THE EFFECTS OF WATER DEPTH AND SUBMERGED AQUATIC VEGETATION ON THE SELECTION OF FORAGING HABITAT AND FORAGING SUCCESS OF WADING BIRDS

Successful foraging by avian predators is influenced largely by prey availability, which encompasses not only the density of prey but also its vulnerability to capture. For wading birds (Ciconiiformes), habitat features such as water depth and density of vegetation are thought to affect the vulnerability of their aquatic prey. In January and April 2007 we experimentally manipulated the depth of water and density of submerged aquatic vegetation (SAV) in enclosures (10 x 10 m) with equal densities of fish to determine their effects on wading birds' selection of foraging habitat and foraging success. Analysis of the results with Manly's selection index showed that wading birds preferred habitat with shallow water and SAV. The two habitat components had little effect on the birds' foraging success, however, as capture rate did not vary with water depth or SAV density. Capture efficiency did not vary by SAV density and was actually lower in shallow water, contrary to our expectations. Our results suggest that birds selected habitat on the basis of environmental cues such as water depth and SAV but that these factors did not affect foraging success strongly. We hypothesize that wading birds were selecting habitat with shallow water and SAV because of an anticipated benefit to foraging through elevated density and vulnerability of prey, but the relatively high and uniform density of prey stocked in the enclosures, as well as the scale of the enclosures, effectively equalized the vulnerability of prey across treatments.