Mystery Bird: Flying Royal Tern, Sterna maxima

tags: Royal Tern, Sterna maxima, birds, nature, Image of the Day

Below the fold is a detailed analysis for how to ID this species ..

Rick Wright, Managing Director of WINGS Birding Tours Worldwide, writes:

The taxonomically savvy will have noticed that these quizzes are proceeding in roughly checklist sequence: first a bunch of shorebirds, then a gull, now a...well, now a what?

Long-winged and white, this bird might at first glance be mistaken for another gull; but our quiz bird is "sharp" everywhere gulls are "blunt": the bill is finely pointed, the wingtips acute, the tail feathers--particularly the outermost ones--noticeably attenuated. These are the characters of structure and shape that make terns, as Roger Tory Peterson quotably observed, "readily distinguishable" from their fatter, clunkier cousins, the gulls.

Lingering over this tern's structural features will let us refine our identification even further. The bright orange-yellow bill is very heavy at the base, but fairly rapidly to the tip; though it's hard to gauge from this angle, there's no obvious droop. The wings are fairly narrow and long, as is the tail. This description eliminates both Caspian and Elegant Tern: the former is obviously heavy, the latter exaggeratedly slim.

We're left with Royal Tern, and what we can see of this bird's plumage fits that species perfectly. The outer primaries are dark above and pale below, with a smudgy blackish trailing edge. The head is almost entirely white, with a streaky "scarf" on the nape; that patch does not surround the eye. The dark upperwing coverts and secondaries identify this as a first-winter individual, a determination made easy in the field at any distance by the almost continuous whining whistles and bleats young Royal Terns utter.

Terns have wings and they fly; normally thought of as exclusively coastal, Royal Tern has occurred in such surprising inland localities as Arizona and Nebraska in the last few years. Other, even more outlandish outlanders have to be looked for wherever terns congregate, but fortunately, Royal Tern is a very distinctive species. Lesser Crested Tern is -- to judge by photographs and paintings -- smaller, stouter, and shorter-billed; just how difficult some of the large-to-medium terns can be was nicely demonstrated this spring by a bird at Bolivar Flats (!) this spring -- variously identified as an Elegant, a Cayenne (Sandwich), and a Lesser Crested, and ultimately and sensibly let go as undiagnosable. Birding is all about the challenge.

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