Why Do Crows Flock In the Winter?

I'm an amateur birdwatcher, which in lazy-person-speak means "sometimes there are birds outside, which I look at if I don't have to turn my head that much."  Despite a complete lack of commitment to what is evidently a fairly serious hobby, and the fact that I spend most of my time in biodiversity-free metropolitan areas, I still occasionally get to see some weird bird-related stuff, like a red-tailed hawk that couldn't take off because its foot was stuck in a pigeon carcass that I ran across back when I was an undergrad.  The latest is something I've observed for two straight winters in our not-quite-urban Minneapolis neighborhood.  Every night for the past couple of months, thousands of crows descend on the skeletal trees on our block around twilight.  They spend about an hour blackening the trees, yelling at each other, driving my cat insane, and generally adding to the postapocalyptic vibe that comes with being in the dead of a Minneapolis winter.  Once they've accomplished whatever it is they came to do (attempt to poop on every car, house, and unlucky human or animal caught outside on the block?  Discuss who to endorse in the Republican primary?  Plot the downfall of the human race?) they all fly off en masse, which is sort of terrifying in itself.  Like I said, the whole process takes about an hour, and only seems to happen in the winter.  What are they up to?  Is it as simple as trading tips on where the day's most accessible garbage cans are located or something much more sinister?  I feel like it's in my best interest to figure this one out.

This is every tree in our neighborhood right now

The first thing I learned while researching crow behavior is that a group of crows is called a "murder," which had the understandable effect of making the project seem even more urgent.  You don't generally call something a "murder" when it has only your continued happiness in mind, right?  Luckily that turned out to be a false alarm; groups of crows only got that name because they have a habit of occasionally swarming and killing what I can only imagine are the more irritating members of their group.  While that's a bit weird and unsettling, I'm not a crow so it didn't really seem like my problem.

The second thing I learned about crows is that they're incredibly intelligent, social birds that do things like construct their own tools and may even have a rudimentary language.  Whether or not that's a good or bad thing at this point is still up for debate (you know who else was incredibly intelligent, social, and constructed his own tools?  Hannibal Lecter), but it means their social behavior is somewhat more complex than, say, your basic stupid pigeon.

While crows tend to be mostly solitary, territorial birds in the summer, winter will find them congregating in "roosts" of a few hundred to a thousand or more birds at night.  The reasons for this are not entirely understood, but common sense would dictate that it's some combination of predator protection (in the winter they don't really have the option of sleeping camouflaged in a leafy tree), warmth, and congregation near a reliable food source in order to get a guaranteed breakfast-- in other words, all the usual reasons animals hang out together.  There may also be a social/sexytime component to the behavior; winter isn't really mating season, but a roost of thousands of crows wouldn't be a bad place to find potential mates if you're a single crow (interestingly, crows appear to mate more or less for life). 

So OK, fine, crows are roosting in my neighborhood.  Except that they're not-- like I said, they usually hang out for an hour or so jabbering about whatever, then all head off en masse.   This is apparently also normal, well-observed crow behavior-- crows from all over the city will come to a designated "meet-up" spot, hang out for a bit until everyone gets there, then all fly to the final roosting site, which may consist of dozens of these meet-up groups, together in a pretty spectacular display of sky-darkening feathers.  None of the crow-related sites I found in 15 minutes of Googling had an explanation for this, although it's likely a safety-in-numbers thing again; crows in a given area make the sometimes-longish flight to the roost in a group, rather than by themselves where they'd be easy prey for owls and hawks.  It's also entirely possible that they just like socializing in the evening before they go to sleep, since they apparently get settled in and fairly quiet not too long after reaching the roost proper.

So it would appear that the crows' nightly invasion of our neighborhood is just typical crow behavior and not indicative of some larger, more insidious agenda.  As crow invasions go, it's also pretty minor (like I said, an eyeballed count puts their number in the thousands) since it only represents a fraction of the final roost population; there are anecdotal reports all over the internet of American Crow roosts containing hundreds of thousands, or even millions, of individuals.  Interestingly, crow roosts are showing up more and more in urban and semi-urban areas like mine, possibly because the lack of natural camouflage and large amount of artificial light helps them spot predators more easily, or possibly just because if, like crows, you're willing to eat pretty much any damn thing, a garbage-strewn American city is basically a gigantic buffet. 

Even though this all gets filed under normal crow behavior, it does seem worth noting that crows are:

1. Congregating in groups of thousands to millions, via a surprisingly orderly and organized process, on a daily basis, for reasons science is currently unclear on
2. Making a lot of noise during said congregating, which may or may not constitute language
3. Increasingly doing both of these things in places with lots of humans

I'm not saying they're planning to kill us all, but I'm also not saying they aren't.

I'm sure they have only your best interests at heart

Thanks to Wikipedia, crows.net, Cornell University and various other sites for the information.

Here's a handy map of major known crow roosts, just in case.