The availability of resources varies over time in fluctuating wetlands, and is closely linked to hydrological variation. In the subtropical freshwater wetlands of South Florida, the wading bird breeding season is restricted to the dry season, when water levels in wetlands recede and prey become concentrated. The hydrological variation of the region’s wetlands, particularly those of Lake Okeechobee, which is enclosed with levees, is strongly influenced by water management regimes. Because the density of wading bird prey is a function of controlled hydrologic fluctuations, we expected that water management regimes might also influence wading bird reproduction. We used model selection to investigate the influence of hydrological parameters on the daily nest survival rates (DSR) of wading birds on Lake Okeechobee. Because lake stage and water recession rates, in particular, can be adjusted by managers during a nesting season, we focused on the influence of those parameters on daily nest survival.
We monitored the nests of 218 Great Egrets (Ardea alba) and 1348 small herons during the 2011-2013 and in 2015 nesting seasons. Small herons included Tricolored Herons (Egretta tricolor), Snowy Egrets (Egretta thula), and small heron nests with unhatched eggs that could not be identified to species. Model selection identified the quadratic form of lake stage as the only important parameter that influenced DSR of Great Egrets, but this model was only marginally better than the null model. The quadratic forms of recession, lake stage, and an interaction between recession and lake stage were important parameters for predicting DSR of small herons, regardless of species or nesting stage. The relationship between daily survival and lake stage was slightly convex for Great Egrets and strongly concave for small herons. Our results suggest that lake stage influences daily survival of Great Egret and small heron nests, albeit in opposite directions. Small herons were more sensitive to lake stage, and they were sensitive to another hydrological parameter, water level recession rate. Future analyses will more directly measure the influence of water management on nest success by including separate parameters for inflow/outflow (management driven) and evapotranspiration/rainfall (climate driven).