Black Rail Occupancy and Detectability in the Texas Mid-Coast National Wildlife Refuge

Abstract

Abstract. The federally Threatened Eastern Black Rail (Laterallus jamaicensis jamaicensis) is difficult to detect and may not be well monitored by existing survey methods. The goals of this project were to explore occupancy and detectability of Black Rails along the Texas Gulf Coast and to determine the best months and time of day to conduct surveys. We conducted surveys at 90 points in San Bernard NWR and Brazoria NWR from 5 March through 31 May 2014. We visited each point six times—twice each at dawn, dusk, and night and used playback surveys to elicit Black Rail vocalizations. We measured habitat characteristics for each point, including the vegetation height, litter depth, water depth, stem density, canopy height, species composition, and time since last prescribed burn. Occupancy rates averaged 0.75 ± 0.25 and was greatest where the average number of stems between 0 and 10 cm in height was six or greater. The average canopy ceiling height (i.e., height of canopy ceiling above layer of dead vegetation), canopy floor height (i.e., height of canopy ceiling above soil), and number of stems between 20 and 30 cm also affected occupancy. Detection probability averaged 0.11 ± 0.03 and was highest at night, increasing over the course of the spring. Due to the low probability of detection, repeated call-broadcast surveys will be required to detect this species. The results of this study are important for determining the timing of surveys in Texas, and we suggest that future surveys for this rare species will have the greatest success if they search for birds approximately 2.5 hours after sunset.