Abundance estimates of eastern black rails in southeastern Colorado

The eastern black rail (Laterallus jamaicensis jamaicensis), a federally threatened subspecies, is cryptic and difficult to study. Once found across the eastern United States, eastern black rails currently occupy only a few interior regions and are largely limited to coastal marshes. Black rails elusive nature and patchy distributions have made abundance estimation difficult and occupancy modeling has been the primary method for monitoring population trajectory. In southeastern Colorado, one of the most prominent interior regions still occupied by eastern black rails, they are relatively common. We used detections from surveys conducted in 2019, 2020, 2022, and 2023 to estimate eastern black rail abundance along the Arkansas River in Colorado. Estimates were produced using a zero-inflated N-mixture model in a Bayesian framework. Our study confirmed predictors from previous work on black rail occupancy and abundance also predict detection, occupancy, and abundance of black rails in southeastern Colorado, though the relationships differ for predictors relative to other parts of the range. We found detection probability increased later in the season and later at night but decreased with ambient noise. We found occupancy increased with residual vegetation height and detection of Virginia rails (Rallus limicola). We found abundance increased with the height of live emergent vegetation and surrounding wetland cover but decreased with distance to a large wetland. We estimated 129 to 180 ((Formula presented.) = 160) eastern black rails at our surveyed point locations along the Arkansas River. Our results indicated that southeastern Colorado is hosting high numbers of eastern black rails at survey locations. The results of our work will inform future monitoring efforts and will allow for an improved understanding of management priorities for eastern black rails in southeastern Colorado.