Habitat Choice and Niche Characteristics Under Poor Food Conditions. A Study on Migratory Nearctic Shorebirds in the Intertidal Flats of Brazil

Abstract

The tidal flats of the Bragantinian Peninsula in the tropical part of Brazil harbour wintering and migratory nearctic shorebirds in very high densities, whereas the benthic food supply is very low. In 2001 and 2002 a study was conducted on the habitat choice and distribution of shorebirds to investigate the formation of niches and the spatial distribution under the constraint of a limited food supply. We showed that shorebird densities were not correlated with a range of abiotic variables or with prey density within sites, but that between sites birds tended to select sites with peak prey densities. Two foraging guilds were differentiated: Whimbrel Numenius phaeopus, Willet Tringa semipalmata and Short-billed Dowitcher Limnodromus griseus preferred to forage in the water or on wet sand at the water's edge; while Red Knot Calidris canutus, Grey Plover Pluvialis squatarola, Ruddy Turnstone Arenaria interpres, Semipalmated Sandpiper Calidris pusilla and Semipalmated Plover Charadrius semipalmatus, were found predominantly on dry substrate. Although the birds showed species-specific preferences for characteristics defining microhabitats, niches were broadly overlapping. The results of our study suggest that, in case of a very low food supply, individuals might not be able to react to competition by niche segregation but might be forced to feed in a broad range of available niches.