With the exceptionally warm weather this March, I have been seeing quite a few reports of early migrants. I'm not too surprised to see early reports of species that winter in the U.S. (short-distance migrants). This is probably due to both favorable weather and the fact many more people are outdoors this year looking for birds. But reports of Neotropical (long-distance migrants) deserve more scrutiny. What follows is a version of a message I sent to our local birding listserv.
What are we to make of reports of Neotropical migrants arriving many weeks ahead of schedule? Why would these birds, wintering in the tropics with no "knowledge" of an early spring here, be arriving so early? The migratory movements of these birds are typically triggered by photoperiod.
For a Wood Thrush, for example, to be present in Michigan right now means that it departed its Central American wintering grounds in mid-February. Recent studies have indicated Wood Thrushes take about a month get here. The average arrival date here in southeast Michigan (based on 16 years of data) is May 1, so the typical departure from Central America is early April. While favorable weather patterns can accelerate migratory flights, the birds have to be on the move to begin with.
There are many things we still don't know about the interactions of weather, climate, and birds. Yet it seems hard to conceive why so many individuals of various species (and for us to find even a handful having survived to arrive in Michigan, the starting number would have to be larger) would begin migrating so prematurely. The annual cycle of birds has been intricately choreographed through evolutionary time. Remember, these birds have to spend a period of time (also coincident with resources in the tropics) gaining weight in order to migrate north, just like they do to go south in fall.
While the data is not infallible, an excellent assessment of where migrants are currently being located can be found at eBird. Go to Explore Data, Range and Point Maps, choose a species, and specify March to March, current year (direct link here). There have been only five reports of Wood Thrushes in the U.S. so far this month. Most are still in Central America.
I suspect that some (most?) of the Neotropical migrants being reported at northern latitudes, if accurate, may actually be birds that did not migrate last fall and managed to survive the very mild winter. Black-throated Green Warbler and its close relatives, orioles, and tanagers, for instance, are occasionally reported in winter. I've also given some thought to the fact that every year there may be a few exceptionally early migrants which routinely do not survive jumping the gun that might be having more luck this year. Still, this is probably pretty rare and might be more applicable to short-distance migrants.
At any rate, just because it's warm outside doesn't mean we can expect birds too many weeks ahead of schedule. When they do arrive, detection and viewing this year will be greatly diminished by leaves -- this is a good year to learn your bird songs!
Update: Marshall Iliff, one of the eBird coordinators at Cornell, has written a couple of long but excellent posts on the Massachusetts bird list regarding early arrivals, including hummingbirds. He's someone who has a vast experience in examining migration patterns through his work. I strongly encourage you to at least read through this one. An earlier post deals more with common early migrant ID problems.