Watch 02 A)The Fowl Friend, B)The Fast And The ... !LINK!
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.
Watch 02 a)The Fowl Friend, b)The Fast and the ...
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