Factors Affecting Nest Success of Colonial Nesting Waterbirds in Southwest Louisiana

Subsidence and accelerated sea level rise impact nesting area availability and flood probabilities of breeding islands for colonial nesting waterbirds. In 2017 and 2018, we monitored 855 nests of four species of colonial nesting waterbirds on Rabbit Island, LA, to determine factors affecting nest and chick success. Based on logistic exposure models of nests, tricolored herons had the greatest likelihood of survival to hatch (mean (95% confidence interval)) (77% (65.9-83.1%)), followed by brown pelicans (70% (59.9-98.5%)), roseate spoonbills (70% (38.9-83.8%)), and Forster's terns (12% (10.7-12.2%)). Likelihood of survival to fledge was highest for tricolored herons (32% (12.8-40.7%)), followed by brown pelicans (28% (19.5-28.6%)), roseate spoonbills (47% (43.7-53.3%)), and Forster's terns (0% (0.005-0.01%)). Nesting strategy and nest timing impacted survival rate; however, the effect depended on timing of inundation events as the timing of inundation events varied across years. Flooding was the primary cause of nest failure for most species. In 2003-2012, rapid expansion in brown pelican colony numbers and significant chick production occurred at Rabbit Island, but hydrologic records indicate no island inundation occurred during the breeding season from the beginning of the hydrologic record (2006) through 2011. Thus, our results contrast with those of previous studies conducted under different hydrologic conditions and demonstrate the challenges of short-term studies informing coastal restoration in a system that is influenced by multi-year to multi-decadal climatic cycles.