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One good tern deserves another and another and another…

October 10, 2007

Here’s an update on how my museum bird aging project is going. There’s no great insight in this post on bird identification, just some interesting things I’ve noticed with regard to tern plumages. North Carolina has a long coastline, and therefore a lot of terns, and when they die and people find them, many end up at the museum. So the museum has an excellent collection of many of the terns that spend time along the Outer Banks. It seems odd for me, who only got my life Bridled Tern last month, to see drawers upon drawers of specimens of that very bird, most collected in the very place where I had seen mine. Hardly a rarity then.

Aging terns requires a keen eye for very subtle variation in plumage. Most bird books only show a small bit of this, usually a juvenile, an immature bird of indeterminant age, and a typical adult. Like gulls, terns can be two or three year birds with several plumages before sexual maturity with the onset of what we know as “adult breeding” plumage, called Definite Alternate. Bridled Terns are pretty typical in this regard. You can see the examples of the birds of three ages to the right. A juvenile bird, showing the white tips body, wing and tail feathers giving a scalloped appearance, a first year bird in Basic I bird one year old with white tips limited to the back, and a typical adult bird in full alternate plumage with solid gray back and nice clean black cap. Most of the Bridled Terns in the museum collection were Basic I, birds who are in their first full year of life and most likely to disperse up the Gulf Stream into North Carolinian waters. This was the plumage of the individual I saw on my pelagic. They’re the most common museum birds because they’re also the most likely to have difficulty finding food, starve and wash up on the beach. For the same reason, the museum’s collection of Sooty Terns, another species I looked at, is nearly entirely of juvenile birds.

Bird that breed on the NC coast are also well represented and also pose their own plumage problems. After I had finished the Bridleds and Sooties I moved on to Sandwich and Royal Terns, both rather common on any NC beach. Sandwich in particular is a very tough bird, as most of the plumage knowledge is from the European subspecies sandvicensis, and less from the North American one, acuflavida. Whether this is because the European version is better studied or because the American version simply doesn’t show the same immature plumage characteristics (possibly indicating they’re actually separate species) I couldn’t find out. In any case it was more than a bit confusing, especially as I found some birds that seemed to best fit the European subspecies.

Royal Terns were more straight forward but they typically molt into Basic (winter) plumage very early. In the collection were several birds that were collected as early as late June who had lost their black crown entirely. The birds to the left for instance, are both adults collected one month apart to give you an indication of how quickly they go from summer to winter. Add this to the 2nd summer birds who never completely grow into a full black cap and you have some birds that are very difficult to figure out.


I’m working on Common Terns now, as you can see to the right. They’re more complex than Bridled and Sooty but not as tough as Royals and Sandwich. Mostly because adult birds always have an orange bill and juvis and winter adults never do. I’ve had a couple birds which were obviously immature birds (2nd year) by date collected (always a nice tip), but by and large the birds in the museum’s collection are adult birds in Definite Alternate (adult breeding) plumage. I should shoot through the Commons in the next week, but then I get to work on the Black Terns, an altogether tougher species. More on that in a future post.


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