Partial Migrants

Crows, she says, are what's known as "partial migrants." Every year, some members of the population migrate between breeding grounds and their overwintering grounds--like parking lots . But others just stay put. So Townsend and her colleagues wanted to know if that urge to migrate was something individual crows can turn on and off. To find out, they captured 18 crows from overwintering spots in California and New York. They fitted the birds with little backpack satellite tags, and tracked them for several years. Overall, three quarters of the birds migrated, an average of 300 miles. And more importantly, if they migrated once, they did it every year--suggesting traveling is not a habit they switch on and off. The researchers also found that migrating crows returned faithfully to the same breeding grounds every year--but were more flexible on where to overwinter. Which could be a good thing.