They’re coming our way. And others are bidding us farewell. The spring bird migration is under way and will really gain steam in early May. Are you curious about how many birds flew through the night over Oakland County yesterday? Here’s a great tool from Cornell University’s Ornithology Lab that can give you a data-based estimate! (Thanks to birding friend Vinnie Morganti for the link!)
The Dark-eyed Juncos (Junco hyemalis) and Tree Sparrows (Spizelloides arborea), who nest in colder climates, are beginning to depart from the hedgerows and from under our feeders as they wing their way to their northern breeding grounds. Bufflehead dabbling ducks (Bucephala albeola) showed up on Cranberry Lake in mid-March as they made their way through Michigan to their nesting grounds that extend from Ontario to Canada’s Northwest Territories. And of course, the hoarse, ancient cries of the Sandhill Cranes (Antigone canadensis) now draw our eyes skyward. (Click on photos below to enlarge.)
I’m always impressed by the ability of birds to survive the ordeal of migration, successfully navigating their way twice each year across the country and sometimes far beyond. This spring a lot of questions bubbled up in my winter-weary mind.
How does the tiny hummingbird beat its wings thousands of times on its way to Central America without expiring from exhaustion? How do fledglings find their way when they travel without adults, which happens more often than not? How do birds flying nonstop over oceans eat and sleep? And what’s up with birds using the earth’s magnetic field to navigate? I sure can’t perceive the earth’s magnetic field!
Recently, while doing some spring cleaning, I came across an article that I’d saved which gave me some possible explanations and some resources to go further. (See references below.) So I wanted to share with you the astounding and somewhat bizarre adaptations that allow our avian neighbors to successfully make such arduous journeys.
First, a big Thank-You! My photographer friends, Paul Birtwhistle and Bob and Joan Bonin have again generously shared some of their wonderful photos for this blog. My heartfelt thanks to all three!
So How Did This Whole Migration Pattern Get Started, Anyway?
Doesn’t semi-annual migration seem a bit extreme? I mean, why don’t birds just stay in warm regions all year ’round happily eating and breeding? (I’m glad they don’t, though!) The Cornell Lab of Ornithology describes two theories: the Northern Home theory that northern bird ancestors moved south little by little as ice ages advanced, or the Southern Home theory that southern bird ancestors followed the ice north as it retreated. According to a recent University of Michigan study of the evolutionary lineage of 800 species of North American song birds, the Northern Home theory seems more likely. (Very cool detail about this subject at this link! Click on the words “Evolution of Bird Migration” at the top left.)
Birds have continued to migrate for eons because of the basics: food and breeding opportunities. Temperate zones like Michigan are very buggy places. Our inland wetlands and shorelines produce a glorious abundance of insects and their caterpillars each spring and summer. Just look at the meal this Song Sparrow (Melospiza melodia) found in one of our parks! Quite a haul!
So when daylight lingers in spring or shrinks in autumn, birds notice the change and start feeling restless. Even captive birds in scientific studies evidence migration restlessness, which is known among researchers by the German name zugunruhe. As the season approaches, they eat more and later into the evening. Their sleep decreases by as much as two-thirds in some species. (Sounds familiar somehow. Maybe human “snow birds” are experiencing zugunruhe?)
The other big draw is mating, of course. Favorite stopovers bring together migrating birds of the same species, which means a more diverse choice of mates. Let’s hear it for diversifying the gene pool!
And then there’s the “housing market”; early birds enjoy a greater selection of the preferred, sometimes scarce, nest sites.
OK, So How Do Birds Prepare for Migration?
It’s tough to generalize about bird migration. Consider that some travel long distances, like the Ruby-throated Hummingbird (Archilochus colubris) who flaps its tiny wings to reach Central America, or the Magnolia Warbler (Setophaga magnolia) who sets off for the Caribbean. Others travel short distances, like Eastern Bluebirds (Sialia sialis) and American Robins (Turdus migratorius) who simply move just far enough in winter to find open water and more food.
Many songbirds, like the Rose-breasted Grosbeak (Pheucticus ludovicianus), make their way at night to avoid migrating predators like the Cooper’s Hawk (Accipiter cooperii), which migrates in daylight. Also, according to the Audubon Society, “Free of daytime thermals [rising warm air], the atmosphere [at night] is more stable, making it easier to maintain a steady course, especially for smaller birds such as warblers that might fly as slowly as 15 miles per hour.”
