Why do crows congregate in large numbers to sleep?
One of the great animal phenomena of the world is the congregation of large numbers of birds into a single group to sleep together. Such communal sleeping groups are known as "roosts." Many species roost in groups; such things as crows, robins, starlings, blackbirds, swallows, and herons. Most do this only outside of the breeding season. Some species, like starlings, also forage together in great numbers. Others, such as herons, disperse out from these gathering areas to forage singly. For crows, roosts are primarily a fall and winter thing. Numbers peak in winter and then decrease near the beginning of the breeding season (usually in March). It appears that all crows will join winter roosts, even territorial breeding crows. Most breeding crows sleep on their territories during the breeding season, but join the roosts afterward.
Just why birds congregate in such large groups is still largely a matter of conjecture. A number of hypotheses have been constructed to explain it:
* One is that the birds simply are congregating in the most favorable spot (protection from predators, protection from the elements, the only trees suitable for roosting, etc.), and they don't mind doing it with a bunch of other birds. This idea is kind of analogous to a crowded hotel: everyone has the same needs being met at the same place, but no one is really interacting with anyone else.
* Another idea is that the birds get some protection from predators by being in a large group. This is the "wagontrain" analogy: safety in numbers. Crows are most afraid of large owls, and sleeping with a bunch of other crows could afford some protection for an individual crow.
* Another idea is the information center hypothesis, where information about profitable foraging areas is transmitted. The idea is that an individual that did poorly foraging for itself on one day can watch for other individuals coming in to the roost that look fat and happy, that obviously found some rich source of food. Then the hungry individual can either backtrack the happy ones' flight paths, or follow them out first thing in the morning to the good food source.
* Another food related idea is the patch-sitting hypothesis. This theory is similar to the first one mentioned, in that roosts congregate around a large, non-defendable, reliable food source. So, first thing and last thing in the day, food is available. It need not be the best food, but it is something to eat to get them going. The birds can then disperse out and do whatever they need to do, having had some kind of breakfast first. Roosts, then, will form in suitable roosting habitat near these large food sources. For crows, such abundant sources might be landfills, commercial composting facilities, or certain types of agricultural fields.
Crows have been congregating in large roosts in the fall and winter for as long as there have been crows. Crow roosts can range from small scattered roosts of under one hundred individuals to the spectacularly large roosts of hundreds of thousands, or even more than a million crows! A roost in Fort Cobb, Oklahoma was estimated to hold over two million crows (Gerald Iams, 1972, State of Oklahoma Upland Game Inventory W-82-R-10). Most roosts are much smaller, but roosts of tens of thousands are common.
Before heading to roost, crows will congregate in some area away from the final roosting site, usually an hour or two before complete darkness. Here the crows spend a lot of time calling, chasing, and fighting. Right at dark the main body of the group will move toward the final roosting spot. Sometimes this final movement is relatively quiet, but usually it is still quite noisy. I have seen crows coming together from several separate congregation areas, heading to one final staging area where they all coalesce, then everyone heads to the final roost. The final roost can be a cohesive group in a single woodlot, or it can be rather diffusely spread out over quite a wide area of suitable trees.
Many, perhaps most, people who witness large roosts or the flight lines to them are reminded of Alfred Hitchcock's movie "The Birds." I think this association is unfortunate. It makes the allusion that somehow what we are watching is sinister, unnatural, and threatening. In fact, it is none of the above, but one of the most natural things in the world. I would prefer to replace this association with the idea that such roosts are something to be marveled at. To me they always bring up the idea of Passenger Pigeons. When Europeans first came to North America, the Passenger Pigeon (Ectopistes migratorius) was the most abundant bird on earth. Migrating flocks were said to darken the sky for hours as they passed. Despite their incredible abundance, they are completely gone now, driven extinct by the early years of the 20th century. A combination of habitat destruction (the complete devastation of the eastern hardwood forests) and hunting for sale as meat in commercial markets destroyed one of the greatest natural spectacles on earth. Not a single Passenger Pigeon remains on earth today, nor do any people that remember seeing their massive flocks. I would like for people to look at the large congregations of the similarly-sized American Crows going to roost and think that, despite how impressive they might be, they are but the slightest hint of what the Passenger Pigeon flocks must have been like.