SHOREBIRD COMMUNITY PATTERNS IN A SEASONALLY DYNAMIC ESTUARY

I examined the influence of environmental factors (exposed intertidal habitat, river volume and invertebrate abundance) on nonbreeding shorebird communities in the Mad River, California, an estuary in which intertidal habitats are affected by seasonal variation in river discharge. The shorebird community exhibited the following temporal (July-April) variation: (1) species composition exhibited a 50% turnover between October and November; (2) species richness peaked in summer, declined in winter and increased in spring; (3) shorebird abundance peaked in summer, when most birds foraged in cobble habitats; and (4) during winter a large percentage of birds roosted, and a small proportion fed in low numbers in aquatic habitats. Exposed intertidal habitat explained significant variation in both community patterns and the abundance of five species; these species were characterized by use of exposed intertidal habitats. Compared with other studies, the relatively weak associations between environmental variables and variation in the shorebird assemblage suggests that shorebirds occupying seasonally dynamic estuaries may exhibit space use and activity patterns different from those reported for birds occupying coastal estuaries where habitat availability and food resources are more predictable.