NONRANDOM SHOREBIRD DISTRIBUTION AND FINE-SCALE VARIATION IN PREY ABUNDANCE

Spatial variation in the abundance of nonbreeding shorebirds can be predicted to varying degrees by the density of their prey species; strongest relationships obtain from studies encompassing large spatial scales (e.g., entire estuaries). We examined variation in shorebird distribution and abundance within microhabitats of the Mad River estuary, California, with the following null hypotheses: (1) shorebird spatial distribution was random, and (2) no relationship existed between bird abundance and invertebrate densities. Shorebirds exhibited nonrandom spatial distributions; species were highly clumped within the study area. Most foraging calidridine sandpipers (Calidris minutilla, C mauri, and C bairdii) aggregated in sandy areas within 1 m of the tide edge, where they foraged by probing for a burrow-dwelling amphipod, Corophium spp. By contrast, Semipalmated Plovers (Charadrius semipalmatus) and especially Ruddy Turnstones (Arenaria interpres) foraged by pecking in drier, coarse-grained substrates greater than 1 m from the tide edge. Corophium densities in sand exceeded those in cobble; Corophium densities explained significant variation (r2 = 0.36, 0.31 and 0.22) in the abundance of Least Sandpipers, Western Sandpipers and all shorebirds, respectively. These findings emphasize the importance of understanding variation in processes across spatial scales.