); Department of Agriculture, Forest Service (Young and Hutto 2002); Department of Interior, Fish and Wildlife Service (Somershoe and Chandler 2004); and Department of Defense (Althoff et al. 2004).