Michael,

What is that saying? There are no stupid questions, only stupid... (I forget
what).
Anyway, I don't know if it holds for Crows, but some years ago I read an
article on Cormorant roosts and the function in "exchange of information" (the
authors didn't put it that way). It turned out birds who had "bad" foraging
days were likely to follow birds who looked like they had been feeding well
(I'm not sure how they gauged this) to their feeding grounds.

I immediately see a problem with this concerning the communal crow roosts I
know, there doesn't seem to be a perceptible change in the number of crows
going in different directions. I. e. about 200 fly from the Foster Island area
to Green Lake each morning and then disband around the area, until it's time
in the late afternoon to do the reverse. If there was an important foraging
information exchange function for the communal roosting behavior, would we
humans be able to detect a change in directional flights?

A quick check of Lawrence Kilham's The American Crow and the Common Raven
(1989, Texas A&M University Press, College Station) didn't produce anything,
besides the suggestion that crows may feel safer when in larger groups.

See the discussion of communal roosting in Ehrlich, Wheye and Dobkin The
Birder's Handbook (p 615-619). They say that birds of different age join
communal roosts for different reasons. Older birds occupy more central
roosting positions and gain safety from that, while younger birds gain the
opportunity to follow older birds to better foraging places (very loose
summary). They also mention the thermal advantage although they doubt this is
one of the main reasons for the behavior.

Martin Muller
Seattle, WA
MartinMuller at msn.com