Winter shorebird communities of Humboldt Bay : species diversity, distributions, and habitat characteristics

Patterns of species assemblages are often influenced by habitat variables. I used nested subset analysis to determine if wintering shorebird assemblages at 19 sites on Humboldt Bay have a significantly nested pattern. I then analyzed species diversity, individual species' densities, and species' incidences in relation to the following habitat characteristics: mudflat width, channelization, standing water, ebb height, and sediment particle size. An additional variable, length of tide-line (analogous to area surveyed) was analyzed for diversity and incidence, and variability of the sediment was added to the diversity analysis as a measure of habitat complexity. I found a significantly nested pattern that was not caused by random sampling. Killdeer (Charadrius vociferus), American Avocet (Recurvirostra americana), Spotted Sandpiper (Actitis macularia), and Black Turnstone (Arenaria melanocephala) were species that detracted disproportionately from the nested pattern. These species have specialized habitat requirements suggesting a link between habitat use and nestedness. Diversity was most strongly correlated with tide-line and substrate variability. Standing water, ebb height, sediment particle size, and tide-line were the most frequently occurring variables in the density and incidence models. All species analyzed had significant (P 0.1) incidence models. In the two most significant density models yellowlegs (Tringa melanoleuca and T. flavipes) correlated negatively with ebb height (P = 0.0026) and Long-billed Curlew (Numenius americanus) correlated positively with channelization and ebb height (P = 0.014). Two of the most abundant species, Marbled Godwit (Limosa fedoa) and Dunlin (Calidris alpina) had the poorest correlations with habitat variables. I found that wintering shorebird communities using Humboldt Bay are influenced by physical characteristics of the mudflats. I recommend additional study on the mechanisms that affect shorebird use of sites to aid in protecting Humboldt Bay as an important wintering and migratory stopover site for large numbers of shorebirds.