The number of Crows spotted in Salt Lake City has been increasing, from a somewhat rare occurance to the estimated 500 individuals we spotted at twilight last eveing. So, what's going on, why are they "suddenly" showing up in such large numbers into cities? "All About Birds" contributor Kevin McGowan offers these possibilites
A number of possible explanations exist for the relatively recent influx of roosting crows into urban areas. The birds are not making drastic shifts in behavior; crows have been gathering into winter roosts for as long as there have been crows. We know, for example, from work done in the 1930's by John Emlen at Cornell University that approximately 25,000 crows were gathering in a roost near Auburn, NY in the winter of 1932-33, and that a large roost was present in 1911-12 (Emlen, J. T., Jr., 1938, Midwinter distribution of the American Crow in New York State, Ecology 19: 264-275). The big difference is that they were roosting 3 miles south of town then and are roosting smack in downtown Auburn today. Any increase in size of the roost would be imperceptible, compared to the change of locale.
A couple of things may have worked together to get crows into town (both for nesting and roosting):
1) The 1972 extension of the Federal Migratory Bird Treaty Act of 1918 to cover crows. At this point the hunting of crows became regulated. No longer could anyone anywhere take shots at crows, but had to do so (theoretically) within proscribed guidelines and hunting seasons. It is possible that this change may have resulted in the decrease of shooting pressure on crows, allowing them to become more tolerant of the presence of people.
2) A prohibition on the discharge of firearms within city/village limits. It is conceivable that crows somehow stumbled across the fact that they could not be shot in cities because of local ordinances against shooting in town. So, in fact crows might have somehow figured out that the best thing to do to live with their enemy was to get as close as possible, not stay away. Many crow hunters do most of their hunting along flight lines of crows moving to roost. These flight lines through urban areas are protected, those in rural areas are not.
Once crows overcame the urban barrier, a number of possible advantages could extend to them:
a) Cities are warmer than rural areas. In most places a difference of 5-10 degrees F exists, sometimes referred to as a "heat bubble" over cities. Because roosting is a winter phenomenon, warmer spots could be important.
b) Great Horned Owl (Bubo virginianus) populations should be lower in urban areas. Next to people with guns, Great Horned Owls pose the largest danger to an adult crow. Great Horned Owls take adults as well as nestling crows with great regularity. (That is why crows hate them so much!) Owls probably are regular attendants at crow roosts, as owls wake up as the crows are heading into the roosts, and sleeping crows should be pretty easy picking.
c) Artificial light assist crows in watching for owls. I have noticed that many urban crow roosts are not located in nice dense trees where the crows would have microclimate advantages, such as protection from wind or cold. Rather, the crows perch out on the tips of bare branches of leafless deciduous trees. I was quite surprised by this at first, but then I noticed that many (most?) roosts are located near sources of bright illumination, such as streetlights and parking lot lights, like the lights at the Auburn prison and Syracuse University. It makes sense for crows to like "nightlights" to protect them from their biggest bogeyman, the Great Horned Owl. Crows don't see well at night; owls do. Crows near street light could see approaching owls. Also, if a crow gets scared out of its roost in the middle of the night (presumably by an owl taking crows), in lighted urban areas the crows can see where the predator is, and perhaps more importantly, can see to find another perch. You can imagine that flying blindly into the dark is not something any bird would choose to do. I was surprised at the amount of activity at the Auburn roost well after dark. The crows were still making a lot of noise and even flying from tree to tree. In other roosts I have watched that were in darker locations the crows quieted down rather quickly and no movements between trees were seen shortly after complete darkness.
d) Urban areas provide large trees for roosts. In many places some of the largest trees to be found are in urban areas. Many trees in parks and cemeteries were protected from the severe logging of the end of the last century, and are some of the oldest trees around. These large trees may be especially attractive to crows.