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Winter Crow Invasion!

Do you have a metric tonne of crows flooding into your neighborhood/town/city each night?  Do you feel like the crow population is getting out of control?  Are you worried that Alfred Hitchcock’s “The Birds” is about to be a reality?  Do not fear!  What you are experiencing is perfectly normal winter behavior for crows!  In this post we will discuss ROOSTS!

Thousands of crows coming in to roost for the night.

What is a roost?

A roost is a place where birds sleep.  In the context of crows, a roost usually consists of many birds gathering in one location, at dusk, to sleep for the night.  In the winter American crows (Corvus brachyrhynchos) in particular gather in the hundreds to thousands to even millions, in some exceptional cases, each evening.  It’s quite a sight to see and might be a bit disturbing for anyone unfamiliar with this winter behavior!  However, the behavior is just that, a winter one and is therefore temporary.  Crows form massive roosts from November (at the earliest) to about March, when they go back to their home territories to start the nest-building and breeding process.

Why do they gather in such large roosts?

We don’t know for sure, but it’s something American crows have done forever.   One hypothesis as to why they gather like this in winter, is for safety.  One of the main predators for American crows is the great-horned owl.  With longer nights in the winter, having a large group to sleep with after dark might help your chances of not being attacked by an owl.  Having so many other birds means there’s a greater chance it will be a different individual that gets attacked, and also means you have more birds that can detect a predator and send up the alarm to everyone else.  Other hypotheses involve possible information exchange, about foraging sites, but it hasn’t been studied in crows, and many crows just return to their home territories during the day.  It could be that visiting crows (that may have migrated from the north, for the winter) might find out about good foraging sites, but it’s probably not a large contributing factor to roosting behavior.  Finally, roost sites may be conveniently close to good foraging sites, and therefore a good place to sleep.

Crows coming in over an industrial area.

Why do they gather in my town/city?  And not the woods?

I’m sure there are roosts in the woods that people don’t know about, but the biggest conflict with roosts and people are in towns and cities.  Ever parked your car under some trees filled with thousands of birds?  Ever walked under thousands of birds?  Neither the car, nor yourself, likely come out without a fair deal of poop covering you.  Roosts are also quite noisy before the birds all settle down to sleep.  And even then, if they get disturbed in the night, they all sound the alarm, which is quite loud coming from thousands of already-noisy birds!

There are no studies saying exactly why the roosts tend to be in towns, but one hypothesis is that the crows like the light.  Crows see about as well as us humans at night, and the added light might help them spot and evade the dreaded great-horned owl a bit easier.  Great-horned owls are also less abundant in towns.  Urban areas are slightly warmer than rural areas as well, which is an added bonus in the cold winter.  Thirdly, most urban areas prohibit the discharge of firearms, which protects crows from being shot.  Urban areas may also provide better actual sleeping sites, such as taller or specific kinds of trees.

In some cases a roost may have been near a town before the town was even built up, and then the town itself became suitable for roosting (due to the reasons I just gave).  In the case of one roost the Crow Research Group studies, the roost has been documented in the same rough location for over 100 years.  It used to be three miles south of the town, but is now in the middle of downtown, and it likely moved due to the reasons I stated above.

Crows gathered in a group of tall trees.

Doesn’t a roost of so many birds indicate that crows are overpopulated?

No, and there are a couple of reasons why.  Remember that I said crows have been doing this forever?  Well, even if there were only 10 crows in a 100-mile radius, they would come together in the winter to roost.  Additionally, some crows migrate.  In the winter here in New York we tend to get migrant birds down from Canada that join our roosts and communal foraging sites.  So the number of crows in a roost is NOT indicative of the population in a particular area.  The roost contains birds from many miles around and possible migrants.  In the case of roosts further north, they may see fewer crows, as many of theirs have taken off to warmer climes for the winter.

There’s a roost in my town or nearby, what should I do??

Go watch them an enjoy yourself!  Seeing such large aggregations of birds is thrilling and you get to see all sorts of great flights, behaviors, and hear a variety of sounds.  It’s quite comical watching crows vie for space on a branch in a particularly over-loaded tree.

I say bring revenue into the town by having a crow festival in the winter!  Do crow-themed foods, crafts, attractions, competitions, etc.!  Make an eco-tourism site and bring in the money as well as educate people about crows and how wonderful they are!

Is there current research on roosts?

Glad you asked!  The Crow Research Group is trying to get some funding via Experiment.com (crowd-sourcing) to do some movement and roost research!  My colleague, Ben Eisenkop, is interested in how these large roosts affect nitrogen cycling in local environments (since crow poop is chock full of nitrogen).  If you would like to contribute, please click the following link! (The fund raising is over, but please still visit the link to learn more about crow roost research!)

What are the patterns and effects of American crow movements?