Misuse of data from mist-net captures to assess relative abundance in bird populations

Capture data from mist nets are used frequently to quantify the relative abundance of birds. In spite of obvious confounding variables, most of which have been mentioned previously in the literature, relative capture of birds typically is equated directly to relative abundance. Through modeling, we quantify the potential magnitude of the effect of those variables among species and between age/sex categories of the same species. We demonstrate that differences in proportional use of vertical-height categories, including differences below the resolving power of visual estimates, can produce substantial differences in the capture rates of birds with identical abundance. To simulate capture on the horizontal plane, we designed a computer program that models how frequently birds strike nets with respect to home-range size and overlap, number of flights, and mean flight distance. The quantitative results of these simulations show that differences in spacing system, flight distance, and Bight frequency have strong effects on capture rates. We also list additional problems with interpretation of differences in capture data. We think that these influences on capture data combine to preclude quantitative comparisons of relative abundance of birds, either among species or within species in different habitats, by use of mist-net capture data under most current research protocols. Although our analyses refer directly only to birds and mist nets, the outcomes of the analyses are relevant to any method that estimates relative abundance from captures of mobile organisms by stationary traps during brief sampling periods.