Molt of Bristle-Thighed Curlews in the Northwestern Hawailan Islands

Abstract

-I studied molt of Bristle-thighed Curlews (Numenius tahitiensis) on Laysan Island in the Northwestern Hawaiian Islands from 1988-1991. Adult curlews underwent a complete prebasic molt between August and December. Duration of primary molt was about 92 days, which is rapid compared with other shorebirds that molt in tropical and southern latitudes. Adults replaced large numbers of primaries and secondaries simultaneously, and about 50% of the birds became flightless during molt. The prealternate molt began in winter and ended in early spring; it involved the body feathers and variable numbers of rectrices but no remiges. Juveniles molted body feathers and some rectrices during their first autumn and winter but did not replace their juvenal primaries until the spring and summer of their second calendar year. Second-year curlews began replacing their new first basic primaries in late summer, in some cases before the outer juvenal primaries had been dropped. The delayed first prebasic primary molt is probably an adaptation allowing inexperienced birds to devote energy expenditure in their first winter to obtaining food rather than to molting remiges. Because second-year birds remain on the wintering grounds throughout the year and do not prepare for migration, there is no selection against replacing new primaries. Unlike most shorebirds, adult Bristle-thighed Curlews gained mass steadily throughout the autumn and winter. Their rapid prebasic molt in autumn may be an adaptation to allow the birds ample time to build up fat reserves during winter. I suggest that the absence of rich intertidal feeding areas and the frequency of winter storms make it difficult for curlews to take on large fat stores in short time periods as do species that winter on continental coasts. The lack of predators and the small size of remote oceanic islands obviate the need for curlews to maintain peak flight efficiency, allowing birds to become flightless during molt in autumn and to carry increasingly large fat stores throughout the winter. Received 8 October 1992, accepted 15 December 1992. SEVERAL GENERAL PATTERNS in the timing and duration of the prebasic molt of shorebirds have been identified. For example, most shorebirds do not overlap molt of flight feathers with either breeding or migration, although molt of body feathers may begin on the breeding grounds (Ferns 1978, Johnson and Johnson 1983). Notable exceptions include some populations of Purple Sandpipers (Calidris maritima; Sutton and Parmelee 1955, Bengston 1975) and Dunlins (C. alpina; Holmes 1966) that replace their primaries while breeding, and Common Redshanks (Tringa totanus; Pienkowski et al. 1976) that molt while migrating. Also, shorebirds wintering in tropical and southern latitudes tend to initiate molt later, and to molt more slowly, than do species that winter farther north (Prater 1981). Despite substantial infor' Present address: Cooperative Wildlife Research Unit, University of Montana, Missoula, Montana 59812, USA. mation on the molt of shorebirds (e.g. Boere 1976, Morrison 1976, Pienkowski et al. 1976, Prater 1981), the ecological factors responsible for these general patterns have been difficult to elucidate. Consequently, there remains a need to document molt in additional species. Bristle-thighed Curlews (Numenius tahitiensis) are one of the rarest and least-studied shorebirds in the world. They breed in remote areas of western Alaska and winter on small islands in the tropical and subtropical Pacific Ocean. The only published information on their molt comes from a small sample of subadults collected in the Marshall Islands during summer (Johnson 1977). Bristle-thighed Curlews exhibit several unusual traits that make a study of their molt particularly interesting: (1) they are the only migratory shorebird whose winter range is restricted to oceanic islands, which they reach after nonstop flights of at least 4,000 km (Marks et al. 1990); (2) they have high annual survivorship and extreme longevity (Marks 1992); (3) most subadults remain on the wintering