varminthunter06...
While there individual preferences, the concenses on them is from #9 to #7-1/2. They are hard to hit because of their aerobatics, but not hard to kill... they weigh only about a pound under all that black fluff. You need pattern density.
Many guys shoot skeet loads, 27yd trap shells, or 1&1/4 "Live Pigeon" loads, or handload the equivelent.
A 12ga with a modified choke and decent trap loads will kill them easily as far as you can hit them, but you have to get them to come in to your call... HA! That's the hard part.
6's and 4's will give you large holes and thin patterns, and with a good hold, you may have many just laugh at you as they fly off.
Go to:
http://www.crowblasters.com
... and do a search of their forum... you will get more info on shooting crows than you ever thought existed.
You don't have to be a member... just go to the middle of the first page and click on "Bulletin board"... and you can browse and search for free.
There are people on that site that have shot over 100,000 crows, and consider a morning of 200 crows just a walk in the park... they know what they're talking about!
CatShooter.