Long stretches of the Pacific coast of North America are rocky or sandy, punctuated by only a few large bays and river estuaries. During migration and winter, most shorebirds (suborder Charadrii) use these bays and estuaries (Senner and Howe 1984, Page et al. 1992), which are also the centers of human activities that place shorebird populations at risk (Myers et al. 1987).