August 20, 2014
Nazca Booby in the Pacific Ocean near the San Diego/Mexico border
Observation narrative and commentary on identification issues
On Sunday, August 17, 2014, I took an 8-hour whale watching trip with Pacific Nature Tours departing from San Diego, California, travelling to the 9-mile bank, across the U.S./Mexico border and to the Corodano Islands. Birders on board were Pete Ginsburg, Kerry Ross, Rachel Smith, Stan Walens, Susan Smith, Justyn Stahl, Justin Bosler, Elizabeth Copper, Supeena Insee Adler, and myself. Approximately 2 miles south of the border, in Mexican waters, we encountered the first booby of the trip. Kerry Ross first spotted the bird and called "booby!" and I quickly had the bird with binoculars, and my heart jumped as I saw the strong black/white contrast suggesting that wasn't the Brown or Blue-footed Booby most likely to be present, just as Kerry shouted "Cameras! Get cameras ready!" having surely thought the same thing. The bird approached the boat from the rear and circled over the rear of the boat--so close that the bird would not fit within a single camera frame, in my case--and then headed away not to return. I concentrated on taking photographs rather than detailed sight observations--the time stamps on my photos span only 20 seconds. Including the approach, the entire encounter lasted probably just 40 seconds. The bird was observed by most of the birders on board the boat and all those presumed an identification of Masked Booby.
According to Justyn Stahl, the precise location of the observation was 32.47486, -117.32561, shown on this map (labelled MABO/NABO) which he prepared.
Upon posting the photographs, in particular this close image showing some orange coloration in the bill, San Diego birders Brennan Mulrooney and Stan Walens both suggested that it should be considered as a possible Nazca Booby.
Nazca Booby Sula granti was split from Masked Booby Sula Dactylatra by the AOU in 2000 based on research by Robert L. Pitman and Joseph R. Dehl, Jr., published in 1998, establishing differences in morphology, plumage, bill color, nesting preferences, foraging habits, and breeding range. The split was supported by genetic evidence by Friesen, et al. (2002).
Review and discussion of the photographs taken by multiple birders aboard the boat included all those birders as well as ornithologists Guy McCaskie, Paul Lehman, Alvaro Jaramillo and Robert Pitman. What follows is my attempt at a summary of points raised in the discussion as well as other relevant sources:
This bird is aged as a subadult, either 2nd-cycle or 3rd-cycle, based on still having an irregular speckling of brown feathers on the rump and on the lesser wing coverts, both upper- and lower-, visible here and here. According to Howell, et al. (2014), most of this brown speckling is lost by the 3rd cycle "but some show scattered dark marks on rump and underprimary coverts". And according to Howell (2010), molts in boobies are protracted, and so it may not be possible to distinguish 2nd from 3rd cycle birds based on appearance. (As an intereting side note, Howell also points out that it was a study of Masked Boobies by D. F. Dorward published in 1962 that first recognized the stepwise wing molt progression shown by some larger birds.) None of the photos show missing or growing primaries, nor do I see any obvious signs of wear on the primaries that might help determine age.
At no stage of development should the bill of a Masked Booby attain orange tones. According to Howell, et al. (2014) it is not known when orange tones begin to appear on the bill of Nazca Booby, but it is "probably late in the first cycle." This bird shows orange tone at the base of both mandibles grading into the yellow, with the orange more extensive on the lower mandible. Pitman and Jehl note average differences in bill measurements, and some have suggested that the culmen of Nazca appears more concave. However, the measurements in Pitman and Jehl (of length and depth) do not describe curvature or shape, and in any case show a significant amount of overlap. Looking through photos on line of adult, yellow-billed Masked Boobies, I see variation in the appearance of the culmen shape. Some birds seem to have straighter and culmen and some show as much concavity as the subject bird. So, I suspect that this may not be a useful feature for field identification.
This bird also shows partial white on the inner retrices of the tail, visible here and here. The illustration of adult Nazca and Masked Boobies in Howell, et al. (2014) shows a distinction between the entirely dark tail of Masked Booby and the mostly-white central tail feathers of Nazca, although it is not explcitly mentioned in the text. This feature is described by Pitman and Jehl as the central retrices appearing "as if dusted with flour [with] the extent increasing with age, so that some older sub-adults appear white-rumped", whereas for Masked Booby the more limited white on the tail feathers is usually covered by the upper tail coverts.
Both the bicolored bill and partially-white central tail feathers of the observed bird are consistent with a subadult Nazca Booby.
Pitman and Jehl did note hybridization between Masked and Nazca Boobies. Vanderwerf, et al. (2008) review evidence for hybridization at various locations and note the possibility that this would result in intermediate orange-yellow colored bills. By e-mail, Robert Pitman provided photographs of adult hybrids showing bicolored bills and central tail feathers that are white at the base grading to brown at the end. As both of these features are consistent with the appearance of the subadult Nazca, it is therefore not possible to rule out the possibility of a hybrid in the subadult stage.
Dorward, D. F. 1962. Comparative Biology of the White Booby and the Brown Booby Sula SPP. atascension. Ibis, 103B: 174–220. [link]
Friesen, V. F., D. J. Anderson, T. E. Steeves, H. Jones, and E. A. 2002. Schreiber. Molecular Support for Species Status of the Nazca Booby (Sula granti). The Auk 119(3):820–826. [link]
Howell, Steve N. G., Ian Lewington, and Will Russell. 2014. Rare Birds of North America. Princeton University Press.
Howell, Steve N. G. 2010. Molt in North American Birds (Peterson Reference Guide). Boston: Houghton Mifflin Harcourt.
Pitman, R. and J. Jehl. 1998. Geographic variation and reassessment of species limits in the “Masked” Boobies of the eastern Pacific Ocean. Wilson Bulletin 110: 155–170.
Vanderwerf, E.A., Becker, B.L., Eijzenga, J. and Eijzenga, H. 2008. Nazca Booby Sula granti and Brewster’s Brown Booby Sula leucogaster brewsteri in the Hawaiian Islands and Johnston and Palmyra atolls. Marine Ornithology 36: 67–71. [link]