Eight Intriguing Migration Mysteries Solved With BirdCast and eBird

Artwork by Luke Seitz
October 9, 2014
Migration Mysteries solved Artwork by Luke Seitz

Scientists are learning more than ever about autumn bird migration, but there are still plenty of unsolved mysteries. What forces dictate their departures and arrivals? Why do they choose the routes they do? And why do some birds end up far off course?

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In the past, rare sightings have been chalked up as vagrants—anomalies of bird movement that couldn’t be predicted and might never happen again. But as time and data points have piled up, it’s become clear that many rare sightings are the result of regular, if infrequent, patterns. As we gain an understanding of these patterns, we can use them to unravel migration mysteries and know when and where to look for rarities.

Two of the programs leading the way in this effort are BirdCast and eBird. The eBird project provides the raw sightings data (when bird watchers like you report your sightings to the project). BirdCast combines the sightings data with meteorological data and weather surveillance radars (which can “see” birds just as they can “see” raindrops—as in the map above). These data sources allow BirdCast’s team to develop weekly, region-specific predictions to let North American birders know which birds to look for and when—as well as to understand rarer phenomena like these eight migration mysteries:

What’s that “peep” in the night? Ever go outside on a crisp September evening and hear what sounds like a spring peeper in the sky? Except, it’s fall… and frogs don’t fly? Lots of songbirds migrate at night and call to each other with very short, faint notes. The calls of Swainson’s Thrushes are among the most distinctive of these “night flight calls,” and they are loud enough that you can actually hear the birds as they fly overhead. Your best bets for hearing this peep in the night sky will come immediately after a cold front passes, particularly when birds fly into areas with poor visibility (like fog) and light pollution where calling increases dramatically (birds tend to call more frequently when disoriented). See this post to hear the call notes and learn how to identify them.

NLapwingWhere did all those Northern Lapwings come from? After Hurricane Sandy in late October, 2012, dozens of Northern Lapwings showed up in the northeastern U.S. Never before had a tropical system’s passage brought such a bounty of this visitor from Eurasia. In fact, most hurricanes tend to take American birds and deposit them in Europe, not the other way around. But by analyzing meteorological data, the BirdCast team discovered that the hurricane’s counterclockwise circulation was augmented by high pressure over the North Atlantic and winds blowing from Europe. The result was east-to-west winds that delivered many American birders a new species for their life list. See the full story on vagrants from Sandy in the BirdCast archives.

FTFlycatcher2What drives cameo appearances of Fork-tailed Flycatchers in fall? These spectacular birds are one of the species birders look forward to seeing when they plan a trip to Central or South America, where they are fairly common. But almost every autumn (especially in September) the tables turn and a few Fork-tailed Flycatchers come to visit birders in the eastern U.S. On closer examination, these rarities usually belong to the savana subspecies, which breeds from Brazil to Argentina and winters in Amazonia. In other words, these birds don’t belong in the U.S. The evidence suggests that when most Fork-tailed Flycatchers are migrating, some individual birds overshoot and wind up over the ocean, where they then fly downwind and end up making landfall in the U.S.

Orange and white colors show where Blackpoll Warblers migrate in the spring and then in the fall, en route to and from Canadian breeding grounds. The spring route covers much of the East, whereas in fall most birds leave from the Northeast and Mid-Atlantic, and fly directly to South America. See a larger map at eBird.

Where did all the Blackpoll Warblers go? Blackpoll Warblers are fairly common spring migrants all the way up through the eastern states. Not so in fall, when they are rarely sighted in states south of the mid-Atlantic. That’s because their main fall migration route takes them out over the open ocean. The birds often fly nonstop over the Caribbean en route to their winter grounds in South America.

CSparrowWhy do Chipping Sparrows migrate twice? We often think of migration as a straight shot from breeding grounds to wintering grounds. But quite a few species—including the familiar Chipping Sparrow—take a detour to accommodate their molt. These so-called molt migrants take a short migration trip to one area where they grow new feathers. Then they resume their migration to wintering grounds. For instance, Chipping Sparrows in the Front Range of Colorado migrate eastward to molting sites such as the Pawnee National Grasslands, then continue to Mexico.

