The above molt discussion dealt with wing molt. A further shot (right) also confirms the fresh black look of the outermost 3 primaries, and presumably ageing this bird as early second cycle (© Blake Matheson).

Obviously there is substantial body molt also underway at 1.5 years old, with new white feathers replacing brown juvenal-plumaged body feathers across the head, neck, back, rump, and wing coverts. Slash has very extensive white on the underwing coverts but much brown remaining in the upperwing coverts and on the rump.

Compare our booby Slash (above) with these photos of similar-aged boobies in the eastern tropical Pacific (below). The upper left booby on the first row below is a Nazca Booby (from the Galapagos on 24 Sep 1989); the other 3 are Masked Boobies taken at various locales southeast of Hawaii on 5-7 Sep 1989 (all © D. Roberson). You can get a feeling of the yellow to yellow-green color of the bills of Masked Booby at this age. The Nazca does show a white central rectrix on this individual in late September.

We can see that it looks like these rectrices are pretty worn. This could be consistent with still-retained juvenal rectrices (A. Jaramillo, in litt.). It is one of the central rectrices that is missing. We probably can see the tip of the new rectrix growing in just slightly left of the base of the one current central rectrix, especially in the second photo (above right). We can see that the very base of many (all?) of the rectrices is white. Pitman & Jehl (1998) had this to say about tail differences between adults OB [orange-billed = Nazca] and YB [yellow-billed = Masked] boobies in their study:

"The central rectrices in OB birds tend to be pale at the base, as if dusted with flour, and the extent increasing with age, so that some older sub-adults appear white-rumped; in YB populations the rectrices average darker, and whitish bases, if present, are usually concealed by the upper tail coverts. In definitive plumage, OB and YB forms are similar, except that the dark areas tend to be a rich chocolate brown with a reddish tinge in OB birds, compared to dark brown to blackish in YB birds, and the central rectrices average paler and may be almost entirely white, a condition that is rare in all YB populations."

Of course all this applies to rectrices that are second-cycle or older; juvenal rectrices are all-dark in both species. [Also, according to Pitman & Jehl, some small minority of Masked tails will have white centrals, while some percentage of Nazca tails will be all-dark. There is a photo of a pair of Nazca from Tower Island, Galapagos, in Nelson (1978) that shows one dark-tailed Nazca paired with another Nazca with white central tail feathers. So tail pattern is not a "diagnostic" character — just a helpful one.] If Slash still has 13 juvenal rectrices, it doesn't help us identify it at all! We would need to hope Slash stayed for months and molted in both central tail feathers. And even then ....

So let's step away from tail pattern and move on to bill color. Howell et al. (2014) says that in the first-cycle Nazca has a bill "pale greenish gray to greenish yellow; age unknown at which pinkish or orange tones appear on bill base, but probably late in 1st cycle (study needed)." Roberson (1998) says that "juvenal boobies have dull gray or bluish bills" but as subadults the color changes towards adult condition: "Masked Boobies have quite greenish-yellow bills by the time they appear 'half-and-half' dark and white [in body plumage] while the orange or pinkish-red of Nazca Booby should be apparent on the basal half of the bill."

Our initial impression in viewing Slash through a scope was that the distal two-thirds of the bill was yellow but the basal third was some pale, dull, non-yellow color, described variously as "ivory" or "bone" or "dusky-chalk." Here are over-enlarged photos taken at different times over 4 different days by 3 different cameras, and showing both sides of the bill. These are all unadjusted for color or saturation [top row, left, backlit on 18 July © Cooper Scollan, who walked out farther on the rocks at low tide to get closer than most of us ever got; top row, right, direct sunlight on 21 July © D. Roberson; bottom row, photos on 19-20 July © Bill Hill].

You can see the black or blackish bare facial skin for which the original "Masked Booby" was named; some see a blue or dark purple tinge to that bare skin. You can see the yellow eye on at least two of the photos. You can see a bill whose outermost (distal) half or two-thirds is a shade of yellow and a basal half or third that is another color. In general, we would expect a Masked Booby to be yellow, greenish-yellow, or (sometimes) bluish at the base of the bill. We would expect Nazca to be orangeish, pinkish, or dull rose at the base of the bill. I suspect that different folks will use different terms for that part of the bill.

Finally, Pitman & Jehl (1998) discussed some other differences between Masked and Nazca Boobies which they called, respectively, YB ("yellow-billed") and OB ("orange-billed") in their paper:

Discriminant function analysis showed that OB birds are generally smaller, having shallower bills, shorter tarsi, and longer wings. The smaller size of OB birds is further indicated by body mass, which averages 12-14% lighter than S. d. personata. They are also more sexually dimorphic in bill, wing, and tarsus than YB boobies.

They go into further depth on bill shape differences in their paper. To my eye, a small but perceptible 'dip' in the culmen of Nazca occurs about two-thirds out to the tip, and the bill narrows at that point, providing a different bill aspect than the more straight-culmened heft of Masked. It is a bit subtle and there is likely to be overlap, so it is more of a suggestive point, similar to the tail topic.

