Overview:Across the world’s intertidal ecosystems, daily tide cycles place severe constraints on where and when organisms can feed. This is particularly true for shorter-legged wading birds like the Little Blue Heron (Egretta caerulea) which is restricted to foraging in shallow water during the day (Figure 1). The expected acceleration of sea level rise threatens to drastically alter the timing and areal extent of foraging habitat for wading birds and other species whose reproductive cycle is inextricably linked to natural hydrologic fluctuations. This project investigates the resource selection of foraging habitat and prey by Little Blue Herons in the Florida Bay and Florida Keys to develop a predictive habitat suitability model for wading birds in intertidal systems.
Our first goal under this project is to improve and calibrate a habitat selection model for Little Blue Herons in the Florida Bay ecosystem, which was previously developed for the Great White Heron National Wildlife Refuge (GWH) in the Florida Keys (Calle et al. 2018). We are evaluating the performance of the hydrologic component of the model, developed for the Florida Keys (TiMSA; Calle et al. 2016), for application in Florida Bay. The calibrated TiMSA will be used to simulate tidal cycles and estimate shallow-water availability in Florida Bay (Figure 2). Our second goal is to refine existing resource selection functions for Little Blue Heron foraging habitats in GWH and calibrate model parameters for Florida Bay. A predictive model and maps of foraging habitat availability and selection will serve as tools to monitor high-use wading bird foraging areas, assess inter- and intra-annual fluctuations in resources based on real-time conditions, evaluate outcomes of hydrologic restoration regimes, and model sea-level rise scenarios.
Our goal under this project is to quantitatively link wading birds to the food resources at lower trophic levels, which are managed and affected by the Comprehensive Everglades Restoration. A critical component of this project is identifying Little Blue Heron colonies whose individuals feed within the GWH and Florida Bay study areas. Little Blue Herons have historically nested in Florida Bay, but their current nesting status in coastal South Florida largely unknown. We identified two nesting colonies within foraging distances to Florida Bay and GWH study sites. We sampled diets of Little Blue Heron nestlings in the two colonies in Florida Bay and GWH to identify key prey species (Figure 3). Despite the array of Little Blue Heron diet studies, ours is the first to quantify the available prey community and diet concurrently to evaluate prey selection in their coastal range. Quantitatively evaluating available prey gives an ecosystem context to wading bird diet, which can be used to assess causes of change in wading bird productivity and direct management decisions.
In our study, gulf toadfish and prawns were the most important prey items of nesting Little Blue Herons, though selectivity differed between colonies and years. Prawns were consistently the most important prey item among colonies and years and occurred frequently in bolus samples. On 10 September 2017, Hurricane Irma made landfall over GWH as a Category 4 storm. Heron diets at both study sites were more variable and more dispersed post-Hurricane (Figure 4). Pre-Hurricane diets at GWH were dominated by marine prey while Post-Hurricane diets were comprised more of terrestrial prey. At Florida Bay Pre-Hurricane, most herons selected for gulf toadfish, suggestive of population level specialization, and post-Hurricane, the population shifted to a generalized feeding strategy. These findings may support the expanding specialist diet strategy where foragers select their preferred food until it is depleted to a critical level in a foraging patch and then expand their diet to opportunistically include less desirable food (Brown and Mitchell 1989).
Brown, J. S. and W. A. Mitchell. 1989. Diet selection on depletable resources. Oikos, 33-43.
Calle, L., D. E. Gawlik, Z. Xie, L. Green, B. Lapointe, and A. Strong. 2016. Effects of tidal periodicities and diurnal foraging constraints on the density of foraging wading birds. The Auk 133:378-396.
Calle, L., L. Green, A. Strong, and D. E. Gawlik. 2018. Time-integrated habitat availability is a resource attribute that informs patterns of use in intertidal areas. Ecological Monographs doi.org/10.1002/ecm.1305.