PGallinuleWhat are Purple Gallinules doing in Newfoundland? When Purple Gallinules—which typically live in subtropical and tropical marshes—began showing up in Portugal, Maine, Newfoundland, Iceland, and Ireland, the BirdCast team took notice. They think an autumn drought in the Caribbean caused many gallinules to disperse in the winter of 2013–2014. Because Purple Gallinules are very strong fliers, and sensitive to changes in wetlands, they are very prone to wandering way out of their traditional range. And thanks to the now worldwide network of birders entering their sightings into eBird, the BirdCast team is able to convert these unusual sightings into data.

What’s a Blue-footed Booby doing in California? The Golden State boasts a huge collection of cool birds, but the Blue-footed Booby is not usually one of them. It’s a tropical seabird typically found along the Mexican coast and south to South America—but in fall of 2013, many dozens of Blue-footed Boobies were reported along the California coast. When the BirdCast team looked into meteorological patterns, it appeared that unusually warm sea surface temperatures may have caused the move, along with a similar pattern in Elegant Terns. The suggestion is that prey fish moved north to find cooler waters, and so the boobies moved with them.

Why do Long-billed Curlews winter in both California and Mexico? Long-billed Curlews breed in grasslands and open country of the central and western U.S. In winter, you can find them in coastal and interior California, as well as in landlocked wetlands of the Southwest and Mexico. By putting together species distribution models from eBird data, it appears these two populations may employ different migration routes. Curlews from the Great Basin may travel westward to winter in California, while Great Plains curlews travel south to reach Mexico.



(Illustrations by Luke Seitz. BirdCast is a joint project of computer scientists and bird biologists at the Cornell Lab, Oregon State University, University of Massachusetts, Amherst, and other partners.)


  • Sampson

    I live at about 4,500ft in the Southern Sierras. I have lived here for 20 years. I have only seen Mountain Chickadees maybe twice down at this elevation and just for a day or two. I have seen and heard them up higher on our mountains at around 6,000ft quite often. This is the first year I have a cute little Mountain Chickadee that has stayed around for over a week and is eating out of my birdfeeder! I fill it with safflower and black oil sunflower seeds and see lots of species feed but never this one until now. While my husband and I enjoy seeing this darling little bird we wonder why it has decided to stay so long.

  • David Davis

    Interesting article–thanks. We have property in the mountains of NW Virginia where we do most of our birding and eBirding. I have often wondered what proportion of fall migrants there come from the north (northern tier states and/or Canada) vs. higher altitudes in the Appalachians (and obviously, I’m referring only to those species that have breeding populations in both, like Magnolia and Canada Warblers, Swamp Sparrows, juncos, Rose-breasted Grosbeaks, among several others). I suspect that both show up but probably at different times as the Appalachian birds only have to travel 30 or 40 miles, mostly downslope, to reach us while the northern birds must traverse hundreds of miles. In a potentially related phenomenon, in late summer (August) we usually have a number of species show up in our little habitat-rich valley that breed locally but at higher elevations–some less than 5 miles away. Examples are Ovenbirds, Worm-eating Warblers, and Am. Redstarts. I suspect that they come into our valley for the richer food supply and more abundant water than that available on the higher, drier slopes. Has anyone explored these two questions? If not, it seems to me that they would make interesting topics of scientific and conservation value. Keep up the great work.

  • MiTmite9

    Not recently, but in years past I have been outside at night and heard migrating birds peeping to one another. It is a magical and thrilling sound for me.

  • Tom

    I so enjoyed your comment. Where can I experience the same thing as you have?

  • Bill Pretlow

    We live in the Sonoran Desert near Phoenix. Abert’s Towhees commonly visit our feeders. One year, a Spotted Towhee showed up even though we are far from its natural range. Normally, the Abert’s are peaceful, shy creatures, but they harassed the Spotted bird unmercifully.

  • Ellen

    When Northern Lapwings end up in NE America what happens with them ? Do they find their way back to where they would normally migrate to?

  • Ellen

    I have recently seen some loons in our neighborhood ponds in Chandler AZ. That’s a first for me. Is that a common occurance?

Eight Intriguing Migration Mysteries Solved With BirdCast and eBird