> >So, an immy eagle on the ground signifies "no food"? If "food's here!"
> >were the selection factor driving the contrasting plummage I'd think
> >juvies would share it.

> First of all I should have said that an "adult" eagle on the ground "often"
> signifies food. After all, eagles taking a bath for instance, are also at
> ground level. But to get back to the point I was trying to make; It is my
> understanding that immature plumage possibly evolved to signify lack of
> experience in a younger bird.

Dominance appears to be attached to adult plumage for redtails, at least.
I don't know if this has been extrapolated (by research) to bald eagles.

Dominance then translates to possession of a breeding territory.

This fits in with the tolerance of immy/adult eagles/'tails on wintering
grounds.

Though there appears to be evidence that adult redtails hog better
wintering territories, too, at least in milder climes. In Nevada,
there are vast communal roosts of all ages of RTs (sound like winter
baldies?) but they may still feed differentially.

> I don't think that an immature eagle on the ground signifies "no food" per
>se, but that it sends a different message to other eagles than an adult on
>the ground.

Well...data would be good. Another argument would be that adults are
more skilled at finding food, thus spend less time looking, and more
time sitting in trees, or the ground, or whever, hanging out and
being lazy raptish things. I know from banding migrant raptors (not
baldies, but most else) that kiddies are thin, and will come in
incompetently but reliably to a trapping station. A much less
percentage of adults will come check out the bait, though those
that do are more skilled at nailing it.


> My experience with mud-gathering Cliff Swallows is limited to Umtanum Canyon
> (eastern Washington) where, like Don describes, out of colonies of hundreds,
> dozens of swallows gather along the bank of the river. The "mud hole" is not
> an obvious darker hole contrasting with paler soil around it. Perhaps my
> observations in this single locality influence my interpretation too much.

Yeah, I was just out a Malheur and marveling at how deep a hole they were
digging, and at how different it looked (black vs. dust) than the surrounding
terrain. It's been a wet year, and I was there when it first got hot,
and things were starting to dry up rapidly.

>Does the tail movement show & hide the
> rump patch? Or is it indeed simply a ways to keep it clean?

My slides show muddy cliff swallows, mud smeared about the head and
mouth, though not the rump. I get the impression, though, that
mud-gathering outweighs cleanliness (and the later energy expended
to re-claim it).

> Perhaps the placement/anatomy of the legs & feet and the total length of the
> legs and possibly distribution of weight is different enough in the two
> species. According to Godfrey in The Birds of Canada, tarsus length for (adult
>
> male) Cliff Swallows is 12-13 mm with a 12.3 mm average. For (adult male) Barn
>
> Swallows 10.5-11.5 mm with an 11.3 average. That doesn't strike me as a
> significant difference in tarsus length.

I think social explanations make more sense...the rape-avoidance argument
seems tittilating, if nothing else.

>Do you suppose the wing quivering is a way to ward
>
> off forced copulation? But then one would have to assume that most, if not all
>
> birds seen gathering mud are females...

But they aren't (visually) sexually dimorphic, so perhaps the rapers are
less concerened with the sex of the rapee as they are with getting (ouch)
in and out quickly. Perhaps being an aggressive rapist outweighs the
benefit of selecting the right sex?

All in all, this is fascinating stuff.

- Don Baccus, Portland OR <donb at rational.com>
Nature photos, on-line guides, at http:://www.xxxpdx.com/~dhogaza