Influence of Survey Length and Radius Size on Grassland Bird Surveys by Point Counts at Williams Lake, British Columbia

Abstract

We examine the effect of survey length and radius on the results of point count surveys for grassland birds at Williams Lake, British Columbia. Four- and 8-minute counts detected on average 68 percent and 85 percent of the number of birds detected during 12-minute counts. The most efficient sampling duration was 4 minutes, as long as travel time between points was under 15 minutes. Density estimates derived from 4-minute counts were significantly lower than 12-minute counts for most radius sizes. A larger radius yielded a larger number of detections but not always proportionally with the increase in area. This resulted in lower estimated density with an increase in radius size, especially when using maximum values at a given point. However for the Horned Lark (Emeophilia alpestris), the most abundant species, estimates of densities derived from individual counts did not differ significantly with radius size. A 100-m radius yielded nearly as many detections as an unlimited radius for most species, suggesting that it may be the most efficient radius to use in open habitats.

Parent Publication

• Monitoring bird populations by point counts

Citation

Savard, Jean-Pierre L.; Hooper, Tracey D. 1995. Influence of Survey Length and Radius Size on Grassland Bird Surveys by Point Counts at Williams Lake, British Columbia. 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. 57-62