Birdman of Lauderdale: How do migrants find their way?
By this time of the year, you’re already seeing American robins in your yard, perhaps ruby-throated hummingbirds visiting your nectar feeder and chimney swifts chittering through the sky at sunset. When you consider where these birds have spent the winter, it must take some major navigational skills to get to Minnesota.
Robins return from the southern United States and Mexico. Chimney swifts winter in the Peruvian Amazon, northern Chile and Brazil.
How does a migrating bird find its way north in the spring?
Those birds that winter in the tropics may follow the Central American “land bridge,” or they may be like some hummingbirds that strike out from the northern coast of South America and fly over the Caribbean Sea.
Scientists have learned that birds have a multitude of tools to help them migrate. Birds essentially have to determine which direction to fly and then how far to fly in that direction. That sounds simple enough, but many of us can’t find our way with all the clues, tools, maps and GPSs that we have—and we have much bigger brains than these tiny critters.
A ruby-throated hummingbird, for example, weighs an average of 0.11 ounce, less than the weight of a nickel. And that hummer can find its way over hundreds of miles of land and sea. So clearly, these small brains can perform some sophisticated mental tasks.
Birds that migrate at night often rely on star patterns to guide them. They know their constellations and where they should appear in different seasons and times of the night. They’ve been shown to be able to identify the center around which the Northern Hemisphere night sky appears to rotate (Polaris, the North Star), and align their flight direction based on that. They know what angle they have to fly relative to true north to return to their breeding grounds.
Birds also have tiny crystals of a mineral called magnetite, or iron oxide, concentrated above their nostrils. With these lodestones, they can sense the earth’s magnetic field. It helps them to maintain a proper north-south orientation and allows them to navigate when the stars are obscured.
An important navigational aid for birds that migrate during the day is, of course, the sun. Birds pay attention to its relative position at sunrise. And they have an internal clock that they check throughout the day to instinctively calculate, for instance, that the sun should be 30 degrees east of south at 10 a.m. local time. They adjust their direction calculations as they travel farther north, knowing that sunrise position changes with increasing latitude. That is really sophisticated.
But even if the day is overcast, the birds can read the polarized light coming through the clouds and discern sun position from that. They were doing that long before we had Ray-Bans.
Other guides to direction and location include the landscape itself. With a bird’s-eye view, birds can see river valleys, patterns of hills, location of cities, coastlines and other topographic features that we may not readily recognize at ground level. They compare what they see to their internal map, learned on prior migrations, for a sense of where they are.
Birds can also use sound waves to keep them on course and determine how far they’ve gone. It’s thought that Sandhill cranes migrating up the central United States can hear the infrasound of wind on the Rocky Mountain range. That keeps them east of the Rockies and headed north.
Along the coasts, birds can hear the sound of waves breaking on the shore and determine direction from that.
At the other end of the sound spectrum, birds hear the calls of other birds with which they’re migrating, which helps to keep them on the right track, especially first-time migrants on their first southerly migration as part of a flock. And they can hear calls from frogs and other amphibians that reside in marshy areas. This would help them confirm where they are on their mental map.
And how about smell? It’s believed that seabirds, for example, can tell where their colonial nest sites are by using their sense of smell, and if you’ve ever been downwind from a seabird colony, you’d agree that it certainly would be a powerful directional guide.
But birds can also identify the smell of meadows, marshes, lakes, woodlands and, unfortunately, even industrial smells to use as a checkpoint on their journeys.
Birds know what latitude they’re aiming for, probably confirmed by the direction of the sun at sunrise, for example, or their mental star map. When that target latitude is reached, the northerly flight changes to an east-west exploratory quest for a favorable breeding territory.
All of these tools are innate in the tiny head of a tiny bird, some of which weigh less than a nickel.
Clay Christensen watches and writes about birds at his home in Lauderdale and blogs on his website at www.BirdmanofLauderdale.com.