Wednesday, January 16, 2013


Four winters ago, I was first treated to a sight that I have come to be very fond of: hundreds of crows raining down into the forest behind my house.  That winter and the winter after that, this tremendous gathering of crows spent all night (or so I thought) roosting in my backyard or close by.
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.

So, the woods behind my house weren’t used for roosting after all.  What was going on?  Were the crows gathering just before sundown only to disperse again?  I didn’t think so, because when they all few off, they did so in one direction and with purpose.  I thought that the woods around me, instead of being a real roosting area, were a kind of “staging grounds” where all of the crows coming from a certain direction, a place for a flights of crows to gather, regroup, and fly toward the actual roost or even another “staging area.”
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.

            Every five to ten minutes or so, all of the crows, every last one of them, would suddenly rise out of the trees and instead of heading off to the roost, fly off in a single direction and then circle two or three times and land again.  I have come to nickname such an occurrence a “vortex” for the powerful swirling motion it creates.  It is absolutely amazing, sometimes even dizzying to watch hundreds or thousands of crows take flight all at once and land again, often on the same branch they were perched on before (Good 1952).
"Vortex" at a preroost.

            A true roost site is where flights from several preroosting sites converge to stay throughout the night. Compared to the preroosting sites, roosting sites usually hold incredible numbers of crows.  Un like a preroosting site, where most birds fly in from one or a few directions, crows fly into a roost site from all directions (Good 1952).

Crows flying in from all directions toward the roost site.

            About a week ago (January 6, 2013), my mom and I followed crows leaving a preroost site in the car.  We zigzagged through town until I had the luck to come up on the true roost site of the Lansing-area crows.  It was spectacular!  There were thousands of birds in the air, perched in trees, on buildings, and on the ground.   When they rose up into a “vortex,” they dwarfed any vortex that I had ever seen at a preroosting site.  The crows just kept streaming in in huge numbers.  At the roost site, I was able to count 11,100 crows, although there were definitely much more than that.  There were so many crows that sometimes it seemed that the noise was deafening.  The only way to communicate with my mom was to shout!

See all those black specks in the trees?  They're all crows.  This is just a small portion of the entire roost.

           Another notable aspect of this phenomenon is the behavior of the crows as they are about to land.  Some veer up and circle around before they land.  Although this usually occurs during a “vortex,” a bird will sometimes veer up and away to circle around for no apparent reason.  Usually, a crow about to land goes into a stoop, diving headlong towards the middle of the roost, and going into a glide when it presumably knows where it will land.  In the photo below, the birds labeled "A" are in a stoop, the birds labeled "B" are gliding, and "C" has veered off and will circle once or twice before landing (NOTE: the perspective in the photo is warped so that it looks like the birds are going up, but they really are going down).

          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.

            Although I have attempted to use photographs and words as a means to portray the wonder and sheer size of crow roosts, neither pictures nor words can do a crow roost, or even a preroost, justice.  To really experience it, you truly have to be there for the real thing.


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.

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