Linnea Saukko deserves congratulations for looking closely at the variations
in American Robins in a central Ohio neighborhood and for posting comments
on this yesterday.  One of the best things that birders can do to improve
our skills at identification is to increase our understanding of individual
variation in birds.  For several years I was teaching a series of intensive
five-day courses on bird I.D. at various points in the US and Canada, and
one of the exercises that we did was to find a flock of some species of bird
and start looking closely at individual variation.  Without fail, we would
find that no two individuals looked exactly the same.  This seems to be a
universal phenomenon among birds:  if we look closely enough, no two
individuals look exactly alike (and it follows that no bird will ever look
exactly like the picture in the field guide!).

Part of the variation in American Robins is owing to age and sex
differences.  At this season the adult males (more than a year old) tend to
be the darkest and most richly colored, while the young females (hatched
just last summer) tend to be the palest and drabbest.  Males tend to have
blacker heads than females, while adults tend to have larger white tail
spots than young birds.  But individual variation is so major that even with
a good look, we can only guess at the age and sex of of a given bird.

Some of the variability in American Robins seen in Ohio could also be owing
to geographic variation, with birds originating from different populations,
but this is very hard to prove for any given individual.  The phenomenon of
subspecies is widely misunderstood, partly because it's an artificial
category.  Any given individual either is or isn't an American Robin -- the
definition of that species is pretty clear -- but you often can't assign an
individual to a particular subspecies.  Here's a way to look at it.  If you
leave Ohio in early summer and drive north into Canada, by the time you get
to the coast of Labrador you may notice that the American Robins there tend
to look a little darker, and some of them are quite blackish on the nape and
back.  Most technical references will call this a different subspecies (T.
m. nigrideus) from the one common in Ohio (T. m. migratorius).  But on the
drive north, you will not have noticed any abrupt change from one subspecies
to another; rather, it's gradual and clinal.  (And some variants of our
local American Robins can look very black-backed, like the Labrador birds,
so identifying this subspecies out of range is very difficult.)  Likewise,
if you drive south from Ohio in early summer, by the time you get to the
Gulf Coast states, you may notice that the American Robins there tend to be
slightly paler and very slightly smaller.  These are usually called a
different subspecies (T. m. achrusterus), but again you wouldn't notice a
sharp division between the subspecies, because the change is gradual and
clinal.  (Some past authorities have said that T. m. achrusterus gets into
southern Ohio, but it's a matter of opinion, depending on where you draw the
line in the middle of a gradual cline.)

So what does it mean when we see variation in American Robins in Ohio?  If
we see an obviously mated pair in summer, the one with the blacker head and
richer rufous chest is almost certainly the male.  In a winter flock, it's
almost impossible to be certain of what the variations mean.  But it's still
extremely worthwhile to study the individual variations in common birds.
It's a great way of increasing our total understanding of birds, and
increasing our chances of spotting those that are actually of different
species.

Kenn Kaufman
Rocky Ridge, Ohio

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