UPDATE: Our Pt. Pinos booby ["Slash"] was present 18-21 July 2017. Since then three more large boobies in this set were seen in California:

• a more advanced subadult than the Pt. Pinos subadult was seen from a Shearwater Journeys trip just west of San Francisco SF on 6 Aug 2017. It was identified in the field as a "Masked Booby" but a review of photos taken by Chris Hartzell [see below] strongly suggest to me that it was a Nazca Booby.
• a "fully adult" Nazca Booby "with a bright orange bill" was photographed on 19 Aug 2017 in the Golden Gate channel from Ft. Baker SF; see eBird checklist.
• a nearly adult Nazca Booby with a fully adult-colored bill was photographed 24 Sep 2017 on a San Diego Co. pelagic trip; see eBird checklist.

Both the 6 Aug bird offshore San Francisco bird and the San Diego pelagic trip bird had white central tail feathers [less certain on the Golden Gate channel bird]. As set out in the discussion above, white central rectrices are a feature of Nazca Booby. While my photo-based review in Roberson (1998) suggested that at least 60% of adult Nazca Boobies had white central rectrices, Alvaro Jaramillo found on a recent visit to the Galapagos that all the adult Nazca Boobies he saw had that feature, and so did a percentage of younger Nazca. There is evidence (e.g., in Roberson 1998) that a minority of adult Nazca Boobies have all-black tails, but it may be a comparative small percentage. We have been unable to quantify for sure what percentage (if any) of adult Masked Boobies have white central rectrices, although Pitman & Jehl (1998) did state that it was a "rare" feature in Masked Booby.

Some observers have concerns about hybrids. Robert Pitman, the primary author of Pitman & Jehl (1998), advised me this year that hybrids are a tiny percentage of Masked/Nazca Booby populations, stating:

"I think the number of Nazca x Masked hybrids is probably considerably less than 1%. There are no Masked Boobies at the two major Nazca colonies (Malpelo, Galapagos) that I am aware of. And there are only a handful of Nazcas at Clipperton, San Benedicto and Alijos, combined, and most of those find their own kind. I have seen several hybrids at all three of those locations, however."

With a mere handful of hybrids over a worldwide population in the hundreds of thousands, I consider that possibility to be negligible (and we have no information of the color of rectrices on hybrids, or their survival rates). Accordingly, while an all-dark tail does not rule out Nazca Booby, a booby of this type that has both white central rectrices and an appropriately colored bill should be identifiable as a Nazca Booby.

We turn again to bill color. Please recall that Roberson (1998) pointed out that subadult boobies in this set are yellow on the distal half of the bill but that "Masked Boobies have quite greenish-yellow bills by the time they appear 'half-and-half' dark and white [in body plumage] while the orange or pinkish-red of Nazca Booby should be apparent on the basal half of the bill." Another way to say this is that subadult Masked Booby has either an all-yellow or a yellow-green bill as a subadult, but that Nazca Booby has a bill that is yellow on the distal half but should be changing towards adult color on the basal third or half. As full adults, a male Nazca has an orange bill, often with a yellow tip, but adult female Nazca has a rosy-red to coral-red bill, again often with a yellow tip (Roberson 1998).

Here is, again at below left, Bill Hill's photo of the Pt. Pinos booby, enlarged to see the bill better (© Bill Hill). I see a bill with a yellow distal half and a dull chalky-pink color on the basal half. At below right is Chris Hartzell's photo of the S.F. booby of 6 Aug — the bird that is a more advanced subadult than the Pt. Pinos booby (© Chris Hartzell) — whose bill is yellow on the distal third but the basal two-thirds appear to me to be a dull rosy-pink. I believe that such a bill pattern is diagnostic of Nazca Booby.

If the S.F. booby can be identified as a Nazca Booby by a combination of tail pattern and bill pattern, could the Pt. Pinos bird be identified on bill color alone? Maybe. It certainly seems to be more likely a Nazca Booby based on the pale rosy tone to the basal half of the bill. At this point in time (Oct 2017) there is no consensus that subadults can be identified by bill color alone — although I would say that if the bill was entirely bright yellow-green at this age, as shown by some of the photos of subadult Masked posted above — then it should be a Masked Booby. Here, on "Slash," I am of the opinion that the bill color is consistent with Nazca at that age. And that's about all we can do at this point in time. Further research should make more progress on this topic.

Literature cited:

• Grace, Jacquelyn and David J. Anderson. 2009. Masked Booby (Sula dactylatra), fascicle 73 at The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology.
• Howell, S.N.G., I. Lewington, and W. Russell. 2014. Rare Birds of North America. Princeton Univ. Press.
• Nelson, J.B. 1978. The Sulidae: Gannets and Boobies. Univ. of Aberdeen, Oxford Univ. Press.
• Pitman, R.L., and J.R. Jehl, Jr. 1998. Geographic variation and reassessment of species limits in the "Masked" Boobies of the eastern Pacific Ocean. Wilson Bull. 110: 155-170. [on-line pdf]
• Roberson, D. 1998. Sulids Unmasked: which large booby reaches California? Field Notes 52: 276-287. [on-line pdf]

I am grateful to Stan Walens, Alvaro Jaramillo, and Mark Kudrav for discussions on this bird; to all the photographers for use of their images; and to Rita Carratello for editing out some mistakes and typos (but if you find another, let me know!).

Photos: All photos © Bill Hill, Beth Hamel, Cooper Scollan, Fred Hochstaedter, Blake Matheson, Chris Hartzell, and Don Roberson, as credited; all rights reserved.