Variation in shorebird use of diurnal, high-tide roosts: How consistently are roosts used?

In coastal environs during the non-breeding season, many shorebirds (suborder Charadrii) congregate at roosts, long considered to be traditional sites where flocks of individuals coalesce when high tides inundate feeding areas. Humboldt Bay, California was surveyed (9.5 months at roughly 10-d intervals) to assess temporal variation in incidence (proportion of 28 surveys birds used a roost), proportional abundance, concentrations, and repeatability (of seasonal average proportional abundances) of shorebird use of diurnal, high-tide roosts. Two hundred and forty roosting locations were identified and observations were made of 30 species. Fourteen species accounted for over 99% of observations. Shorebirds occurred at most roosts infrequently (<20% of surveys) and only 4% of roosts had roosting birds present on more than 80% of occasions. Abundant species occurred at more roosts (20-141 roosts per species) compared with less common species. Even at the most-used roosts, abundances at the species level varied greatly. Repeatability of roost use among seasons was high. At Humboldt Bay, roost use formed a continuum from ephemeral locations used by a few birds to sites used consistently by large numbers of individuals.