Sunday, August 23, 2009

But aren't all terns Elegant?

Obviously, I've got at least one more San Diego post in me. This is a series of tern pics from the same mudflat where the western sandpipers and the large shorebirds were. I'd been photographing the sandpipers for probably 20 minutes before I suddenly realized I'd been hearing tern vocalizations for several minutes and looked up to see my lifer Elegant Terns settling onto the saltmuck.

Elegant tern is well known to have a lot of variability in the bill, though this one definitely emphasizes the long, relatively narrow bill with fairly typical coloration. An article from the San Diego Atlas project notes that an Elegant Tern's bill is about 5 1/2 times as long as it is thick whereas Royal (and Caspian) have bills about 3 - 3 1/2 times as long as they are thick.

The bill heft was probably the most helpful mark for me. The bird on the right, and the front-most of the birds on the left, are clearly Elegants. The juvenile is a Royal Tern. The rear-most bird on the left (I'm pretty sure) is a Royal Tern based on the bill. Once it reaches full winter plumage it will have even less black on the head, currently its crown is fairly comparable to the Elegants:

The pinkish hue to the underparts was also more pronounced than I've seen before in terns (I've never seen a roseate), and is apparently fairly unique amongst the larger terns.

Here's a juvenile. These have a yellower bill than the adults. I think I over-cropped this image for the blog. On the original there's a nicer symmetric X formed by the reflection of the spread wings:

The bird on the right is the one that stimulated me to write this blog as I did a fair amount of reading on the web. I'm sure glad I saw it in San Diego and not at Tiscornia.
I doubt this bird would be accepted as a certain Elegant anywhere but in the core range (and maybe not even there?). There are a number of things that make it a significant outlier from the others that I photographed. The first thing that pops out is the all yellow bill, yellower than the juveniles that I saw, despite being at least a year old given that it has a nearly all-black cap. It's clearly at a different point in its moult than any other Elegant I saw. No other tern had a nearly all-black cap, the rest were all nearly to winter plumage. Furthermore, its mantle and wing coverts are much rattier than the other birds which had very clean plumage. The legs are a different color. If this bird were at Tiscornia (or on the Atlantic Coast), the possibility of Cayenne Tern (from South America) or Lesser Crested Tern (from West Africa) always gets raised on the listservs. However, per Paul et al, 2003, the Cayenne is the size of Sandwich tern, so would be noticeably smaller than the Elegant next to it. Lesser Crested Tern apparently has a gray, not white, rump and tail so I think both birds are ruled out. Monroe, Jr, 1956, reported that by August 14th, 95% of the birds were in complete winter plumage. It doesn't say, though, whether the other 5% are close or if they're way behind like this bird. These photos were taken on August 8th. A gem of a paper from about a hundred years ago describes leg color of Elegant Tern as ranging from sepia to black, usually splotched (especially on the underside of the webbing) with yellow or salmon similar to the bill color. (It also reports that some of the specimens were attracted into shotgun range by repeatedly lobbing an already dead willet into the water. Apparently Elegant Terns don't pay much attention to the typical feeding habits of willets).
My best guess is that this is a one-year-old bird that's for some reason way behind on its moult. I don't have the Olsen and Larsson big tern book or the waterbird Pyle however.

Finally, one last comparison of Royal Tern on the left and Elegant Tern on the right, showing comparison of size, bill shape, and the typical amount of black on the crown.

Since I couldn't get blogger to link directly to papers found in the Sora database, here's the other citations I used:
author unclear. Notes on the Elegant Tern as a Bird of California. Condor: 1920's?
Monroe Jr, BL. Observations of Elegant Terns at San Diego, California. Wilson Bulletin: Vol 68, No. 3, 1956.
Paul, RT, and AF Paul, B Pranty, AB Hodgson, and DJ Powell. Probably hybridization between Elegant Tern and Sandwich Tern in west-central Florida: The first North American nesting record of ELegant Tern away from the Pacific Coast. North American Birds: Vol 57, Num 2, 2003.

1 comment:

Jerry Jourdan said...

Nice writeup, Matt! I learned a bunch!