This sequence is from a complete roll of film showing posturing/vocalization. The crow is about 10 feet away from me, responding to my (poor) attempt to mimic the rattling sound.
Note that the crow's neck is normal - that is, there's no obvious evidence of food temporarily stored in the throat sac.
The neck area starts to thicken as air goes into the sac and the head is bent downwards.
The head and beak are at about the final 'intake' stage and the throat sac is quite full
The beginning of the 'release stage as the beak opens to allow air out. Note that the neck is still 'fat'.
Half-way through. Note the neck sac is now much smaller.
Just about finished, the beak is almost now closed.
And it starts all over again.
"In courtship on ground or in tree, male faces female, fluffs up body feathers, partly spreads wings and tail, and bows repeatedly while giving a short rattling sound."
According to Kenn Kaufman, in "Lives of North American Birds:
For whatever reasons crows make this wonderful and strange sound, you'll know you're hearing something unique.
There are times that this distinct behaviour can be seen and heard. Crows occasionally have 'gatherings' where numbers can be a few hundred to thousands; if you are near one of these, you'll suddenly see crows converging from every direction. At these times, it seems the local crow group or groups suspend territorial rights. I have seen crows apparently practicing this vocal sound, individually and in small groups. Since the 'bowing' process is obvious, it's easy to locate individuals and approach to listen.
This is where it becomes obvious that crows are practicing this; it's like a parrot first attempting to learn a phrase or a tune. It's frequently garbled until eventual practice and experience produce that unique dry 'rattle'.