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 are most active 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...