Tuesday, December 1, 2009

Arctic Tern

An Arctic Tern (found by Kip Miller) has been spending the last few days at 3 Oaks. It was bright enough today that digi-scoping was sort of effective, though I'm not great at digi-scoping flying birds (admittedly however, that's why God created the SLR).

The main ID feature for a first cycle bird are the bright white secondaries; a Common Tern has darker gray there. The cap is a little more extensive (almost giving a Franklin's gull like facial look), the bill a little thinner, and the black edging on the primaries much less extensive than a Common Tern.
A couple of my series of shots suggests a gap in the secondaries, I'm not convinced this isn't just from feather mis-arrangement however. Kaufman Advanced Birding states that primary moult in Arctic terns occurs exclusively on the winter grounds; nothing is said with regards to the secondaries.

Here's the bird perched; the short legs are unfortunately hidden. Tim and I discovered today that one of the structures it perches up on is easily visible from Schwark Rd when we were driving away and saw it from the truck.
Note the difference in the primaries' gray shade in the above pic when in oblique sunlight and with them shaded below. Apparently tern primaries have a powdery bloom which slowly wears off causing the feathers to darken as they age.

As the bird dip-dives downward orangey feet were revealed. The bill has essentially lost the color on the lower mandible which Sibley shows in a fresh juvenile. However, a reddish color is apparent especially at the lower base when the bird has its bill ajar while preening.
I spent a little time on the searchable database, which revealed 18 records. A definite pattern exists, all 19 records (counting this one) come from corner counties of the state, Chippewa (8 records), Berrien (5 records counting this bird), Monroe (4 records), and Bay (2 records).
There are three May records; all that were aged in the summaries were adults. There are eight summer records from June, or more commonly, July. There are no UP summer records, although this likely reflects the migration survey periods and observer coverage. Conversely, all of the Monroe birds are from the summer. The summer birds are the most heterogeneous in terms of age. Some were in adult alternate plumage, presumably failed breeders. Some were in basic plumage, and at least a couple were considered subadult birds (most terns (and a lot of sandpipers) spend their first full summer on the "wintering" grounds and so don't come north to breed until they're at least 2 full years old). The fall records are almost exclusively first cycle birds no longer in full juvenile plumage. There are 5 Chippewa records from September or October, but no fall birds have been found in the LP before November. No fall birds have been found in Monroe Co at all (perhaps a bird for the radar screen of hawkwatchers?).
Clearly terns in November need to be scrutinized just like a myarchid flycatcher where Ash-throated would probably be more likely that Great-crested. That being said, the Berrien Field Notes compiled by Jon and edited by Kip, report county late dates for Common Tern of Nov 10 in 2001, Nov 15 in 2006; we had one Nov 10 this year (assuming that all of these late birds were Common, in the other years the late date is mid-late October).
At any rate, I'm looking forward to continuing to study the terns when they return next year. One thing I definitely want to check is how Common Terns hold their tails. This bird has always struck me as holding its tail somewhat creased upwards in the fashion of a grackle or the tail fins on an F-117 stealth fighter. I'll be curious to see if Common's hold their tail that way or not, and if I get to Maine as planned next summer for a family reunion check it on more Arctics.
[note: post edited 12/7/09 thanks to a clarification from Adam Byrne regarding the number of accepted records]

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