Thursday, August 23, 2007

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

3 comments:

Joseph said...

I went through my older pictures and added 3 to the web site including a picture from last year of 2 young sandwich terns with unmarked orange yellow bills

I went through my older pictures and have several more sandwich youngsters.

This sick or injured bird on the Texas City Dike on July 12, this year shows color just starting to infuse the beak. The legs are yellow but darkening
http://www.pbase.com/joseph_kennedy_36/image/84631460

This bird from Bolivar Flats on July 28, 2006 has a similar beak but almost yellow legs.
http://www.pbase.com/joseph_kennedy_36/image/84631461

I wonder if the color is effectively rusting where the original pigment darkens with exposure to the air over time.

Finally, is a picture from from July 6, 2006 again at Bolivar Flats with the holy grail of tern beaks. Both birds have essentially unmarked beaks with the right bird showing just a tinge of dark color starting to appear while the left youngster is pure orange-yellow. Note both birds have totally black feet and legs.
http://www.pbase.com/joseph_kennedy_36/image/84631462

Joseph said...

I tried posting this yesterday but missed the antispam entry

In light of the recent discussions concerning sandwich terns, I took a lot of pictures of the birds featuring the beaks and feet on the Texas City Dike last Friday.

By coincidence, the terns cooperated with the photographer with the exception of one bird with the most yellow bill that only posed when someone forget to reset the camera after a dark cloud passed. Young sandwich terns were scattered along the dike in groups made up of both young and old with other tern species mixed in. Parent birds were still feeding the youngsters. Each time a parent approached with a fish, a youngster would run away from the crowd and competition and be fed. Thus each tern could be verified as a sandwich tern youngster.

These pictures and those from the last month show the progression of beak and foot color from the time that the birds first leave the nesting area. 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 as the underlying structure also changes color. 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.

The yellow tipped beak of the adult is not part of the color change but is a part of the growth process. As the beaks get longer you start to see a hint of color at the tip which eventually becomes the yellow tip of the adult. Some adults maintain or develop yellow patches on the beak other than the tip which, at the extreme, would give rise to the cayenne type of tern which are seen in Florida on occasion.

For every statement above, there are probably exceptions as the progression appears to follow no real pattern unless random is a pattern. Note that the leg color also changes as the birds age. Very young birds have legs and feet almost yellow which becomes spotted until they also are black.

Those not interested in beaks etc can best skip the following

This chick with the parent has gotten to the point the the beak is basically black with just some hint of yellow
http://www.pbase.com/joseph_kennedy_36/image/84569727

But when it opens its mouth to whine, the edges and interior are still bright yellow
http://www.pbase.com/joseph_kennedy_36/image/84569726

This bird has more yellow, an apparent yellow tip developing and shows the carpal bar on the partially spread wing
http://www.pbase.com/joseph_kennedy_36/image/84569723

But the same bird still has very yellow feet and legs
http://www.pbase.com/joseph_kennedy_36/image/84569719

This bird has an upper bill that is largely dark but a lower bill that has a yellow base with some yellow developing or remaining on the tip
http://www.pbase.com/joseph_kennedy_36/image/84569689

While this bird has a largely dark exterior of the beak with just a hint of color at the tip although the interior is yellow
http://www.pbase.com/joseph_kennedy_36/image/84569683

This bird is the same with a little more yellow at the tip
http://www.pbase.com/joseph_kennedy_36/image/84569630

This bird is showing the finely pointed top of the beak starting to yellow while the edge is dark
http://www.pbase.com/joseph_kennedy_36/image/84569671

While this one has a little more yellow
http://www.pbase.com/joseph_kennedy_36/image/84569673

And the beak every sandwich tern wants to own
http://www.pbase.com/joseph_kennedy_36/image/84569664

http://www.pbase.com/joseph_kennedy_36/image/84569675

And a beak where the tip is mainly translucent without any yellow coloring yet
http://www.pbase.com/joseph_kennedy_36/image/84569668

And an adult beak with a little yellow at the base
http://www.pbase.com/joseph_kennedy_36/image/84569685

And a couple of yellowish younger birds taken a month ago
http://www.pbase.com/joseph_kennedy_36/image/82082340

http://www.pbase.com/joseph_kennedy_36/image/82082339

Lots more pictures that can be browsed toward the bottom of the page starting at and wading through the other terns and dike specials.
http://www.pbase.com/joseph_kennedy_36/inbox

Joseph said...

Some further comments on tern leg colors.

We recently had a discussion here concerning the varying coloration of the young sandwich tern's bills and feet. It was mentioned that the royal tern was more of a fixed commodity.

This is what the standard royal tern should have, nice orange beak and black legs and feet
http://www.pbase.com/joseph_kennedy_36/image/85038597

I found a young bird at east beach that had partially orange-yellow legs
http://www.pbase.com/joseph_kennedy_36/image/85038598

So there can be some variation in royal terns as well as sandwich terns

I have received a note Stu Wilson concerning the black tern pictures posted from the same date. I guess that I should check feet on the rest of my tern pictures. According to Sibley, young black terns should have light colored legs as would adult non-breeding birds. My picture of the young black tern has black legs
http://www.pbase.com/joseph_kennedy_36/image/85038602

As does the bird that has assumed most of its winter plumage
http://www.pbase.com/joseph_kennedy_36/image/85038600

Sibley shows all plumages of white-winged terns and adult black terns as having yellow legs, if this is actually a difference, it would be an easy way to id all of the stray white-winged terns that pass through the UTC. Perhaps this is a carryover from European birds and US birds are different.