Some migrators fly in single species flocks, like the Snow Buntings (Plectrophenax nivalis) that arrive in the fall from the Arctic to spend winters in Michigan. Their plumage is whiter in the snowy north to camouflage them while breeding. During their winter visit here, their plumage includes more brown, making them less visible in fallow farm fields and open prairie. Others, like the Greater (Tringa melanoleuca) and Lesser Yellowlegs (Tringa flavipes) make their way north in mixed flocks during spring migration.
According to Scott Weidensaul, author of A World on Wings: the Global Odyssey of Migratory Birds, even if huge numbers of birds are aloft together, a migrating songbird flying at night, “does not fly in cohesive, coordinated flocks; each is migrating on it own.” House Wrens (Troglodytes aedon) and Red-eyed Vireos (Vireo olivaceus), for example are lone nighttime migrators.
Given all that that diversity, here’s what I’ve gleaned so far about how birds prepare.
They Fatten Up Big Time!
Birds really lard up for migration. Experienced naturalist and local bird bander, Allen Chartier, checks the weight of each bird he bands and gently blows the breast feathers aside to actually see the fat layer. He told me in a helpful email that our Ruby-Throated Hummingbirds, which normally weigh about 3 grams during breeding season can weigh more than 5 grams as they leave Michigan. They need to keep bulking up along the way and amazingly can double their normal weight in about a week just before heading across the Gulf of Mexico in the autumn or the Yucatan in the spring.
Blackpoll Warblers (Setophaga striata) transit across our state twice a year and Allen says they, “… undergo a long water crossing, sometimes from the mid-Atlantic coast down to Venezuela, non-stop. Normally they weigh 10-11 grams, but can put on enough fat to more than double their weight for these multi-day flights.” He reports that “in the Great Lakes, I have had Blackpoll Warblers that weigh more than 20 grams.” Fat is clearly the essential fuel for bird migration!
As author Scott Weidensaul points out, “By any typical measure, a migratory bird ready for travel ought to head to the ER, not the skies.” But he says, unlike seriously overweight humans, fattened-up migrators are not plagued by increased risk of hypertension, heart disease, diabetes or stroke. Weidensaul says that “Researchers hope that insights from avian physiology may help may help unlock new treatments and preventive approaches in people.” I hope so too!
They “Grow or Jettison their Internal Organs on an As-needed Basis!”
The quote above from Scott Weidensaul’s book just blew me away. The internal organs of birds actually shrink and expand for migration? Yes! “Internal flexibility is actually common among migrants … a thrush or catbird, feeding on the dogwood berries in a corner of the backyard, has undergone a late summer expansion of its intestines to squeeze every calorie from lipid-rich fruit.”
Migrating birds, which need to travel non-stop over oceans or deserts for long distances, shrink their digestive organs since they’re expendable when they can’t stop to feed. But their hearts, lungs and pectoral muscles grow larger, and do so without exercise! On arrival, the digestive organs make a comeback that allows them to start feeding again. Imagine! Transforming organs!
The little Chestnut-sided Warbler (Setophaga pensylvanica) below, photographed by Paul Birtwhistle in Costa Rica, may not have needed such drastic transformation for its nonstop trip across the Gulf of Mexico. I couldn’t find a definitive answer to that but it definitely burned a lot of fat! According to Cornell Lab of Ornithology’s subscription website, Birds of the World, a Chestnut-sided Warbler in non-breeding plumage could have weighed 10 -12 grams when it left the U.S. coast. But these birds are recorded as weighing only about 8 grams when they reach Central America, having lost most or all of their stored fat on their nonstop flights across the Gulf of Mexico.
Weidensaul also reports that both male and female birds shrink their sexual organs for fall migration. Anything to make flying lighter and easier, I guess! Allen Chartier wrote, “Ever since humans began preparing “study skins” for museums, more than 200 years ago, it was discovered that during the non-breeding season the gonads of birds shrink in size, to maybe 10% of the size that they are in the breeding season. It was figured, eventually, that this was to reduce weight for migration, and allow for more body fat to be laid on for migratory flights.” Their gonads are ballooning right now, which is why we are beginning to hear that robust morning chorus in the spring!
Once They Start Moving, How in the World Do They Navigate?
In her lively and well-researched New Yorker article, “Where the Wild Things Go: How Animals Navigate the World,” (April 5, 2021), Kathryn Schulz wrote: “A bird that migrates over long distances must maintain its trajectory by day and by night, in every kind of weather, often with no landmarks in sight. If its travels take more than a few days, it must compensate for the fact that virtually everything it could use to stay oriented will change, from the elevation of the sun to the length of the day and the constellations overhead at night. Most bewildering of all, it must know where it is going — even the first time, when it has never been there before – and it must know where that destination lies compared with its current position.” Wow.
