MISUSE OF DATA FROM MIST-NET CAPTURES TO ASSESS RELATIVE ABUNDANCE INBIRD POPULATIONS

Capture data from mist nets are used frequently to quantify the relati ve abundance of birds. In spite of obvious confounding variables, most of which have been mentioned previously in the literature, relative c apture of birds typically is equated directly to relative abundance. T hrough modeling, we quantify the potential magnitude of the effect of those variables among species and between age/sex categories of the sa me species. We demonstrate that differences in proportional use of ver tical-height categories, including differences below the resolving pow er of visual estimates, can produce substantial differences in the cap ture rates of birds with identical abundance. To simulate capture on t he horizontal plane, we designed a computer program that models how fr equently birds strike nets with respect to home-range size and overlap , number of flights, and mean flight distance. The quantitative result s of these simulations show that differences in spacing system, flight distance, and Bight frequency have strong effects on capture rates. W e also list additional problems with interpretation of differences in capture data. We think that these influences on capture data combine t o preclude quantitative comparisons of relative abundance of birds, ei ther 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 abu ndance from captures of mobile organisms by stationary traps during br ief sampling periods.