Eastern black rail (Laterallus jamaicensis jamaicensis) occupancy and abundance estimates along the Texas coast with implications for survey protocols
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Eastern black rails (Laterallus jamaicensis jamaicensis) are a subspecies of conservation concern. These birds vocalize infrequently and inhabit dense vegetation making them difficult to detect. I conducted the first large scale study of black rail occupancy and abundance in Texas. I repeated point count surveys at 308 points spread across 6 study sites from mid-March to late-May in 2015 and 2016. Each survey at a point was a 6- minute call-playback broadcast where birds were detected acoustically. My study sites were located at Anahuac, Brazoria, and San Bernard National Wildlife Refuges, Mad Island Wildlife Management Area, Clive Runnel’s Mad Island Marsh Preserve, and Powderhorn Ranch Preserve. I estimated the fit of 19 occupancy and 19 abundance models that also accounted for imperfect detection. Black rail detection increased with moon phase and temperature but decreased with wind speed and ambient noise. Occupancy and abundance increased with woody, Spartina, non-Spartina herbaceous, and intermediate marsh cover. Black rail occupancy and abundance estimates were similar between years. From the estimated detection probabilities I determined that ~ 16 repeated surveys could establish black rail presence at survey points. I reached two main conclusions. One, black rail management during the breeding season, in Texas, should focus on Spartina cover as occupancy and abundance estimates were highest when Spartina cover was high. Two, effort to establish black rail presence from naïve occupancy estimates is impractical. Monitoring efforts of black rails should design studies that estimate distribution and abundance while accounting for imperfect detection.
CitationTolliver, J. D. M. (2017). Eastern black rail (Laterallus jamaicensis jamaicensis) occupancy and abundance estimates along the Texas coast with implications for survey protocols (Unpublished thesis). Texas State University, San Marcos, Texas.