About a year ago, in December 2011, I decided to count the crows that flew in to the roost. I was shocked to find that, far from my original guess of hundreds of crows, there were actually thousands of crows flying in to roost every night! The roost peaked in February 2012, with over 10,000 crows coming in to roost. After further observation, I found that there was a problem with that statement: the crows were not actually roosting, but taking off just before sunset.
Crows near my home taking off at sunset.
After studying up on the American Crow’s roosting behavior, I found that these woods were indeed a sort of “staging area,” called a preroost. Crows from certain acreage of land take flight some time before sunset towards a designated preroosting area, with the birds farthest away from the preroost starting first. As it gets later in the day, the flight picks up numbers. Since the lowest density of crows is present in areas farthest away from the preroost, the flight gains size slowly at first, then faster and faster as it gets closer to the preroost site (Moore and Switzer 1998, Haase 1963).
Heading toward the roost.
"Vortex" at a preroost.
Crows flying in from all directions toward the roost site.
See all those black specks in the trees? They're all crows. This is just a small portion of the entire roost.
Often, a crow will gain altitude before diving and gain tremendous speed on the way back down. When this happens, the behavior is usually repeated once or twice more. Are these crows climbing to such heights just to have fun on the way down? One might argue that they are especially since corvids, among birds, are well known for their intelligence. At any rate, it is sure fun to watch their free falls.
Occasionally, two or three crows will beat their wings in perfect unison. I cannot tell if this is a purposeful behavior or a coincidence, like the stride of humans walking together. I have never seen it happen with groups of more than three birds. I have come to call this “synchronized flying” because of how perfectly in unison the wing beats are.
Finally, as a crow gets closer to the trees, it abruptly veers off from its downward course and lands on whatever branch, building or parking lot suits its fancy. Eventually, the entire flock settles down for the night, in a tightly packed group as the sun’s light completely fades.
These two were flying in perfect unison. Coincidence or not? I don't know.
Good, E.E. 1952. The life history of the American Crow Corvus brachyrhynchos Brehm. PhD Thesis. Ohio State Univ. Columbus.
Haase, B.L. 1963. The winter flocking behavior of the Common Crow (Corvus brachyrhynchos Brehm). Ohio J. Sci. 63: 145-151.
Moore, J.E. and P.V. Switzer. 1998. Preroosting aggregations in the American crow, Corvus brachyrhynchos. Can. J. Zool. 76: 508-512.