These groups are "winter roosts", and they are nothing new in crow life. Despite what urban residents sometimes think, crows didn't start gathering when we set out cities for them to use. Roosting in groups at any time of year may offer safety in numbers from night predators, such as Great Horned Owls.
In winter, birds living in northern areas that usually have constant snow cover for months do migrate south--Canada, areas of New England. Crows don't necessarily have a particular area they migrate TO. They may go as far as an area that is usually ok for foraging, perhaps one that they are familiar with from previous migrations. There they form flocks that are made up of migrants as well as wide-foraging locals. If it gets unusually snowy and cold, they may move further south. (We really don't know much of the repeat migratory routes of individual crows. We do know that birds tagged in Ithaca in winter are then seen on territories in Canada, VT, New Hampshire in summer, and that some birds RAISED in Ithaca have been observed or shot in winter, in such places as Maryland, West VA, and Pennsylvania, as well as in Cortland, Auburn, Geneva) In the winter flocks, birds are foraging in open fields and off familiar areas. During foraging, flocks offer some safety in numbers to detect predators in day (hawks, hunters, whatever). At night the flocks "flock up" still more in places that offer "good roosting sites", which probably includes wind breaks, places from which owls can be detected at night. So they are probably gathering both for safety in numbers and also because they all agree on what makes a good site. Cities may offer fewer predators, but also the lights may allow them to see the predators. Finally roosting in flocks that include birds that have sampled food sources widely may allow birds to find new food sources, perhaps by following the most assured and directed birds leaving the roost. So--Upstate NY has its own crows and is ideally positioned for northern crows--so flocks become big. They like the agricultural fields interspersed with trees and lots of running water sources (which may be important in cold winters)...and we also offer lots of smaller cities, with large groups of lit trees in their downtowns or college campuses. These seem to be attractive. Mid-late March is the start of the breeding season and flocking crows will be returning to their breeding latitudes. Our Ithaca pairs are already calling on territory during daytimes. As I say, some of this story is surmised from the patterns, not pinned down with hard data on individuals! We know what our tagged birds do, when we can follow them. But we would love to have gps data coming in from our birds, such as the snowy owls and golden eagles give their researchers. Bring on the Tiny Tags! Anne On Mar 4, 2014, at 7:19 AM, Sue Rakow wrote: > I observed the murder of crows on Sunday evening. It was stunning. I would > like to know more about why they gather in such large groups. Are they on the > move or are they local? Can anyone help me understand? > Thanks. > Sue Rakow > -- > Cayugabirds-L List Info: > Welcome and Basics > Rules and Information > Subscribe, Configuration and Leave > Archives: > The Mail Archive > Surfbirds > BirdingOnThe.Net > Please submit your observations to eBird! > -- -- Cayugabirds-L List Info: http://www.NortheastBirding.com/CayugabirdsWELCOME http://www.NortheastBirding.com/CayugabirdsRULES http://www.NortheastBirding.com/CayugabirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/cayugabirds-l@cornell.edu/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/Cayugabirds 3) http://birdingonthe.net/mailinglists/CAYU.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --