Distribution, abundance, and habitat use of Long-billed Curlews in California’s Central Valley from broad-scale surveys in 2007 and 2008

Abstract

The Long-billed Curlew (Numenius americanus) is a migrant and winter resident in California’s Central Valley, where it concentrates primarily in agricultural lands. Despite recent estimates of the size of the North American breeding population of this at-risk species, little is known about its abundance, concentration sites, and habitat needs at migratory stopovers and wintering areas. To help fill this gap, we coordinated three broad-scale surveys of curlews in the central and southern portions of the Central Valley. Survey totals were 19,063 curlews in 121 flocks in September 2007, 10,183 in 77 flocks in February 2008, and 18,775 in 118 flocks in September 2008. The Solano-Yolo region had consistently high numbers in September, whereas parts of the San Joaquin and Tulare basins were important to curlews in either September or February. On agricultural lands, curlews used mainly alfalfa fields and pastures, which together accounted for 83% to 94% of all birds across surveys. For pasture, curlews predominantly used irrigated pasture in September and native pasture in February. Supplemental surveys in Solano County documented much higher curlew numbers in September than in early or late winter. Lower winter numbers may reflect wider dispersal of curlews after the soaking winter rains enhance the foraging suitability of previously parched grasslands or other habitats. Curlews might also be shifting to Sacramento Valley rice fields after the early fall harvest period, or some may migrate further south to wintering areas in the Imperial Valley or in Mexico. A more complete survey of the entire Central Valley is needed to refine estimates of overall curlew abundance in this region and to better determine the importance of various subregions, particularly the Sacramento Valley, to curlews. More work also is needed to document seasonal movements of curlews, examine local and seasonal variation in habitat use, study the use and persistence of nighttime roosts, and explore ways to maintain and improve curlew wintering habitats while ensuring economic viability of agricultural lands.