This gull was part of an unprecedented invasion of larids on the rivers of Pittsburgh, Pennsylvania in early 2007. An
extended bout with temperatures far below normal in February froze lakes throughout Pennsylvania in short order, leaving the
rivers of Pittsburgh as one of the few patches of open water anywhere in western Pennsylvania. In similar situations like
this in recent years, a few hundred to a few thousand gulls have sometimes be found on Pittsburgh's rivers,
but the event of 2007 set several records and brought rare gulls to the ice-bound rivers of Pittsburgh in numbers never
before seen. Though we don't know for sure, the assumption is that these February gulls are genenerally northbound, and upon
making their way into Pennsylvania are forced onto the only reasonably open water they can find... the head of the mighty
Ohio in downtown Pittsburgh.
In 2007, at least six Iceland Gulls were identifed between Feb 8 and 18; only three had ever been found here
before 2007. Two separate Glaucous Gulls were present between Feb 8 and 18, the first two in Pittsburgh's history. Lesser
Black-backed Gulls and Great Black-backed Gulls, both rare here but the former much more so, were also recorded.
On Monday, February 12, I and few others discovered a very dark Kumlien's/Thayer's-type bird, which shows many of
the "classic" marks of a first-winter Thayer's Gull. It proved difficult to pin down and for the first few days it was
only seen near dusk, and distantly. Finally, on the 16th and the 18th, it was discovered well enough before sunset to allow
for some photos, which are shown below.
In most respects this bird looks fine for a Thayer's. In flight, it shows a dark bar along its secondaries, and
its outer primaries have prominent dark leading edges. From underneath, the primaries have a dark trailing edge. When
at rest, the bird shows very dark brown primaries with thin pale fringes, and dark, mostly unmarked tertials, both of which
are considerably darker than the rest of the bird's plumage. Overall, the bird is an even medium brown. Structurally,
the bill and head also both appear acceptable for Thayer's.
|Possible Thayer's Gull -- Upperwing detail in flight
|Possible Thayer's Gull -- Underwing detail in flight
However, this bird's mantle is a mystery. The mantle feathers each seem to have a dark subterminal bar, making the mantle show an
overall "wavy bar" pattern that is usually associated with Kumlien's Gull, rather than more checkered or mottled
appearance typical of a Thayer's.
Digging a little deeper, it can be seen in these photos that the tertials are not completely unmarked. Some of them do
show small markings along the fringes, which, if extensive enough, would indicate Kumlien's Gull. The bird overall also appears
to be at the pale end for a Thayer's. However, each of these characters seems to be within the safe range for Thayer's, and
probably are not a major concern.
The question is, considering everything that appears correct about this bird for Thayer's, how much of a concern is this
Kumlien's-like mantle pattern? Is it completely outside the range of variation of Thayer's, or is this a acceptable? Another
way to ask is this: if this bird showed up in California, would it stick out among the crowd and cause a stir?
|Possible Thayer's Gull -- mantle and tertial detail
|Possible Thayer's Gull - mantle and color detail
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