Spring migration is in full swing here in Ithaca, NY.
(Guess this eastern beauty…)
Just today while lunching in the yard consuming a pizza made by the Red Baron himself, I was greeted by a number of travelers spending the day consuming the insectivorous vermin so common in the Spring. Baltimore orioles, Tennessee warblers, Canada warblers, black-and-white warblers, American redstart, northern parula, and the (for once!) aptly named yellow-rumped warbler, all graced the leafy bits beginning to grow.
These small puffs of yellow, white, and black (with a little orange thrown in) complement the somber colors of the other migrant yard birds: the mewling gray catbirds skulking in the underbrush, the raspy eastern phoebe investigating the porch roof for nesting, and the plump worm-picking robins that love a recently tilled plot. When added to the titmice, chickadees, and woodpeckers that have persevered through the entire winter, my yard has begun to feel impressively full (I saw 35 species during that lunch!). And each year my wonder returns with these small birds, many of them in the 10-15 gram range, that are able to migrate thousands of miles to return to my backyard. Despite its ubiquity, migration remains a challenge for researchers to study-how can the birds physiologically fly for so long? How do they stay on course? Where do they go?
When it comes to orientation, birds are thought to use all sorts of cues to keep on track: the stars, the sun, the inclination of the magnetic field of the earth, the polarity of light, even smells and local landmarks! While all these things are potentially available as tools for birds to use during migration, little is known about which tools they trust more.
One way to think about this quandary is to picture yourself going on a trip to visit a friend. You have a road map from AAA, some directions from Google Maps, a handwritten note with your friend’s address; maybe even a GPS in your car. You also have road signs and the occasional gas station attendant to help you on your way. So what happens when the AAA triptik says to go one way, and Google Maps says to go another? Your GPS might suggest a different route as well, and as anyone who has driven on a road heading south that is labeled north, road signs aren’t always trustworthy either. So who do you trust?
I had the opportunity to work for a week with Dr. Rachel Muheim, one of the leaders in the study of how birds orient during migration. While this might sound like an endeavor requiring glamorous high-tech equipment, the reality is somewhat more scribbler-esque. Studying bird migration relies most heavily on one thing: Zugunruhe (i know, I know, not another funny German word). In English, this translates as “migratory restlessness”, a phenomenon that has been studied by placing a bird into an Emlen funnel: picture an upturned lampshade, wide side facing up, with a screen on top. On the inside of the funnels is typewriter correction paper (or an inkpad and paper), and into the funnel you place a migrating bird. There’s no room for it to fly away, so instead, the Zugunruhe manifests through the bird’s hopping in the direction it wants to travel, and the data looks something vaguely reminiscent of chicken-scratch, with the highest quantity of hopping equal to the direction the bird would like to travel. Birds that have to travel the furthest also hop the most.
(An example of an inkpad Emlen funnel with migration orientation data illustrated on the right)
This technique, alone, may seem sort of interesting, catching a glimpse into the nighttime motivations of a songbird; but the real fun begins when Rachel starts to mess with the cues that the birds use to figure out which way to go. It turns out that birds can actually “see” (in some fashion) the magnetic field, and that they can also see polarized light. Rachel can mess with a bird’s perception of sunrise and sunset by placing a polarized filter over the funnel; furthermore, she can even change the direction of the magnetic field by using a Helmholtz coil, which is basically a charged wire coil (big enough to set an Emlen funnel inside) that generates its own magnetic field in whatever direction she chooses.
Her research has demonstrated that birds will use whatever tools they have at their disposal, allowing for the most accurate orientation. Yet one tool trumps all, and is used to calibrate the others: the sun. Birds use the sun’s rise and setting to calibrate their magnetic compass, and keeping them on track despite changes in declination as they move north or south. Since the sun rises and sets at symmetrical points in the sky, birds average between the two to determine where absolute south lies. But if you give a bird completely erroneous information, like the sun rising in the west and setting in the north, it will actually disregard it and follow the program from the night before (kind of like the response I had to listening to my parents as a teenager).
(Female and male black-throated blue warblers migrate from the Caribbean and Central America to breed in the southeastern Canada and northeastern U.S.)
S0-next time you wake up in the middle of the night to a warm spring evening, listen carefully for the chip notes of migrating songbirds overhead–who knows? Maybe your house is one more landmark (like the Wall Drug of the bird world) that birds don’t want to miss.