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On autumn nights in North America, millions of migrating birds take to the skies unseen. AsÌýthe water in their bodies reflects radar beams, their movements pop up on weather maps across the country.ÌýMeteorologists typically erase these biological imprintsÌýfrom forecasts, but ornithologists do the veryÌýopposite: They mask storms 
to reveal bird activity.

Once theÌýweather patterns are removed, all that remains are signs ofÌýbirds, bats, or bugs. Experts largely depend on circumstantial evidence toÌýdifferentiate between the three. Bats just before sunset in the summer months, while birds appear just after sunset in fall and spring. And becauseÌýthey’re so much smaller, bugsÌýare only detectableÌýwhen birds and bats are absent. So by process of elimination, ornithologistsÌýknow to collect nighttime radar data during migration seasons—they've used Doppler in this way since 1941, around whenÌýradar technology was invented.Ìý

Spotting large flights, however, is the the easy part; it's much harder to identify which avian species are actually on the move.ÌýBut now, scientistsÌýthink they mayÌýfinally have a way to turn birds’ radar signatures into near-real-time tracking tools. Kyle Horton, at Cornell University, is combining with to break down these mysterious migratory clouds. When birders log sightings, they clue Horton into which species may be passing through an area, allowing him to roughly match patterns seen on weather maps.

Although theÌýtwo technologies were made for different purposes (eBird for birding convenience;ÌýDoppler for general human convenience)Ìýthey marry well, Andrew Laughlin, an ornithologist at the University of North Carolina, Asheville, whoÌýuses radar to , says. HeÌýsees a lot of promise in the combination of weather data and crowd-sourcedÌýobservations.

Horton and his collaborators at Cornell and the University of Oklahoma agree. TheyÌýplan to use their algorithms to define new flyways for songbirds. (The current flyway system, Horton explains, is solely based on waterfowl migrations.) He hopes that someday birders can wake to maps that show which species arrived overnight. Those tools are still several years away, but the team can already trace continent-scale migrationsÌýfor any given date (illustrated in the map above).

Horton can also pick out frequent flyers by examining which speciesÌýare most reported onÌýeBird checklists.ÌýThis wealth of information helps Cornell scientists understand shifts in flight behaviorsÌýsuch as speed, direction, tendency to drift on winds, and phenology; and it allows them to estimate the likelihood of encountering certain speciesÌýon a one-hour, kilometer-long morning walkÌýanywhere in the country. Below areÌýtheir projectionsÌýfor four migratory songbirdsÌýon September 25, 2016.Ìý

Pacific Flyway:ÌýYellow-rumped Warbler

Central Flyway:ÌýWilson's Warbler

Mississippi Flyway:ÌýSwainson's Thrush

Atlantic Flyway:ÌýCommon Yellowthroat

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