Cocos vs. Brown Booby field marks?

Nature Talk
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I realize that this split just came down, and the field guide folks haven’t had a lot of time to digest it, but do we have any information regarding distinguishing Brown and Cocos Boobies? In most cases the range should be enough, but in the central Pacific, most notably Hawaii, both forms are found. Many adult males are readily ID’d by head color, but the differences in females and immatures are apparently much more similar.

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Last I heard the differences still haven’t really been worked out. As you mentioned most adult males are pretty obvious, but beyond that it’s pretty much still a big question mark. This is a research opportunity for someone!

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@raymie a fair amount has been published on field identification of (primarily adults of) these taxa in recent years—for a couple general summaries of what’s currently known, see the NACC proposal leading to the split and the 2024 eBird taxonomy update article. Additional field marks are still being worked out, true—but overall it’s not the case that

or

.

More details below…

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A rough summary of some (mostly adult-focused) ID features (some diagnostic, others less so), cobbled together from a few different sources:

Underwing coverts: adult Cocos booby has a dark brown bar on the lesser secondary coverts, cutting into the white on the underwing. Adult brown booby (both subspecies) usually has white lesser secondary underwing coverts, creating a more solidly/extensively white underwing; note, though, that some adult brown boobies can have variable amounts of dark on the lesser coverts, approaching Cocos (and immature birds also have more dark on the underwing coverts than adults, likewise more Cocos-like)

Bill shape: Indo-Pacific brown booby (ssp. plotus) typically has a larger/thicker-looking bill (with a straight to slightly concave culmen), while Cocos booby has a smaller/more slender-looking bill (with a more concave culmen). (Atlantic brown boobies [ssp. leucogaster] are maybe intermediate??)

Head/neck color: dark brown in adult brown booby (both subspecies), generally a bit darker than the base head/neck color of adult Cocos. In Atlantic leucogaster, the head and neck are slightly darker brown than the back, forming a subtle hood, which isn’t so much the case in Indo-Pacific plotus (where the head and back are about the same dark brown color). (Immatures aren’t as dark brown)

Iris color (from the Birds of the World account): not diagnostic, but usually lighter in adult brown booby (in females averaging paler in leucogaster than plotus), usually dark with a narrow grayish outer ring in adult Cocos booby. (Can be pale in juveniles of either species)

The eBird update article also notes that Indo-Pacific brown (plotus) might differ from Cocos in the extent of bare facial skin behind the eye (a narrow band of bare skin behind the bare orbital ring in plotus, vs. feathering touching the back of the bare orbital ring in Cocos)

Adult male head and face/bill color are distinctive:

  • Cocos: distinctive pale/whitish coloration on the head (ranging from whitish mostly restricted to the crown, to white across the head, to an entirely whitish head and neck; seems it’s not 100% clear how much of the variation is geographic vs. individual vs. age-related). Dark bluish face and gular pouch, sort of grayish/dull pale bill
  • Brown (both subspecies): no pale coloration on the (dark brown) head.
    • Indo-Pacific brown (plotus) shows some geographic variation in bill and face color, but in the Pacific, the face and gular pouch are rich blue, and the bill is generally yellow to pale greenish
    • Atlantic brown (leucogaster): blue is restricted to around the eye, with the rest of the face and gular pouch yellow; bill pale (grayish-greenish)

Adult females of the different taxa also show differences in (mostly) bare parts color. (All have yellowish faces and typically a dark patch in front of the eye.)

  • Cocos: mostly pinkish bill (outer 2/3 or so), sometimes duskier toward the tip. From the eBird update article, the dark patch in front of the eye averages larger than in female plotus. Pale yellow face. At least some females can have a pale tinge to the forehead plumage.
  • Indo-Pacific brown (plotus): the bill is usually pale greenish to yellowish, unlike Cocos. (However, it can apparently be partially to largely pink, particularly in Hawaii per BotW—but if pink, it typically lacks the duskier tip of Cocos.) The dark patch in front of the eye, when present, averages smaller than in Cocos (and Atlantic brown?), and can apparently sometimes be lacking entirely (unlike in Cocos or Atlantic brown).
  • Atlantic brown (leucogaster): pink bill

For both males and females, bare parts coloration is brighter on breeding individuals and duller on nonbreeding individuals. Immatures start to show the face pattern of their respective sex (albeit duller than breeding adults) by around a year or two old, and male Cocos boobies also usually start showing some pale on the crown around this time

Per BotW, many/most juveniles aren’t currently separable—but maybe sometimes subtle details of underwing pattern, bill shape, and (if it ends up holding up as a field mark) amount of bare skin behind the eye could provide clues. (Per Oceanic Birds of the World, juvenile/immature Atlantic brown boobies can have a brighter yellow throat than other brown+Cocos booby taxa, and some can already show signs of a darker hood.)

Like you mentioned, Cocos boobies have been spreading west across the Pacific, now breeding well into the range of Indo-Pacific brown booby S. l. plotus. (The reverse is also the case, with at least a few plotus breeding in the Revillagigedo Islands off western Mexico, although there aren’t any records of plotus from closer to the mainland yet afaik.) Cocos booby also appears to now meet Atlantic brown booby S. l. leucogaster around the Panama Canal.

Miscellaneous sources in no particular order -

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Was it really necessary to bump all the species-level observations back to genus, though? If these newly split species concepts are geographical, couldn’t the taxon split have used atlasing to keep them at species level?

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I asked @maxkirsch about this, and he pointed out that the system can only atlas “places” – and those are all terrestrial. Once you get a bit out to sea, the system no longer knows how to handle the geography…

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In theory you could make unique places to use for atlasing, that include the ocean, but that would take some extra work.

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That’s not the case—only iNat’s standard places (countries/territories/states/provinces/etc.) can be used in atlasing, not user-created places, so marine observations (that aren’t immediately along the coast) unfortunately can’t be atlased.
(On that note, there’s currently an open feature request to implement standard places for the world’s oceans/seas on iNat)

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