External anatomy, plumages, and historical distribution of the Eskimo Curlew (Numenius borealis)

Abstract

I sought to distinguish between sexes and age classes of the Eskimo Curlew (Numenius borealis), as well as to clarify the historical migration pathways and breeding distribution of the species. Discriminant function analysis revealed that sexes significantly differed in bill and tail length, with bill length the best predictor of sex. I found no significant seasonal discrimination in measurements among individuals, nor did I find any difference between early and later Northeastern fall migrants (i.e., primarily adults vs. primarily juveniles). Breast marking shape and extent of white on throat were uncorrelated with other plumage characteristics, whereas pale abdomens and wide, paler margins of wing covert feathers correlated with low density of breast markings. Individuals with pale abdomens and heavily spotted backs also had wide margins of wing covert feathers. Back spotting was the only significant plumage character related to sex, with heavy spotting predominating among females and light spotting predominating among males. I found few clear seasonal differences in plumage among specimens, although dorsal ground color may have averaged darker on spring specimens. Moreover, more lightly back-spotted individuals tended to occur in later fall, which suggested that these birds were juveniles. The existence of individuals with few Y-shaped breast markings appeared to represent one end of a range of plumage conditions that exhibited no clear relationship to age, sex, or geographic location. Mapping locations of museum specimens clarified details of migration pathway extent, provided evidence of migratory concentration spots, and provided additional evidence that Alaska once possessed a breeding population. Examining dates of last collection indicated that populations collapsed below levels at which birds were regularly located after the first decade of the 20th century.