Many thanks to Melanie Lynch for pointing out that the official term for a 
multitude of turkeys is a rafter of turkeys.

And the origin of Murder of Crows appears to be as follows:
“A ‘murder’ of crows is based on the persistent but fallacious folk tale
that crows form tribunals to judge and punish the bad behavior of a member
of the flock. If the verdict goes against the defendant, that bird is killed
(murdered) by the flock. The basis in fact is probably that occasionally
crows will kill a dying crow who doesn’t belong in their territory or much
more commonly feed on carcasses of dead crows. "

Now to find out what a gathering of birders is called.

Jim Wilson