Xantus’s Murrelets Synthliboramphus hypoleucus nest in loose colonies on the Channel Islands off southern California, USA, and islands off northwestern Baja California, Mexico (Hunt et al. 1980, Murray et al. 1983, Carter et al. 1992, Drost & Lewis 1995). The colonies are vulnerable to extirpation from breeding islands through predation by introduced mammalian predators (Jehl & Bond 1975, McChesney & Tershy 1998). Nest sites of Xantus’s Murrelets occur mainly in rock crevices and to a much lesser extent under plants and artificial structures. At many colonies, a large proportion of nests occur in cliffs and steep slopes that are not easily accessible by humans without extensive climbing skill and equipment. Nest sites are visited only at night during the long incubation period (averaging 34 days), parents take long incubation shifts (one to six days), and eggs are periodically neglected (for one to four days). Chicks are precocial upon hatching and at two days old they depart from nest sites, accompanied by adults, for further rearing at sea (Murray et al. 1983). The birds’ use of largely inaccessible island habitats and of nocturnal and non-daily nest visitations and their foraging far from shore makes it difficult to find colonies, estimate population size and monitor population changes.