American Crows (Corvus brachyrhynchos) are partial migrants, meaning that the most northernly populations (those from Canada) are the ones on the move. The birds that are here in the NE during the summer are also here during the winter. So, the large roosts you see are composed of not only migrant crows but resident crows as well.
These resident American Crows are pretty unusual as far as year-round birds go: there are few birds that use winter communal roosts and still maintain a territory. The territories are used by family groups consisting of Mom, Dad, older sibs and young-of-the-year. Occasionally there will be a totally unrelated crow in the group that has just joined up and been accepted. Sometimes kids will pick up and leave, disappearing either entirely (and possibly dying in the process) or returning weeks to months later. Again, they are usually accepted back.
Neighboring territories may be made of related or unrelated groups. The related groups here at Cook College can often be found foraging on the same field but in separate areas. They seem to be more tolerant of each other. Unrelated groups tend to yell at each other, perhaps reinforcing the territory boundaries. Territories are a precious resource. You don’t have a territory, you don’t reproduce. And if you don’t have a territory that can maintain a family, then you’re less likely to reproduce as well as you potentially could. But a territory during winter might not be as well stocked as during the spring, summer or fall.
Crows from these family units can roost on their territory at night or fly to use the communal roost. (The crows here in central NJ use the Staten Island roost, one that has been in use since the 1860’s [not a typo]. Prior to Fresh Kills Landfill, the roost was over on the Jersey side but now it is between Foster/MaGuire/Brookline Rds past the tolls on 440.) This seems to be an individual choice situation but we suspect that on-territory food availability plays a big role. If a territorial crow joins a winter roost for the night, the rest of his or her family might still be on the home territory. The advantage of going to a winter roost (and obtaining food from superabundant resources) is likely balanced with the increased exposure to predators. But if you’re part of an enormous group, maybe you won’t get snagged by a hungry Great Horned Owl looking to feed her kids.
As far as the ravens are concerned, it’s possible I guess to take down sheep. But nineteen? I’m thinking perhaps something else got the sheep. Ravens, like many crovids, are scavengers when a carcass appears, and they settle down for a lunch when humans are most likely to see them. Ravens on a carcass doesn’t necessarily mean killers. An unkindness maybe…
