Predicting effects of water management on breeding abundance of three wading bird species

Wetland conservation often involves creating hydrological regimes that maximize habitat and resources for wildlife. In the greater Everglades ecosystem in Florida, USA, where wading birds are food-limited in some years, models predicting the influence of hydropatterns on foraging habitat availability are used to guide the management of water levels for wading bird nesting populations. These models are useful but do not consider that nesting wading birds are central place foragers, and thus resource availability is a colony-level measure. We examined long-term nest abundance patterns of the great egret (Ardea alba), snowy egret (Egretta thula), and white ibis (Eudocimus albus) to determine effects of hydropatterns on wading bird nest abundance in a 400-km(2) littoral marsh in Lake Okeechobee, Florida. We developed 2 sets of statistical models for each species: 1 examining variation in nest abundance (1977-1992, 2006-2019) and 1 predicting colony-level nest abundance (2006-2019). Models of nest abundance predicted that great egret nesting will peak when March-April lake stage is 4.3-4.5 m, coinciding with the peak in area of available foraging habitat. Neither recession rate nor stage explained total snowy egret or white ibis nest abundance, though snowy egret nest abundance has increased since the 1990s, when the water management schedule favored higher lake stages for longer duration. For all species, colony-level models predicted that nesting increased with increased habitat availability, faster water-level recession rates, and greater number of days dry in the previous 2 years at nest sites. When applied to simulated hydrological data representing changes to water-level regulations in Lake Okeechobee, our models predicted that management regimes allowing extreme flooding (stage > 5.18 m) in <220 days per 5.0 years and Carolina willow (Salix caroliniana) recruitment (stage < 3.9 m) >3.0 years per 5.0 years resulted in the highest nest abundance for snowy egret and white ibis. Snowy egret and white ibis nesting decreased by 575 nests/year and 465 nests/year, respectively, when regulation schedules increased the management envelope by 0.46 m, whereas great egret nest abundance increased modestly (149 nests/year). Operational rules that allow intermediate drought disturbance in dry years and prioritize increased habitat availability at short-hydroperiod colonies in wet years should result in overall benefits to the wading bird community at Lake Okeechobee.