Conservation and foraging dynamics of migratory shorebirds

Red Knot (Calidris canutus), Sanderling (Calidris alba) and Ruddy Turnstone (Arenaria interpres) populations have suffered drastic declines in Delaware Bay (USA), a critical migratory staging area. The declines are thought to be related to a reduction in their main prey source, horseshoe crab (Limulus polyphemus) eggs. My objectives were to create foraging-behavior models for each shorebird species in order to (1) predict the effects of varying prey availability on foraging efficiency, (2) to predict the effects of inter- and intra-specific social behaviors on foraging efficiency, and (3) use the resultant models to recommend a new management framework to set specific management targets and identify factors important to increasing shorebird foraging efficiency. Using video, data were collected on various foraging metrics such as search velocity, foraging rate, foraging effort, multiple attacks and search interval for 412 Sanderlings, 264 Ruddy Turnstones and 176 Red Knots during 2003-2004 on the New Jersey beaches of Delaware Bay. Concurrent measurements of prey density and prey patchiness were obtained from the foraging area. Additionally, environmental characteristics of the foraging area such as wind speed, air temperature, tide stage and stopover day were collected. Candidate models were tested and the best explanatory models selected using Bayesian statistical methods. Results from prey availability models suggest that substantial increases in horseshoe crab egg density and reductions in patchiness, particularly at the 0-5 cm depth, are necessary for shorebirds to maximize foraging efficiency. Results from social interaction models suggest that the formation of large multi-species flocks and effective gull management may be necessary for shorebirds to maximize foraging efficiency. In order to maximize foraging efficiency of imperiled shorebirds, I recommend that (1) horseshoe crabs in Delaware Bay be managed such that 0-5 cm egg densities in the range of 105 to 106 eggs/m2 be sustainable on Delaware Bay beaches, (2) efforts must be made to ensure the diversity of all shorebirds on Delaware Bay because I found that increased species richness was associated with increased foraging efficiency, (3) management plans should be based on maintaining or restoring community-level interactions between shorebirds, horseshoe crabs and other non-target shorebirds.