Understanding the use of barrier islands as nesting habitat for Louisiana birds of concern

Abstract

Colonial nesting seabirds are threatened by habitat loss and degradation, human disturbance, predation, and climate change. Several species of conservation concern concentrate high percentages of their total U.S. populations in Louisiana breeding colonies. We studied seabirds, including Royal Terns, Sandwich Terns, and Black Skimmers, nesting on Isles Dernieres barrier islands along the Gulf coast of Louisiana. Two of the four islands in this chain host extensive seabird colonies and two do not. We used an experimental approach to test the hypothesis that large terns and skimmers are prevented from nesting on Trinity Island, the largest of the Isles Dernieres, by lack of social stimuli. Decoys and call broadcast attracted Royal Terns to visit experimental sites, but they did not nest. Sandwich Tern and Black Skimmer visits to the sites were not significantly affected by the social stimuli; however, isolated nesting attempts imply interest. Lack of colony establishment in response to the experiment indicates that social factors alone are not responsible for the lack of nesting by these species on Trinity Island. Scent station transects revealed the presence of raccoons, rats, and coyotes on two non-colony islands, and no mammalian predators on two colony islands, suggesting that seabirds avoid predator-infested areas. Least Terns were an exception, nesting on islands with mammalian predators. In 2008 and 2009, we monitored 53 and 80 Least Tern nests on Trinity Island and modeled nest success using logistic exposure. A subset of nests was protected by fences in each year (n= 3 in 2008, n= 19 in 2009). For unprotected nests, model-estimated nest success was 20% in 2008 and 53% in 2009. Fenced nest success was 83% and 49% in 2008 and 2009, respectively. We believe the increase in nest success between years reflects effects of Hurricanes Gustav and Ike on predator populations on Trinity Island. Rats and raccoons declined in surveys and anecdotal field observations. The impact of mammalian predators on this Least Tern population supports the hypothesis that predation limits seabird colonization of Trinity Island.