Watch the sky in the last light of the afternoon and you may notice magpies all flying the same way, in ones and twos and loose strings, toward the same patch of trees. Through the colder half of the year they leave their small family flocks at night and sleep together in communal roosts, sometimes several hundred birds in one stand of dense spruce or pine. They don't huddle: each bird takes its own branch and lets the needles break the wind. A winter-long study of one roost near Edmonton found the birds turn in earlier on colder days and on dark, cloudy ones, so the colder it gets, the earlier the evening commute. In the morning they scatter back across the neighbourhood to spend the day.