Sandwich Tern - bill color of juveniles

-- updated Sep 27, 2008

This photo by Greg Lavaty of Texas, generated a lot of discussion on the Texas Birds listserv recently, and brings up some interesting points and questions.

First, this is definitely a juvenile Sandwich Tern, and the bill color is pretty typical of a recently fledged bird. Here are some photos of similar birds in Texas by Martin Reid. Few birders have the opportunity to see birds at this age, and I've only seen it a few times on the Texas coast in summer, so this is a very instructive photo. In the Sibley Guide I illustrated the juvenile with a bill pattern similar to Greg Lavaty's photo (orange along the cutting edges of the bill), and stated "bill dull orange before fledging, quickly becomes blackish with pale tip". This is not strictly accurate on a couple of points, as Ted Eubanks reports on Texbirds [19 Aug 2007 in Galveston, TX]: "Today I saw young Sandwich Terns with bills from completely yellow-orange to black with no yellow tip. Of course, I found every permutation in between."

Sandwich Terns through their first year typically have a black bill with a much-reduced pale tip compared to adults (contrary to the juvenile illustrated in the Sibley Guide), and their bills can look all black. I did use this species for a comparison image in Birding Basics, where I illustrated the bill pattern of adult vs juvenile correctly.

More importantly, some young Sandwich Terns show a yellow-orange bill with little or no black for a substantial period of time after fledging, and such birds are apparently not that unusual. To the unaware, this can cause some real confusion. Here's a photo of a mostly orange-billed juvenile Sandwich Tern by Greg Lavaty.

From the comments: observations by Joseph Kennedy at Galveston, Texas in August 2007:

Newly flying chicks have bills that are yellowish or maybe yellow-orange, very different from the orange color of the royal tern chicks. As they age the yellow fills in with streaks of tan and brownish from the outside in which becomes black.... This pattern appears to be random which can result in upper and lower beak differences or base and tip differences.

Last to change is the interior of the beak and the edge of the beak which is
still yellow when the outer beak is all dark. At the same time the beak is
growing longer. Originally the beak structure appears more like a
gull-billed tern and as the youngster ages the beak grows at the tip until
it reaches the long skinny beak of the adult.

Distinguishing these orange-billed juveniles from Royal and Elegant Terns must rely on size and shape, the paler gray upperparts of Sandwich, and at least on some birds (fide Ted Eubanks) the more spotted pattern of the back and scapulars compared to juvenile Royal (but compare variation in upperparts pattern of juvenile Sandwich from unmarked to heavily marked, as in other terns). Mostly it should just be a matter of being aware of the possibility and recognizing that juveniles are this variable.

• Key point: Royal and Elegant Terns never show any black on the bill. So as soon as these juvenile Sandwich start to show black on the bill it should be easy to rule out other species. [Birds with mixed orange and black bills could also be hybrid Elegant x Sandwich Terns (which are very rare, but recorded in Texas and Florida), or 'Cayenne' Tern, the South American subspecies of Sandwich Tern (very rare visitor recorded in NC and NY]

Questions that follow from this very interesting observation are:

• Could these birds' bill color be confused with Royal or Elegant Tern, or is the color different somehow? Ted Eubanks reports that the color can be just as bright yellow-orange as Royal Tern, but Joseph Kennedy says Royal is noticeably more orange (and of course some juvenile Royal Terns have very pale and drab yellowish bills). It does seem that most young Sandwich soon develop at least some dark smudges that make the bill look "dirty" or drab.

• What percentage of birds show different variations, especially how many have completely yellow-orange bills with no black? Reports so far suggest this is a small percentage. Greg Lavaty from Texas on 26 Aug 2007 reports "the great majority of them (the young ones anyway) had what looked like all black beaks or black with only the slightest yellow tip."
  

• How long can they retain orange color with or without black smudges? i.e. at what date is it safe to identify an orange-billed tern as "not-Sandwich". [From photos it appears that all develop at least some black on the bill very soon after fledging - July and August. And Martin Reid's photo from September 20th, and photos of a similar bird from Florida on October 19th by K. Dean Edwards indicate that at least some juveniles retain orange on the bill through October]
  

• Could this be a regional variation? It seems unlikely that this is a local regional variation limited to Texas, but it would be great if observers from Florida to the Carolinas could check to see if a similar proportion of juvenile Sandwich Terns there show yellow or orange bills after fledging. This color is not normal in Sandwich Terns of Britain and Europe, where subspecies sandvicensis occurs (North American birds are subspecies acuflavida). Klaus Malling Olsen (Terns of Europe and North America, 1995) highlights a single British record of a juvenile with all yellow bill. So this certainly seems to be a difference between European and American subspecies. The South American subspecies eurygnatha - Cayenne Tern - has a yellow-orange bill as an adult, so one would expect a yellow-orange bill to be the norm for juveniles there.
  

Thanks to Ted Eubanks, Greg Lavaty, and Joseph Kennedy for comments and photos.

If you live where Sandwich Terns can be seen now, and/or have an answer or insight, feel free to post comments here, or to email me directly.
Good Birding,
David Sibley