In personal correspondence, Charles and June Maginley of Mahone Bay, Nova Scotia, mention a large gathering of crows which occured for several winters in Sydney, Nova Scotia.

CROWS! When we lived in Sydney I was fascinated by their behaviour. Each winter ALL the crows in Sydney would congregate at night in a roosting area where there were large trees. This area changed each year....

At dawn all the crows would take off from the roosting area and great clouds of them could be seen crossing the harbour. As evening approached they began to gather on the ice (Sydney harbour is frozen over from January to April). We had a good view of them from our house. They would settle in two or three groups of hundreds of birds; thousands in all.

The groups were always elongated ovals, not round. They did not appear to be making much noise. Some birds would take off, others join and a few go from one group to another, but each patch of birds remained about the same size. A channel of open water or thin recently frozen ice, made by ships, went up the harbour and the crows were near it, but not right at the edge. At sunset they would all take off and fly to the roosting area.

Those roosts, with the accompanying "flocking" behaviour, afford protection against predators such as the snowy owl, or red tailed hawk, and maximize the discovery and exploitation of food sources. Also, flocking tends to reinforce the altruistic qualities of the crow. Corvids, in general, are known for acts of altruism towards their own kind. There are local stories and legends of crows supporting, defending, and coming to the rescue of other members of the flock who have gotten themselves into precarious situations. They are also very protective of the young, baby crows. Charles and June Maginley conclude their letter by noting,

In an earlier house, a pair of crows nested near and eventually the young one got out of the nest and was being fed on the ground. It could manage short hops. (This period of vulnerability is probably the only thing that prevents the world from being overrun by crows.)

The parents, and last year's fledgling guarded it making a big row all the time. We had a small black kitten which was out prowling in the yard, not actually hunting the chick; but the parents started to swoop down on it. The sight of that black kitten streaking for the safety of the house with a crow zooming a foot above it was something to remember! Eventually the young crow made a big hop across the road and the commotion went over there.

I haven't had the opportunity to witness crow parents protecting their young in this manner, but I have seen crows come to the rescue or defence of a comrade who was having a flying confrontation with a hawk. The combatants were flying almost parallel, making passes at each other, when a second crow appeared on the scene, swooping down at the hawk. The latter, being outnumbered, quickly sought the refuge and protection of a maple tree nearby.