Plumages of the Least Tern

Over the past five years, U.S. breeding colonies of the endangered California Least Tern (Sterna albifrons browni •) have been closely monitored throughout the nesting season (Atwood et al., 1977). One result of this close scrutiny has been the observation each year of a small number of birds in prebreeding or subadult plumages (in this discussion, the term subadult is used to represent any alternate plumage that differs from "nuptial," i.e. definitive alternate, plumage). Such birds have often appeared later in the season, after nesting has begun. Although they have sometimes engaged in courtship behavior, these individuals have not been observed actually breeding. Terns in subadult plumage have been seen for many years in breeding colonies of the larger Sterna species. Two rather distinct plumage types, observed in Arctic Tern (S. paradisaea) colonies over 100 years ago, were originally described as separate species•S. pikei (Lawrence, 1853) and S. portlandica (Ridgway, 1874). Both "species" were subsequently determined to be subadult plumages of the Arctic Tern. The names have been retained to refer to stages in the plumage development of terns, portlandica referring to the plumage of 1-year-old birds, and pikei generally used for subadult plumages beyond the first year (Cullen, 1957; Grant, et al., 1971). Common Terns (S. hirundo) in subadult plumage have been seen regularly in eastern U.S. breeding colonies (Palmer, 1941). The plumage sequence of this species was first described by Dwight (1901); his assumption that full "nuptial," i.e. definitive alternate, plumage was attained by 1-year-old birds has been corrected by subsequent data. Palmer determined that the Common Tern has a portlandica phase, and that, even though some 1-year-old birds in this plumage migrated to the nesting grounds, they did not breed. Birds in pikei plumage were also regularly present, and were observed to breed occasionally. He noted one "white-faced" Common Tern for every 150 breeding pairs in a colony in Maine (Palmer, 1941). Nisbet (pers. comm.) reports less than one portlandica-plumaged Common Tern for every 1,000 breeding birds in Massachusetts during recent years. The literature lacks mention of Least Terns in subadult plumage at breeding colonies, although Haverschmidt (1972) briefly described the portlandica plumage from Least Terns collected on the coast of Surinam (where flocks of immature terns summer, but are not known to breed).