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.
References:
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.
No comments:
Post a Comment