Improved Estimation of Bird Abundance and Energy Consumption by Combining Point and Transect Observations

Point counts and transect surveys are both common methods for estimating bird abundance (by species, genus, or family) in a given environment. Sometimes both methods are used in the same study. Point counts reflect abundance directly, while transect observations of bird feeding instances reflect both the bird abundance and the mean energy consumption rate per bird. In this paper we exploit the relationship between these two types of observations to draw inferences about bird energy consumption; to develop an improved estimator of bird abundance; and to derive error bars for abundance estimates. The mathematical methods developed are applied to a dataset consisting of point counts and transect surveys of Ecuadorian frugivore bird species in six different types of environment (three elevations and two degrees of forestation). By comparing transect-based and point count-based estimates of energy consumption across species, we show that mean fruit weight consumed does not have a significant effect on number of feeding instances. We also derive a procedure for forming an optimal linear combination of two relative abundance estimators based on point counts and transect observations respectively. The improvement in accuracy achieved by this combined estimator is verified by simulations applied to bird family data from the above-mentioned survey. Finally, we use simulations to obtain 90% confidence intervals for the bird family relative abundance estimates. These confidence intervals were also verified using simulations.