Arctic Tern at Dashields Dam, Allegheny County
On Saturday, May 29 I was shocked to discover two Arctic Terns passing Dashields Dam in the 5pm hour. Fortunately for
me, I had my camera already handy from a failed attempt to photograph Spotted Sandpipers and I was able to make about 30 in-focus
images of the pair before they disappeared downriver into Beaver County. I won't go much into the story here (perhaps, look
for it in a future issue of Pennsylvania Birds
), except to say that I was totally flabbergasted by the encounter, and that I along with at least 3 other birders were
unable to relocate the terns between Dashields and Rochester in Beaver County after I lost sight of them.
One of the birds seemed to be more clear-cut than the other in terms of the identification. These first two photos show
this individual's upperwing pattern, as well as the overall structure: generally small headed with the wings appearing to
be set far forward on the bird's body. The upperwing is basically a clean light gray, unique to Arctic Tern among red-billed
North American terns. Common Tern at this time of year, as a result of its distinct molt strategy, should show a distinct
dark wedge on the outer 5 or 6 primaries on the upperwing. Note too that the bill is nearly all red, with just a hint of a
On the photo directly above you can also see a dark thin line on the underside of the outer primaries, but this can be
seen much better in the three photos below. Note that the dark tips are very limited, forming a well defined thin line down
the trailing edge of the outer wing. Note also that you can distinctly see dark tips to the outer seven primaries, which a
limited dark tip to the eighth. Again this pattern is unique to Arctic Tern, with Common normally showing a thicker and less
organized area of dark on the trailing edge, which is also normally limited to just the outer five or six primaries. The third
photo also shows the bird in good profile, where it displays a nicely thin, tapered look, compared to the larger and bulky
look typically shown by a Common.
Of course, it wouldn't be an ultra-rare bird in Allegheny County if there wasn't at least one feature that makes you
scratch your head. Arctic Terns generally show a small bill for a Sterna tern, yet this bird shows a bill that is
actually rather large, and also dark-tipped. Though a dark tip on an Arctic is not immediately alarming, a too-large bill
and a dark tip might be. That said, though the bill on this bird does appear to be on the large end of variation (some male
Arctics can show a bill as long as the shortest Commons) I do not think it is "too big". Also somewhat odd about this bird
is the shape of the dark cap. Generally, of the North American terns that might be mistaken for Arctic, Arctic has the
"lowest slung" dark cap on the front of its face, showing the least amount of white between the bottom edge of the cap and
the gape of the bill. In the photo below, you can clearly see that the area of white below the cap is quite noticeable. It's
also interesting to note that the lower cap looks different on this side of the bird compared to the photo above that shows
the bird facing the other way.
Also a bit problematic was the second bird that passed the dam, of which I only made a few photos. As with the other
bird, it basically looks like an Arctic Tern in all respects except the bill. It has the clean upperwing, the thin trailing
line on the underwing, the thin tapered body, and the "top-heavy" look with the wings set forward on the body. But the bill
on this bird seems even longer, with an even more extensively dusky tip than the other bird. Is this in the range of variation
of Arctic? I suppose if it's not, then I don't have any idea what this bird is, since it's plumage and structure are
otherwise totally wrong for Common.
Of course, it goes without saying that this is the first record for Allegheny County.