eWestern BirdsThe Quarterly Journal of Western Field Ornithologists
Vol. 51, No. 2
Western Field Ornithologists
Molt, Age, and Identification of the Masked and Nazca Boobies in California
ABSTRACT: I examined photographs compiled by the California Bird Records Committee to assess bill-color and plumage-color changes with molt of the primaries in Masked (Sula dactylatra) and Nazca (S. granti) boobies recorded in California. I evaluated a total of 134 individuals initially discovered between 5 April 1990 and 19 November 2019. For each individual I scored the stage of molt of the primaries, coloration of seven areas of the plumage, and color and brightness of both the base and tip of the bill. Two distinct bill-color patterns emerged as the birds aged from juvenile to definitive plumage. Birds with dusky, horn color (medium-pale brownish, often tinged mustard or olive), orange, and/or pink in the bill base commonly had brighter orange-yellow or golden yellow but not green in the bill tip. By contrast, birds with dark blue, green, and/or yellow in the bill base commonly had green and/or yellow but not orange in the bill tip, and the tip was less often brighter than the base. I propose that the first category represents Nazca Boobies and the second represents Masked Boobies. A horn-colored base and a brighter golden-yellow tip of the bill of birds in their first and second plumage cycles (including most juveniles) evidently indicates the Nazca Booby. Some birds probably reach definitive appearance by completion of their first primary molt, at 20–22 months of age. The central rectrices of apparent Nazca Boobies (by bill color) average whiter than those of apparent Masked Boobies, the extent of white increasing from juvenile to second basic to definitive plumage. But there is species overlap in this feature. According to provisional identifications in my sample, Nazca Boobies have outnumbered Masked Boobies in California (92 vs. 27 individuals through 2019).