Estimating population size of red-footed boobies using distance sampling and drone photography

The red-footed booby (Sula sula) is one of the most common pantropical seabird species, but its populations have been declining in the Caribbean region and elsewhere. We used distance sampling from point-counts to estimate population size of red-footed boobies in Mona Island, Puerto Rico, USA, before and during the breeding season in 2019. We compared results from early morning and early night surveys to determine the best survey time given that many individuals are foraging at sea during the daylight hours but not at night. We also examined the suitability of drone photography to survey active nests (a measure of breeding pairs) and compared it to point count and transect survey data. Point count surveys show that an estimated 6,130 birds occupied the colony, which is more than double previous estimates for Mona Island. Our results also showed that to avoid underestimates, red-footed booby colonies are best surveyed at night as they yield higher bird densities than daytime surveys. For the same reason, daytime photographic surveys with a drone underestimated population size compared to nighttime point-count surveys. However, drones were more effective than ground surveys in detecting active nests, and thus breeding pairs, which provides a better estimate of the resident population of a colony. We recommend that nighttime surveys are tailored to site-specific conditions to improve estimates of red-footed boobies, while drones can save significant effort and time in monitoring numbers of active nests in the remote and rugged islands where red-footed boobies typically nest.