Wading bird foraging trade-offs in response to the production and concentration of prey

Background/Question/Methods

Wading birds are important indicator species for wetland ecosystems, integrating productivity across trophic levels and over a large landscape scale. However, understanding and predicting wading bird responses to wetland restoration are hindered by their changing preference of hydrological processes depending on the production and concentration of prey.  In the Everglades, populations of species that are visual foragers and tolerate relatively deep water (i.e., Great Egret; exploiter) have disproportionally increased when compared with populations of tactile foragers that require a higher prey density (i.e., White Ibis & Wood Stork; searchers).  Identifying trade-offs among species between components of prey availability will help predict how communities respond to restoration.  Locations of foraging Great Egrets, White Ibises, and Wood Storks were obtained from Systematic Reconnaissance Flight daily distribution data from 2000-2009.  Temporal models grouped foraging observations over the Greater Everglades landscape to predict daily flock abundance. Spatial models grouped observations over time to determine factors contributing to how often a 400-m cell was used.  Hydrological variables over a range of temporal scales were used as a proxy for prey dynamics.  Days since drydown (DSD) quantified long-term prey production dynamics, recession rate quantified 2-week prey concentration dynamics, and daily water depth quantified short-term prey availability. 

Results/Conclusions

For all species, daily flock abundance increases as portions of the landscape with higher DSD become available and are used; an important link to a previous study relating increased small fish density to increasing DSD.  This effect is more prominent when Great Egrets and especially White Ibises are using shallower depths.  Rapid recession rates (i.e., concentration) play a particularly important role for Great Egrets and to a lesser extent White Ibises by maintaining a foraging response when prey production is limiting.  In contrast, concentration promotes Wood Stork foraging under high prey production, but not when prey production is low.  Concentration is more important for Great Egrets feeding in shallower depths and White Ibises feeding in deeper depths, likely better accommodating their opposing foraging strategies.  When Wood Storks are using deeper depths, prey production has no impact on the foraging response; however, storks forage more frequently in cells where high prey production is available in shallow depths.  These patterns indicate important differences among species in their response to the trade-offs of prey production, concentration, and availability.  If the Everglades ecosystem is limiting wading bird populations through the mechanism of prey production and/or concentration it will likely be reflected in these species.