[obol] Iceland or Glaucous Gull

[Cindy Ashy]

I think this gull illustrates why it is important not to depend on just one
guide, not to depend chiefly on one particular trait to identify the gull, and
to not over-simplify each trait in question. Regarding the last point, the
presence or absense of a two-toned bill is not enough information.

Judging from the plummage, this gull looks like a first winter bird. Sibley's
and National Geographic guides show illustrations of a first winter Iceland
Gull as having a mostly solid dark bill or an entirely solid dark bill
respectively, not a two-toned bill as this one obviously has. Sibley's adds the
text, "small usually all-dark bill." NGG says, "chiefly dark bill is short and
thin." Both show a second winter Iceland Gull as having a two-toned bill and
Sibley's shows this developing in the first summer.

Olsen and Larsson have more to say about the bills of first winter Iceland
Gulls:

"...similar to Glaucous Gull, but the shorter bill has broader black tip
grading into paler base. The base may be fleshy, but not the bubblegum-pink of
Glaucous."

"Bill variable: in <2% whole bill is blackish; but normally with paler (dark
brown to grey-brown, greyish-yellow or olive-grey) based 30-45%; in <5% the
basal region is fleshy or pink. Division from black tip normally more diffuse
than in Glaucous, with black penetrating along cutting edges. Precise bill
pattern may be hard to judge according to darkish coloration of base; birds
with pale bill patterned more like Glaucous."