Pacific Southwest

Title: Detectability of Forest Birds from Stationary Points in Northern Wisconsin

Author: Wolf, Amy T.; Howe, Robert W.; Davis, Gregory J.

Date: 1995

Source: In: Ralph, C. John; Sauer, John R.; Droege, Sam, technical editors. 1995. Monitoring bird populations by point counts. Gen. Tech. Rep. PSW-GTR-149. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: p. 19-24

Station ID: GTR-PSW-149

Description: Estimation of avian densities from point counts requires information about the distance at which birds can be detected by the observer. Detection distances also are important for designing the spacing of point counts in a regional sampling scheme. We examined the relationship between distance and detectability for forest songbirds in northern Wisconsin. Like previous investigators, we found that some birds can be heard from much greater distances than others. Within the same species, some individuals (or the same individual under different circumstances) can be heard from greater distances than others. In general, this within-site variation in detectability is similar to variation in detectability among individuals in different forest types. Knowledge about the relationship between distance and detectability can be used to approximate the area sampled from a stationary point. This information can then be used to estimate the density of vocalizing birds. Even under ideal field conditions with accurately measured distances, detectability does not follow a simple threshold relationship. On the basis of our empirical data, we use a statistical probit analysis to describe the attenuation of detection with distance; the resulting sigmoidal function can be used to approximate the effective sampling area. Complications arise because individual birds become increasingly difficult to distinguish from conspecifics at greater distances from the observer. Coupled with variation caused by habitat structure, wind conditions, observer bias, and other factors, we conclude that data from point counts can give only a crude picture of avian density. Nevertheless, such estimates might be the best available, and the costs or ambiguities associated with alternative procedures might out- weigh the disadvantages of the point count method.

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Citation

Wolf, Amy T.; Howe, Robert W.; Davis, Gregory J.   1995.  Detectability of Forest Birds from Stationary Points in Northern Wisconsin.   In: Ralph, C. John; Sauer, John R.; Droege, Sam, technical editors. 1995. Monitoring bird populations by point counts. Gen. Tech. Rep. PSW-GTR-149. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: p. 19-24.