Like humans with our much more limited ability to orient and navigate, birds use a variety of basic navigation strategies, and different species may use a combination of them. Kathryn Schulz lists as strategies: sight, sound or even scent cues, landmarks (mountain ridges, coastlines), compass orientation or vector navigation (stringing together multiple orientations (e.g. south and then southwest for a precise distance) or dead reckoning (calculating based on bearing, speed and time elapsed from a previous location). But she points out, “… to have a sense of direction, a given species might also need to have other faculties, something like a compass, something like a map, a decent memory, the ability to keep track of time, and an information-rich awareness of its environment.” And Weidensaul adds to the list: the patterns of stars around Polaris (the North Star) and the movement of “bands of polarized light that are invisible to us but easily seen by birds.” Wow, again! All that in one small skull!
According to Weidensaul, “Migratory birds grow fresh neurons before autumn migration” and scientists have correlated longer migrations with more neuron growth, presumably as an aid to navigation. The neurons also increase according to whether birds travel individually or in flocks. Warblers which generally fly alone show increases in the hippocampus which processes spatial information and memory. Birds in large migrating flocks see most of the increase in regions of the brain that may be more important for noticing and understanding the actions of other birds.
Researchers believe that a bird’s general destination may be defined by instinct, especially in young birds, many of whom make their first trip alone or with other juveniles. But learning clearly occurs during their first flight with or without adults and plays its part in perfecting the best route from then on.
But what I wanted to know was, how do some migrating birds create and use a mental map of the earth’s magnetic field? I kept finding references to their ability to do so, but no one told me how! Well, Weidensaul had an answer which he says “most experts accept.”
Imagine the little Wilson’s Warbler (Cardellina pusilla) in the photo above glancing upward at the stars as it flies through the night from Central America to where I saw it in Tawas City, Michigan. As I understand it, photons of the stars’ blue light hit specialized cells in the bird’s retina containing molecules of a protein called cryptochrome. (Love the Superman sound of that!) Those molecules react by thrusting one of their electrons into a neighboring molecule and the two become connected (“entangled” in scientific jargon) and magnetic. (That’s the quantum mechanics part which is a bit beyond me.) As light continues to stream in, multiples of these paired molecules build a map of the magnetic field within the bird’s eye. Scientists think the map may appear as a “dim shape or smudge — visible as the bird moves its head, but not opaque enough to interfere with normal vision — that shifts with the bird’s position relative to the ground and to the inclination of the magnetic field lines arcing out of the planet” (Weidensaul). Evidently, birds can orient themselves within that map to help find their way to their destination. Many of us humans have a tough time reading a road map!
If my brief summary leaves you with more questions than answers, you’re in good company. Even scientists don’t completely understand how cryptochrome works its magic and some disagree with the whole theory. But at least that little map in the eye created by starlight and quantum mechanics satisfies my curiosity for now. If you’d like a somewhat what more detailed description, I recommend Chapter Two of Weidensaul’s book, A World on the Wing.
Eating and Sleeping,: How Do They Survive Along the Way?
Eating Strategies
Eating on migrations varies according to what and how a bird eats. Most migrating birds depend on trusted stopover sites for food and rest. Allen Chartier wrote that “Migrating warblers, sparrows, and thrushes migrate at night, and put on fat to fly 200+ miles each night…” to their next stop. Eastern Kingbirds (Tyrannus tyrannus) and Barn Swallows (Hirundo rustica) migrate to Michigan from deep in South America during daylight hours, in order to gobble up flying insects while on the wing as well as at known rest stops.
Amazingly, some birds in other regions of the world fly nonstop for multiple days and nights without eating at all! Weidensaul describes the Bar-tailed Godwit’s “7,200-mile nonstop flight each autumn from western Alaska to New Zealand, a journey that takes them eight or nine days of uninterrupted flight — the longest nonstop migration known.” They are an impressive example of “jettisoning” organs and living on fat. I’m really glad I’m not a godwit – but I am impressed by them!
Drinking in Flight
Birds don’t sweat but they do lose moisture through breathing and excreting. That may account for so many birds migrating at night when the air is cooler and more humid, according to Weidensaul. Of course they look for freshwater wetlands as a basic source of drinking water. But on long ocean or other nonstop flights, for instance, research shows that they can still maintain a healthy amount of moisture by extracting water from their beefed-up muscles and organs while in flight.
