Photo by Jason Benfield

LILA

Summary and Objectives

Successful foraging by avian predators is influenced largely by prey availability, a composite variable of both prey density and prey vulnerability that can be affected by a number of different variables (Fig. 1).  In a large-scale experiment at the Loxahatchee Impoundment Landscape Assessment project within the Arthur R. Marshall Loxahatchee National Wildlife Refuge, I manipulated two components of prey availability, water depth and vegetation density (submerged aquatic vegetation and emergent vegetation), and quantified the response by wading birds in terms of foraging habitat selection and foraging success. Experiments were conducted in 10 × 10 m enclosures situated in the marsh, three of which can be seen in Figure 2.  Fish density within the enclosures was kept constant through mark-recapture using visible implant Elastomer (Northwest Marine Technology) and daily restocking. Manly’s standardized selection index showed that in both years of the study birds preferred shallow water and intermediate vegetation densities.  However, the treatments had little effect on foraging success, measured by both individual capture rate or efficiency.  This was a consistent pattern seen across multiple experiments.  Birds selected for certain habitat features but accrued little benefit in terms of foraging success.  I hypothesize that birds selected habitat with shallow water and intermediate vegetation densities because they anticipated higher prey densities, but they did not experience it here because I controlled for prey density. 

The purpose of this study was to begin quantifying the linkages between hydrology, vegetation, and wading bird foraging.   Because both routine wetland management and large scale ecological restoration projects often concentrate on hydrologic manipulations with little understanding of the implications across the ecosystem, there is a need to understand the effects of hydrologic manipulation of wetlands systems on the response by upper trophic level animals such as wading birds (Gawlik 2006).  The chances of successfully restoring wetland ecosystems can be increased by incorporating wildlife species but only if there is a clear understanding of the linkage with the processes being restored.  The findings of this project suggest that changes in water and vegetation affect the attractiveness of foraging habitat to wading birds, but it is not yet clear how this may affect wading bird foraging and subsequent reproductive success. Declining habitat quality has been linked to declines in wading bird populations (Bancroft 1989, Frederick and Spalding 1994, Ogden 1994,  USFWS 1996) and habitat manipulations clearly produce strong patterns of habitat selection (Gawlik 2002, Master et al. 2005, Gawlik and Crozier 2007), which is thought to be adaptive (Smith and Dawkins 1971, Smith and Sweatman 1974).  However, if these habitat manipulations produce no benefit in terms of foraging success then more work is needed to understand how birds benefit from improved foraging habitat.  This will be a key precursor to using wading birds as indicators of ecological restoration or routine wetland management.