Dear Cobirders,
I hate using all caps, but ...
Today (12/25) from 2:00 PM until 2:30 PM, I observed an adult Black-tailed Gull
at Thomas Reservoir in Boulder County. I was able to observe the Gull
unobstructed for at least 25 minutes at a distance no greater than 400 ft under
ideal lighting conditions. The Gull flushed numerous times showing the extent
of the black on the tail as a dark morph Ferruginous Hawk harassed the birds at
the reservoir. The Gull was in the company of nearly 2000 Gulls, mostly
Ring-billed with about a dozen Herring and at least 1 California Gull with whom
the Gull dozed on the ice. The characteristics I observed that led me to the
conclusion of Black-tailed Gull are:
* an adult plumaged Gull (uniform mantle and upperwing covert color, white
(little, but present) on the primary tips) combined with a wide black terminal
band and white uppertail coverts
* overall size, larger than adjacent Ring-billed Gulls and similar to the
adjacent California Gull.
* dark mantle color, slightly darker than the adjacent California Gull (I
initially thought the Gull was going to be a Lesser Black-backed Gull)
* yellow legs
* a black ring around the bill extending from the gonys to nearly the bill tip;
this ring became duskier at the tip. There was a faded red spot on the lower
mandible, posterior to the black ring and near or on the gonys.
* strong, white scapular cresent and tertial cresent
* very small white tips on the end of the primaries
A few more observations, which don't fit adult Black-tailed Gull so nicely:
* dusky iris
* more extensive streaking on the nape than in most of the photos in Howell and
Dunn's "Gulls of the Americas"
The streaking on the nape was quite extensive, similar to photo 21.2 and 21.15
in Howell and Dunn's. However, I feel that the streaking extended more
anteriorly from the nape than photos 21.2 and 21.15, although the bird
maintained a clean white forehead and crown, consistent with photos in Howell
and Dunn.
The iris was dusky, similar to Thayer's Gull, not dark black like a California
Gull. This is unlike all of the photos and description in Howell and Dunn, but
is similar to some of the photos on this website
(http://www.surfbirds.com/mb/Features/black-tailed-gull.html). That website
also shows some Black-tailed Gulls with more extensive streaking, more like the
bird I saw today.
Hopefully this individual will return to Thomas Reservoir tomorrow afternoon.
Good Chasing,
Walter Szeliga
Boulder, CO
--
Colorado Field Ornithologists: http://www.cfo-link.org/
Colorado County Birding: http://www.coloradocountybirding.com/
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