The chatter on Facebook caught my attention—a Magnificent Frigatebird had been seen and photographed at Cherry Creek State Park, and a Bohemian Waxwing was hanging out across town at Hudson Gardens. While I ponder the wisdom of driving over an hour each way to chase these out-of-place birds, especially with snow in the forecast, I have to wonder—how did they end up here in the first place? The frigatebird is a tropical species—I’ve seen them in Central America and the Caribbean—while Bohemian Waxwings typically hang out in the boreal forests of the northern hemisphere, and are considered to be rare for the Denver area.
Then there are the other birds I’ve chased over the years… Eurasian Wigeons in both Mountain View, California and Cañon City, Colorado. The Pine Warbler at our local nature center. A Purple Sandpiper in the Rocky Mountains. While millions of birds successfully migrate even long distances every year, some seem to get lost, arriving in the most unexpected places—to the delight of birders, but often leading to the demise of the stray bird. Some strays only wander a few hundred miles out of range—such as a Scissor-tail Flycatcher in Colorado instead of Oklahoma or Texas, or the Common Black Hawk—a bird usually found in Arizona and Mexico—that I saw recently at a nearby lake. But often, these birds are thousands of miles from where they belong.
This raises some questions: how do birds navigate? And how do they get so lost? While ornithologists have some answers, much remains to be learned. The “hows” of migration are still a hot topic for researchers.
To some degree, birds in transit use the same clues that we do. In past centuries, sailors navigated by the stars. Birds do too, especially those species that migrate at night. Birds that fly during the day use the sun to stay on track.
Just like us, birds likely use familiar landmarks. Many species return to nest on the same territory every year. If I’m delayed in hanging my sugar-water feeders, the arriving hummingbirds hover right where the feeder was last year, wondering where it went. Orienting by sun and stars will get you close, but to navigate this precisely, it’s essential that you recognize familiar landmarks.
Finally, we might pull out a compass to find which way is north. Birds also use the earth’s magnetic field to orient themselves. The question is how.
There is some evidence that birds have a compass built into their heads in the form of magnetite (Fe3O4). As the name suggests, magnetite is very sensitive to the earth’s magnetic field, and could be used by birds—and fish and turtles and perhaps even humans—to determine both where they are and which way they’re going. But scientists are still looking for proof. According to an article on the AAAS website, “Except in bacteria, … no one has seen magnetite crystals serving as a magnetic sensor. The crystals could be something else—say, waste products of iron metabolism, or a way for the body to sequester carcinogenic heavy metals.” Here’s a terrific opportunity for further research.
Then there’s cryptochrome, a protein that regulates circadian rhythms in both plants and animals. This molecule has now been discovered in the retina of certain birds, such as Zebra finches, where it may enable them to actually see the earth’s magnetic field.
There is some promising research along these lines, such as the discovery that levels of one cryptochrome, Cry4, are much higher in European robins during migration than at other times of the year.
Still, there are questions. How, exactly, does this molecule work? What is the path connecting the molecule to the brain? A more significant issue is that initial laboratory experiments suggest that the earth’s magnetic field isn’t strong enough to elicit a molecular reaction.
However they do it, it’s clear that most birds don’t need to stop for directions. But just like everything else in nature, the system doesn’t always work perfectly. Most of the strays are young birds on their first journey. Sadly, if they run into trouble, odds are they won’t live to make another one. There are a few that manage to end up in a hospitable spot, such as the Eurasian Wigeon I saw in California. The same bird made the same trip to the same wetland for several years, hanging out with the American Wigeon locals for the winter.
Although highly risky for the individual, there’s an advantage to the species for some birds to be explorers. Maybe they’ll happen across a highly desirable habitat, paving the way for the species to expand its range, increasing the odds of its survival.
The answer to last week’s quiz is Northern Harrier.