LAUSR

Abstract. Gull-billed Terns (Gelochelidon nilotica) have historically demonstrated variability in nest-site selection and reproductive success throughout their breeding range. However, few studies have investigated the breeding parameters of Gull-billed Tern populations on barrier islands in the northern Gulf of Mexico, USA. The main objective of this study was to investigate annual variability of breeding population size, hatching success, and causes of nest failure during eight breeding seasons (2009–2016). Annual variation was observed in colony sizes (Range = 1–68 nests), colony locations (among five islands), and hatching success (0–93%). Mean hatching success was 56% and flooding was the main cause of failure, accounting for 64% of all failed nests. Additionally, nest site habitat characteristics were compared between successful and unsuccessful Gull-billed Tern nests. Nest substrate composition differed between successful and failed nests, and successful nests were also associated with higher elevation, greater distance from the high tide line, and less vegetative cover. There was considerable variation in Gull-billed Tern breeding population size and hatching success, which may be attributable to the dynamic nature of barrier island habitats. Long-term monitoring is required to better understand the breeding dynamics of Gull-billed Terns, but conservation efforts should consider maintaining island habitats that are protected from flooding and have adequate nesting substrate, minimal vegetation, and limited disturbance from humans and predators.