Modeling Detection and Density Using Distance Sampling for Three Priority Grassland Bird Species in Texas - Northern Bobwhite (Colinus virginianus), Eastern Meadowlark (Sturnella magna), and Painted Bunting (Passerina ciris)
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In Texas, three priority grassland species have experienced declines due to the loss of native grasslands. These species are the Northern Bobwhite (Colinus virginianus), Eastern Meadowlark (Sturnella magna), and Painted Bunting (Passerina ciris). Due to this, accurate density and abundance estimates are critical for management of these species. This 5-year study identified covariates that influence detection, availability for detection, and density for these species using data collected from 986 points across 11 Texas counties. Points were visited once per year from 2013 to 2017 in May and June, and 5-minute point counts were performed using distance sampling protocols. Models were fit using gdistsamp from the package “unmarked” in R. Covariates that influenced detection for Northern Bobwhite were observer, ambient noise, time of day, and Julian date. Detection was higher later in the season, earlier in the morning, and at lower ambient noise levels. Julian date and observer influenced Eastern Meadowlark detection, with detection declining later in the season. Painted Bunting detection was influenced by ambient noise and observer. The proportion of individuals available for detection was influenced by year, showing trends that are likely influenced by precipitation from the preceding year. However, despite this, availability for all species was not influenced by the Palmer Drought Severity Index. Vegetative cover was selected in the top density model for all three species. Native grasslands positively influenced density of Northern Bobwhite and Eastern Meadowlarks, shrubland positively influenced Northern Bobwhite and Painted Bunting, and cropland positively influenced Northern Bobwhite. Overall, there was high variability across the three species in the effect of the type of vegetative cover on density, indicating that management for one species may ignore vegetative cover needs of other grassland species. Finally, for all species, monitoring should emphasize observer training and attempt to survey only under low ambient noise conditions.