Seasonal Variation in Territory Occupancy of Non-Breeding Long-Billed Curlews in Intertidal Habitats

Abstract

-During the non-breeding season, many shorebirds use coastal habitats where tides influence their dispersion patterns and social systems. We spot-mapped Long-billed Curlews (Numenius americanus) at the Elk River estuary, California to examine variation in territory occupancy from May 1999-April 2000. Curlew abundance was highest (a maximum of 16 territorial and up to 10 non-territorial birds) from July to September, after which time numbers declined progressively to 2-6 in winter. Five curlews were resident on >50% of observation days (N = 133) from June-April; these individuals returned to the estuary earlier and departed later than 11 other territorial curlews that were present 12-37% of occasions and mostly from July-November. Territory occupancy declined in association with precipitation and diurnal low tides ranging between -0.7 and 1.3 m. The absence of two territorial curlews coincided with rainy periods. The absence of curlews from five territories coincided with neap (low) tides; four of these curlews held territories at lower intertidal elevations with gently sloping topographies, such that they were inundated longer than other territories. At the Elk River estuary, territoriality may limit the number of curlews using intertidal habitats, but individual variation in territory residency suggests that numbers, and possibly carrying capacity, changed seasonally with availability of food in intertidal and pasture habitats. Habitat losses, such as those accompanying moderate (25-50 cm) rises in sea level associated with global warming, may reduce the carrying capacity of estuaries where existing levees preclude creation of new intertidal habitat. Received 31 October 2000, accepted 16January 2001.