Hello, Birders.
Thanks to Tony Leukering for exhorting us to precision about molts and plumages
and such. Tony said:
> I just have a slightly belated comment on
> Ted's post about ageing American Robins.
> Like most all thrush species, American Robins
> conduct a complete or nearly-complete molt
> in their first fall, resulting in adult or
> adult-like plumage. This means that once
> the first youngster of the year completes
> this molt, one can no longer age American
> Robins in the field as adults. Youngsters,
> yes; adults, no.
1. Robins, thrushes, and apparently all passerines molt not into an adult
plumage in their first fall, but rather into what is increasingly known as
their formative plumage.
2. In some passerines, formative plumage is dramatically different from the one
or more adult ("definitive") plumages. A familiar example for us in Colorado is
the White-crowned Sparrow.
3. In other passerines, the formative plumage is quite difficult (although
generally not impossible) to distinguish from the adult plumage(s). The Wrentit
of the Pacific slope of central North America comes to mind.
4. And in many other passerines, the formative plumage, although not
dramatically different (as in the case of the White-crowned Sparrow) from the
adult plumage(s), can be separated with reasonable study from the adult
plumage(s). This is the case with the American Robin. American Robins in
formative plumage differ from adults in the following ways:
4a. Their wing coverts show distinct "molt limits," i.e., an abrupt change in
darkness.
4b. The innermost flight feathers of their wings are often darker than most of
the secondary flight feathers of the wing.
4c. The outermost primaries (those most susceptible to the ardors of migration)
are often distinctively pointed and abraded.
4d. The flight feathers of the tail are generally pale and often pointed and
abraded.
4e. The outermost tail feathers tend to show less-distinct white spots.
4f. The birds are paler and duller overall.
4g. Some retain faint spotting on the breast through the first winter.
As robins' plumages wear down through the spring and summer, distinctions
between formative and adult plumages become muted. As early in the plumage
cycle as early January, though, the plumages are distinct and easily separable
in the field.
I'm not certain of anything, but I'm nearly certain that bird we saw in Grand
Junction was an adult.
-------------------------------
Ted Floyd
Editor, Birding
Blog: http://tinyurl.com/2g2staq
Twitter: http://tinyurl.com/2ejzlzv
Facebook: http://tinyurl.com/2wkvwxs
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