I have a couple of friends from here in Missoula that bowhunt bears over bait in Idaho and on a few occasions I've gone and helped them freshen baits and did an evening sit with them. When we'd go in and add bait we dumped bread and meat scraps into the open crib then reloaded the bait barrel with dry dog food and dumped old resteraunt fry oil over it and resealed the barrel.
After recharging the bait stations we'd head back to the pickup and change into clean/scent free hunting clothes then head back to the bait for an evening hunt. When you approached the bait again (probably 1-1 1/2 hrs later) there would be an insane number of ravens on the open bait pile in the crib. 20-30 ravens would probably be a conservative number, they were never noticable in the area when we packed the bait in..
They would all fly off when you approached and climbed up in the tree stand. Once we were in the tree and things settled down, not one of those ravens would come to the bait again. They would all sit in the tree's 50-75 yds away in a complete circle around the area and sqwak a bit and watch the bait but would never come near it. It was usually 2-3 hours til dark and regardless of how still and quiet you were, they just knew you were still in the area and wouldn't come near the bait..It was still enough that the bears would sneak in cautiously but not one of those ravens would come down for the rest of the evening..