Estimating western scrub-jay density in California by multiple-covariate distance sampling

Using multiple-covariate distance sampling with seasonal point transects, we surveyed for the Western Scrub-Jay (Aphelocoma californica californica group) over a substantial portion of its range in California . Our goals were to produce seasonal and habitat-specific estimates of the scrub-jay's regional density and abundance in 2008 and to demonstrate how the concurrent collection and analysis of covariate data may be useful in improving estimates of bird density. Density and abundance estimates implied a significant 38% increase over 2008, from a low of 24 jays km ^-2 (2.3 × 10 ⁶ jays) in February to a high of 78 jays km ^-2 (7.5 × 10 ⁶ jays) in November. Density was greatest in agricultural habitats (98 jays km ^-2) and least in rural habitats (38 jays km ^-2). Averaged over the study, abundance was greatest in agricultural habitats (2.6 × 10 ⁶ jays) and least in urban habitats (3.6 ×10 ⁵ jays). Inclusion of covariates such as habitat type, observer, weather, and time of day often increased the precision of density estimates, and it significantly improved model results in one case. As detailed in this study, the techniques of multiple-covariate distance sampling may have application as effective and noninvasive methods for obtaining more precise estimates of bird density and monitoring the density and abundance of species across broad habitats.