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

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

Spotting large flights, however, is the the easy part; it's much harder to identify which avian species are actually on the move. But now, scientists think they may finally 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 the two technologies were made for different purposes (eBird for birding convenience; Doppler for general human convenience) they marry well, Andrew Laughlin, an ornithologist at the University of North Carolina, Asheville, who uses radar to , says. He sees a lot of promise in the combination of weather data and crowd-sourced observations.

Horton and his collaborators at Cornell and the University of Oklahoma agree. They plan 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 migrations for any given date (illustrated in the map above).

Horton can also pick out frequent flyers by examining which species are most reported on eBird checklists. This wealth of information helps Cornell scientists understand shifts in flight behaviors such as speed, direction, tendency to drift on winds, and phenology; and it allows them to estimate the likelihood of encountering certain species on a one-hour, kilometer-long morning walk anywhere in the country. Below are their projections for four migratory songbirds on September 25, 2016. 

Pacific Flyway: Yellow-rumped Warbler

Central Flyway: Wilson's Warbler

Mississippi Flyway: Swainson's Thrush

Atlantic Flyway: Common Yellowthroat

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