by Pamela Hunt
It’s fall again, and millions of birds are winging their way south to wintering locations that can be as close as New England or as far away as offshore Antarctica. For perhaps half of these birds, this is the first time they’ve even made the journey, and yet they do it without any obvious instruction. Certainly there’s no electronic voice telling them to “turn right” or “recalculating” when they start drifting out over the Atlantic Ocean.
How do they do it? The Socratic approach would be to respond with: “How do we do it?” You seem to have no problem getting home from work every day, or even going back and forth to relatives for the holidays. Granted, if I instead asked you to drive to Sudbury, Ontario tomorrow, you’d be a little less certain, but you would know how to go about solving the problem. Any kind of destination-driven travel requires the traveler to have two things: a map and a compass. The map need not be something you hold in your hand (e.g., by now you’ve memorized the way home from work) and the compass need only be as simple as a general sense of which way is north. These are otherwise referred to as the processes of Navigation (knowing where you are on the landscape and where you’re heading) and Orientation (knowing which direction you need to go to get there).
Biologists know quite a bit about how birds orient. It turns out they have a whole arsenal of tools at their disposal, from using the north star (yes, just like ancient sailors) to proteins in their eyes and iron-oxide particles in their brains that are sensitive to the earth’s magnetic field. The latter are mainly a back-up for when cloudy conditions make using visual cues unreliable. A third option involves sensitivity to the polarization of light that allows a bird to detect sunset (and thus “west”) even under overcast conditions. Most of these discoveries have been made in laboratory settings where researchers could more easily modify birds’ perceptions, including everything from changing the stars in a planetarium to fitting them with tiny polarizing “contact lenses.”
The navigation part is a little more problematic. How does the phoebe that nests on your porch return to the same place three years in a row? It would appear that for young birds at least, the direction they fly in their first migration is mostly innate, with the main purpose being to get them from point A (where they were hatched) to point B (where they winter). This is often expressed as a compass bearing (e.g., “fly southwest”). In fact, when biologists hybridized birds from southwest-migrating populations with those from populations that migrate southeast, the offspring tended to migrate south, suggesting a strong genetic component to orientation. With experience, birds learn more subtle detail along these routes and are better able to fine tune their navigation to arrive at the same territories year after year. Such landmark-based navigation makes sense when individuals are going to familiar areas (e.g., breeding territories), but we still really don’t know how birds replicate the longer journeys between breeding and wintering areas twice a year – and often at night.
Keep all this in mind as you watch southbound skeins of geese this October, or marvel at kettles of Broad-winged Hawks over Pack Monadnock. And send a postcard if you safely make it to Sudbury (then come back without using a map).