Distribution, colony characteristics, and population status of least terns breeding on the Texas coast

Abstract

Seasonal movements, colony site selection, and population status of the least tern (Sterna albifrons) were studied throughout the Texas coast during 1979-81. Existing procedures for monitoring population trend also were evaluated. Leg bands were applied to 1,044 terns of which 426 also received patagial tags. Physical features of 39 colonies were analyzed to discern important aspects of site selection. Reproductive parameters were estimated from data for 646 nests and 126 confined young in 13 colonies. Post-fledging survival was estimated from previous U.S. band recovery data. Demographic status was evaluated using Leslie matrix analyses. Total breeding population was estimated from coastwide, 1-day counts in early August. All age classes moved toward Gulf beaches by mid- to late summer, and concentration areas were found commonly near jetties or natural channels. Fall migration was southward along Gulf beaches. Patagial tags were ineffective for long-term resightings and may have caused mortality; further use on least terns is discouraged. Principal components analysis (73% of variance, PC I-III) revealed that elevational and spatial relationships were most important in colony site selection, while sand content and ground cover were secondarily important. This analysis verified previous subjective perceptions of colony selection. Clutch size, nest success, hatchability in "hatched" nests, and fledging rate averaged 1.92, 60.3%, 93.9%, and 76.8% respectively. Fecundity estimates (95% C.I.), accounting for renesting, were 0.501-0.649 fledglings/adult and estimated annual adult survival was ca. 85%. Leslie matrices indicated near stability for the average case and 6% annual decline for the worst case. Better estimates of survival are needed for clear resolution of population dynamics. The breeding population was at least 3,000 pairs and was estimated between 5,500 and 8,300 pairs. Previous surveys were adequately timed and counts correlated well with actual presence; however, spatial adequacy was poor in that surveys overrepresented island sites, especially those with numerous nesting species. Procedures were not sensitive to disassociative nesting and rapid exploitation of new sites displayed by least terns. Previous data for Texas are not indicative of least tern population trend. Reevaluation of least tern status is recommended for other areas using routine surveys of known nesting areas, especially those in conjunction with counts of other colonial species.