Question: Are Tree Sparrows facultative migrants?

American Tree Sparrow: Strictly a winter bird in Ohio, leaving in spring to go far to the north.  How much is its departure time affected by local weather?  Photo by Kenn Kaufman.

Late March: Today I'm thinking about birds I'm not seeing.  For the last couple of years, I've seen small flocks of American Tree Sparrows around Oak Harbor throughout the winter and into spring, at least into early April.  There were plenty of them around this winter, but most of them seem to have disappeared in early March, about the time we started having record-warm temperatures.  Was their departure partly a result of the weather?

Researchers who study migration talk about different bird populations being obligate migrants -- those that move at a certain time of year, regardless of what the weather is doing -- and facultative migrants, which can vary the time of their migration (and even the distance that they migrate) depending on the conditions of the moment.  These are not two completely distinct groups, but rather two ends of a spectrum.  Most facultative migrants only vary their migration within certain limits -- that is, they won't suddenly start to migrate south if there's a cold day in June; the conditions have to fall within the right general time of year.

The most obvious examples of facultative migrants around here are various waterbirds, especially ducks, in their fall migration.  Many of them will come south to northern Ohio in fall and then linger as long as there is enough open water.  If it's a mild winter, they may stay the entire season.  Sandhill Cranes respond to weather also, so recently they've been migrating south later and later in fall, and staying through the winter farther north than they used to.

My question of the moment involves American Tree Sparrows.  Their breeding range is far to the north of here, in arctic and subarctic Canada and Alaska.  In a typical year, most will be gone from northern Ohio by mid-April, and almost all of them by the beginning of May.  This year, with record high temperatures for days at a time in March, the Tree Sparrows seem to have disappeared a month earlier than usual.  Did they, in fact, start north so much earlier?  I will be interested to find out if areas north of the wintering range had earlier spring records than usual.