> Some birds also
> appear to segregate in the winter according to age. Almost all records of
> Swainson's Hawks (Buteo swainsoni) from the Pampas of Argentina are of
> juvenile birds, the adults appear to stay to the north, west or south of the
> Pampas.

When chasing a "radio-tailed hawk" a few falls ago in Central California
I had the opportunity to explore (rapidly) a big chunk of that part
of California (including, totally coincidentally, a monument marking
the spot where James Dean crashed his sportscar, paid for by a rabid
Japanese fan in the 1970s, but I'm getting away from birds here,
aren't I?)

OK, back on topic. The hawk was lost...lost by the previous trackers,
I should say, and we were scooting everywhere trying to get this
redtail back in our (radiotelemetric) sights. Spent a lot of
time in the valley, and a lot of time up in the hills to east
and west trying to get a bit of line-of-sight altitude. Many
interesting things happened, including a lifer (yellow-billed
magpie), an unexpected sighting (SCREECHHH....a roadrunner! I
was unaware they occured that far north in California), and
road-killing an unfortunate male kestrel that was feeding in
the middle of a gravel road on the wrong side of a blind turn
that I was cornering in four-wheel drift mode. Felt bad about
that. It wasn't even banded so no good came of it at all. Have
to admit I was glad it wasn't a cow, though.

Never found the redtail, but in our three days and several hundred
miles of criss-crossing everywhere a pattern became noticable:

The adult 'tails were all up in the hills. The hills surrounding
the valleys are largely oak (when they aren't houses, etc) and the
adult redtails apparently prefer that habitat. The kids were all
down in the valley, where conversely there were almost no adults.

We imagined it to be a dominance thing: "I'm an adult, and dominant,
and I'm staying up here where the jack rabbits and cottontail are
plentiful. You, child, get thee down to the voles and other little
shit living in the cultivated fields".

It may also be that the adults are just better skilled at taking
larger prey and that the juvies drift to where they can catch
smaller, easier prey.

Anyway, the age distribution was marked. I have no idea if it
is maintained over the course of the winter.

> I do have the reference to that paper (Jaramillo, A.P. 1993.
> Wintering Swainson's Hawks in Argentina: Food and age segregation. Condor
> 95: 475-479. - How's that for blatant promotion of your own work!).

Pretty damned sly, I'd say!

- Don Baccus, Portland OR <donb at rational.com>