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Back in the Skua again

February 14, 2008

As promised, fresh with Steve Howell’s article from the Finnish super birding mag Alula on my brain I headed back into the NC science museum’s collection of jaegers with a new tool, and a new attitude. Howell, with his vast experience, thinks he’s cracked the jaeger code and with his paper in front of me a grabbed a few of the birds in the collection and put together this little primer.

The thing about jaegers is that young birds are almost completely pelagic until they become adults, so molts take place at sea where it obviously would be impossible to follow the birds to observe the plumage. While there’s no way to confirm it, Howell suspects that the jaegers age cycles are similar to those of large terns. It just so happens that I looked at terns, and found the large ones, like Royals and Caspians, to be pretty difficult, especially determining the different between third year and adult birds. I admittedly haven’t delved that deeply into the birds yet, but to illustrate a few of the points that Howell makes a grabbed three Pomarine Jaegers that seemed to closely match the first three cycles.

Ageing jaegers is perhaps most easily done by looking at the underwings, with younger birds showing distinct barring on the coverts in addition to the back and tail. This is pretty consistent with other Larids, but especially terns where several species have a scalloped appearance from the buffy tips to the body feathers. On the second picture you can also see that there is no indication of a cap on the head either, the head is largely uniform in color. The pictures below are from an individual that is pretty clearly a first year bird.

On this second year bird below the back is still scalloped as the bird has yet to go through it’s first body molt, but it has lost the similar pattern on the wings following the first primary molt. The head also begins to show the beginnings of a black cap and the golden-buff nape that is classic jaeger.

This last bird is either a third year bird or a winter adult. The cap and golden cheek are well-defined and it has completely lost the scalloped pattern that we saw on the previous individuals. But this is a plumage we can find easily enough in field guides.

The tails are also a excellent indicator of jaeger age. Jaegers are known for the elaborate central retrices that they grow in the breeding season. These grow not only gradually longer, but thinner, as the bird ages. The pictures below show the birds in the previous picture side-by-side increasing in age from left to right. Note not only the increase in length of R1 but also the decrease in the amount of barring in the upper coverts.

The barring is also evident on the undertail coverts as you can see in the same birds below (this time from right to left). Likely less obvious in the field but well defined on specimens is the variation in lower leg color from pale in young birds to completely dark in adult birds.

So there’s a fair amount of work here to pick out those birds that seem to straddle some of these traits. It’s certainly made simpler by these individual birds what were prepared with their wings spread. I can’t begin to say how much easier this makes this particular project. Unfortunately, most of the birds in the collection are not spread wings. So if all of this commentary does only one thing, I hope I can convince all future bird preparers to spread their jaeger wings! If so, it will all have been worth it.

More to come, including the possibility that everything I’ve written here is wrong.


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