Sleeping on the Wing
Most migrating birds do their journeys in stages, resting during the day or night depending on when they travel. Weidensaul reports that “For migratory songbirds, like White-throated Sparrows and Hermit Thrushes (Catharus guttatus,) the onset of migration seasons … decreases the amount of time they sleep by two-thirds, even in captivity, and well before they start migration. They may compensate by taking micronaps during the day.”
Mallards (Anas platyrhynchos) have evolved to use unihemispheric sleep, a condition in which only half the brain sleeps at a time and one eye stays open. Neils Rattenborg directs sleep research at Germany’s Max Planck Institute and his team’s work centers around birds. According to an article from the Max Planck Society, Rattenborg documented that “in a group of sleeping ducks, those [Mallards] sitting at the edge kept their outwardly directed eye open and the corresponding brain hemisphere remained awake. The birds can thereby rest a part of their brain while keeping an eye out for potential predators.” I’m on the lookout for that phenomenon!
By outfitting birds Great Frigatebirds (Fregata minor) near the Galapagos Islands with tiny transmitters, Rattenborg also discovered something even more important about sleep. It seems these large birds take repeated unihemispheric naps averaging about 12 seconds long while foraging at sea for six days or more. Sometimes these birds’ entire brains slept while slowly gliding up or down in thermals! Talk about power napping, eh?
What Can We Humans Do to Make Life Easier for Migrating Birds?
Clearly, we’ve created big challenges for migrating birds, despite their amazing adaptations over the eons. So here are just a few of those difficulties and how we might help our beautiful migrating neighbors.
- Light Pollution: Birds need a clear view of the night sky even more than we do. (Don’t you miss seeing a sky filled with stars?) So we can turn off outside lights (at home and in workplaces), make the light bulbs yellow or red instead of white, or install shades on outdoor lights that direct the light downward. Inside, we can close curtains or shades where a light is near a window in the evening during migration season. It all helps.
- Reflective glass like picture windows: Birds that crash into windows may fly off but they often do so with concussions. Check out this link for options for preventing bird strikes.
- Cats: Keep our beloved felines indoors. To quote Cornell Lab of Ornithology on this subject, “These are non-native predators that, even using conservative estimates, kill 1.3–4 billion birds and 6.3–22.3 billion mammals each year in the U.S. alone. Exhausted migratory birds and fledglings are particularly at risk.
- Habitat Loss: Restore natural areas and plant native plants at home. Birds count on finding the adult insects, caterpillars, and seeds that make up their diet when they arrive at a stopover or their final destination. If that land is covered by concrete or invaded by non-native plants that don’t provide the nutrition or cover they need, birds suffer along with the rest of the creatures in that habitat.
- Climate Change: Actively, drastically and quickly reduce our use of fossil fuels. Climate disruption effects migratory birds in so many ways. But here are at least two important ones. It causes more severe weather events which vulnerable migrators must negotiate over long distances. Also, insects and plants initiate hatching or blooming by ground temperature; as the ground warms earlier, overwintering insects hatch earlier, plants mature more quickly. Birds, however, initiate migration by the position of the sun and the length of daylight. As a result, tired migrators may arrive in the spring unable to find the insects, nectar or pollen on which they depend. Want evidence? Look here!
Birds Do It, Bees Do It, Even Whales in the Seas Do It … Let’s Do It. Let’s Start to Adapt…
Obviously, we humans need to adapt just like the migrators have – but a lot faster! We don’t have thousands of years for evolution to re-engineer our bodies and nature itself to cope with the new climate we’re creating with fossil fuels. No miracles of transforming digestive systems, cryptochrome maps in our eyes or unihemispheric sleep are on our immediate horizon. Nature already gave us our adaptive tools – our brains and our will. We already know much of what is needed; the trick is, do we find the collective will as a species to do it in time? The changes we need to make are significant, but not insurmountable. Nature is already warning us with tornadoes, floods, droughts, melting glaciers. It’s insisting “You can do this! Use the adaptation tools between your ears that you were blessed with and save us all!” I certainly hope more humans heed that desperate call – and soon!
Main Sources:
- A World on the Wing: The Global Odyssey of Migratory Birds by Scott Weidensaul, W.W. Norton and Company, New York 2021
- “Where the Wild Things Go: How Animals Navigate the World,” by Kathryn Schulz, published April 5, 2021 in the New Yorker magazine
- Birds of the World, a subscription-only website from Cornel Lab of Ornithology, Cornell University in collaboration with the American Ornithological Society.
- “The Evolution of Bird Migration, “Adapted from the Handbook of Bird Biology, Third Edition, on Cornell University’s website “All About Birds”‘
- “Snoozing between the skies and earth” a posting on the website of the Max Planck Society.
Natural Areas Notebook 