Productivity and Nestling Mortality for Three Species of Herons at Natural and Anthropogenic Islands

Anthropogenic islands have become important alternative nesting sites for wading birds by providing habitat where natural sites are degraded or absent. However, anthropogenic islands have higher elevations and experience more disturbance than natural sites, which may alter nest predation risk. Yet, little is known about the degree to which wading bird productivity varies between colony types. We used a generalized linear model approach to determine the effect of colony type and resource availability on the species-specific productivity of wading birds at Lake Okeechobee from 2010 to 2019. Great Egret (Ardea alba) productivity did not vary by colony type, whereas Snowy Egret (Egretta thula) and Tricolored Heron (Egretta tricolor) on average produced 0.2 and 0.7 more chicks per hatched nest at natural marsh island colony-sites (n = 6) than at anthropogenic island colony-sites (n = 4), respectively. This suggests that anthropogenic island colony-sites provide lower quality nesting habitat for these species than do marsh colony-sites. However, anthropogenic islands provide nesting habitat in years when marsh colony-sites are limited, and thus increase long-term productivity. Constraints on the ability of anthropogenic islands to support wading bird nesting can be relaxed by optimizing island location and structure during their creation.