WATERBIRD COMMUNITIES AND HABITAT RELATIONSHIPS IN COASTAL PASTURES OF NORTHERN CALIFORNIA

We examined waterbird assemblages (diversity, composition, and species' densities) in 20 pastures near Humboldt Bay, California, in relation to habitat characteristics (vegetation height, soil penetrability, water depth) abundance of invertebrates (worms and other invertebrates), and presence of livestock. From October 1991 to May 1992 we observed 29 species and 10,776 birds, most (78%) of which foraged. Nonrandom pasture use by birds resulted in a highly clumped spatial distribution (s(2-):($) over bar x ratio = 42.1). Habitat characteristics of pastures correlated with this nonrandom pattern: waterbird diversity and densities of three sandpiper species and one gull species correlated negatively (r = -0.61 and r = -0.44 to -0.67, respectively) with vegetation height; densities of Two plover species correlated negatively (r = -0.39 and -0.45) with soil penetrability; and waterfowl densities correlated positively with water depth (r = 0.97). Species composition varied among pastures Wading birds used pastures with tall vegetation, shorebirds and gulls frequented short-grass pastures, and waterfowl used flooded pastures. Both the presence of waterbirds and their densities increased in association with livestock. In coastal areas where much intertidal habitat has been reclaimed as pastureland, pastures offered valuable habitats to nonbreeding waterbirds. We suggest that grazing in coastal pastures can be used to provide a mosaic of vegetation heights, which would yield greater waterbird diversity as well as higher densities of some species.