Sources of Variation in the Abundance and Detection of the Endangered Florida Grasshopper Sparrow

Information on factors affecting the abundance and detection of the endangered Ammodramus savannarum floridanus (Florida Grasshopper Sparrow) was required to determine appropriate management strategies and evaluate monitoring efforts. We examined annual point-count data, records of prescribed fire (2003-2008), observer variability, and landscape features for Three Lakes Wildlife Management Area to identify sources of variation in abundance and detection. The population of male Florida Grasshopper Sparrows was estimated to be 498 (95% CL = 354-641), which corresponded to 23.9 (95% CL = 17.0-30.7) males per km(2). Over most or all observed covariate ranges, abundance estimates increased with mean elevation above sea level, distance from outer edge of optimal habitat, and with growing-season burns (P <= 0.025). Abundance declined with time since last burn (P <= 0.006). Estimates of probability of detection ranged from 0.041 to 0.101. depending on observer and prior detections. Probability of detection declined with time of day, day of year, and days since last burn (P <= 0.058). The current prairie burn regime of 2- to 3-year intervals should be maintained with preference for increased growing-season burns. Annual point-count surveys should be continued. Surveys should be conducted within 2 hours of sunrise between mid-April and mid-May. Future monitoring should incorporate covariates of abundance and detection during the collection of data and in their analysis for population estimates used for recovery criteria.