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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). The Nazca Booby (Sula granti) was split from the Masked Booby (S. dactylatra) by the American Ornithologists’ Union ([AOU] 2000) after Pitman and Jehl (1998) documented differences in morphology, bill color, and plumage color, along with assortative pairing and habitat selection on islands where both taxa breed. The Nazca Booby breeds primarily on the Galapagos Islands but also on Malpelo Island off Colombia and sparingly north to Clipperton Atoll and the Revillagigedo Islands off Mexico (Howell and Pyle 1997, Pitman and Jehl 1998, AOU 2000). The Masked Booby of subspecies S. d. personata (including S. d. californica; Pitman and Jehl 1998) breeds on islands throughout the tropical Pacific Ocean. In the eastern Pacific, it breeds primarily on Clipperton Atoll and the Revillagigedo Islands and sparingly on the Alijos Rocks off Baja California (Everett and Pitman 1995). Hybridization, or at least mixed pairing, of Masked and Nazca boobies has been reported occasionally on Clipperton Atoll and the Revillagigedo Islands (Pitman and Jehl 1998, California Bird Records Committee [CBRC] 2007) as well as in the Hawaiian Islands (VanderWerf et al. 2008). Boobies apparently lack a preformative molt; the molt that replaces the juvenile plumage following the first cycle is the second prebasic. This molt (and the second plumage cycle) begins when the bird is 7–9 months old, with the dropping of the tertials and the innermost primary, p1 (Pyle 2008, Howell 2010). The primaries are molted sequentially and distally, the outermost pri-

Molt, Age, and Identification of the Masked and Nazca Boobies in California