Pt. Pinos booby in July 2017
a web page by Don Roberson

Mark Kudrav discovered this subadult Masked/Nazca Booby while seawatching on Tuesday, 18 July 2017. He watched it fly in off the ocean and land on the outermost roosting rocks at the tip of Pt. Pinos, Monterey Co. This page is being developed during Days 3-4 of its visit. We now call it "Slash." This project is to present some photos of Slash and compare them with at- sea photos of known-identity boobies of a similar age.

Slash had a "usual" roost, shown above — at the far west end of the outermost roosting rock at Pt. Pinos — but it has also roosted on the taller part of that islet which is at the right end of this photo (©D. Roberson). The rock is inaccessible from shore. Many of us have tried digiscoping from shore; this one (right) is a particularly successful effort from 18 July (© Fred Hochstaedter).

Nazca Booby Sula granti, an endemic of the Galapagos region in the eastern tropical Pacific, was split from the pan-tropical Masked Booby S. dactylatra after research showed assortative breeding, genetic divergence, with plumage and bare part coloration differences; Jehl & Pitman (1998). Slash is clearly one of these species, but which one? Adults are separated primarily by bill color (Pitman & Jehl 1998, Roberson 1998, Howell et al. 2014) but there are other characters of some value, such as the tendency of adult Nazca to possess mostly white central rectrices (Pitman & Jehl 1998, Roberson 1998; the latter suggested that the tendency, based on a review of 33 photos, was 60%-40% white-tailed but others suggest the white-tailed tendency is greater than that (S. Walens, in litt., A. Jaramillo, in litt.).

On 18-19 July, Bill Hill obtained some remarkable flight shots (given the distance involved), four of which are shown below (© Bill Hill). These show that the subadult booby appears to be just completing its first-cycle of the accelerated stepwise molt pattern present in boobies (see Howell et al., 2014, p. 37). In such a pattern there are waves of molt with periods of suspension, leading to a stepwise pattern of newer and older feathers. In the photo below left, first row, note the new (black) outer 3 primaries on right wing contrasting with older (browner) inner primaries. Howell et al. (2014) describe the first wave as starting at 6-8 months of age, reaching p6-p8 before suspending, then a second wave at about 14-16 months of age, sometimes before first wave suspends. The outermost primaries (p9-p10) are replaced to complete the first wave while p1-p6 are being renewed in the start of the second wave. Assuming I am applying the criteria correctly, this booby appears to be in early second-cycle. At this age (about 1.5 years old) there can still be some juvenal feathers retained.

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.

Let's move on to specific details. Two more remarkable photos of Slash were taken by Beth Hamel on 20 July (below © Beth Hamel). They provide an excellent opportunity to study the tail pattern of "our" booby in detail. For one thing, one can count the number of rectrices. Masked and Nazca Boobies have 12-16 rectrices (Grace & Anderson 2009). Rectrices always occur in pairs, so the choices for any bird would be 12 or 14 or 16 rectrices. Our booby Slash has 13 rectrices. This means one is missing and likely just starting to grow in.
To look at the tail in more detail, I enlarged these photos and added contrast, which more easily lets us look at the dark and white areas (below)

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.

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, and Don Roberson, as credited; all rights reserved.

  page created 20-21 July 2017  
all text & photos © Don Roberson, except as otherwise indicated; all rights reserved