Using Double Sampling to Estimate the Population of Least Terns (Sternula antillarum) Nesting on Florida's Rooftops

Gravel rooftops are important for nesting Least Terns (Sternula antillarum) in Florida. A network of partners from the Florida Shorebird Alliance (FSA partners) monitor roof-nesting Least Terns by conducting street surveys - a count of flying adults seen from the ground. We used double sampling to adjust street survey data and produce a population estimate for the 2018 nesting season (1 May–20 June). In 2018, FSA partners conducted street surveys at 96% (n = 138) of the active rooftop colonies in the state. In a random subsample (n = 62), we conducted direct counts of breeding adults and produced a detection ratio of 0.61, a ratio estimate of 59.28 ± 5.01 breeding adults/rooftop, and an adjusted population estimate of 8,180 ± 692 breeding adults (95% CI: 6,825–9,536). Cross-validation tests using data collected before and after the 2018 study produced accurate and unbiased population estimates derived from the detection ratio and street surveys. We used available street survey data from 2015 to 2020 and found a 7% population increase. We suggest validating the detection ratio every three years to develop accurate abundance and trend estimates of Florida's roof-nesting Least Terns.