The Use of Mist-Net Capture Rates to Monitor Annual Variation in Abundance: A Validation Study

Abstract

Constant-effort mist-netting has become an increasingly popular tool in long-term monitoring of bird populations. Monitoring programs often assume that variation in the capture rates of adults reflects variation in breeding densities of the sampled population. We test this assumption by comparing annual variation in mist-net capture rates to known variation in breeding densities of four species breeding in coastal scrub habitat at Point Reyes National Seashore, California. For the period 1980 to 1992, breeding densities for each species were assessed using spot-mapping observations of color-banded individuals. These were compared to capture rates calculated from constant-effort mist-netting. Capture rates of adults correlated significantly with breeding density for Spotted Towhees (Pipilo maculatus), Song Sparrows (Melospiza melodia), and Wrentits (Chamaea fasciata), but not for White-crowned Sparrows (Zonotrichia leucophrys). Analysis of covariance confirmed that the relationship of mist-net capture rates to breeding density differed among species. Independent of the effects of breeding density on capture rates, adult capture rates of the four species were higher in years with high rainfall. Among Wrentits and Song Sparrows, approximately 50% or more of adults caught were not occupying territories at the study site; the same bias may exist among other species as well. We conclude that relative changes in breeding density over time can be inferred from variation in mist-net capture rates for some, but not all, species. Additional studies are needed to validate the use of mist nets to assess spatial differences in abundance and to monitor temporal changes in abundance in habitats other than coastal scrub.