A natural phenomenon that usually goes unseen and unnoticed was seen and noticed over Key West on Monday: the massive nocturnal migration of birds was captured by the National Weather Service’s radar stationed there.

The field of birds was first detected around midnight and stretched over the island city, according to the Weather Service. Meteorologists said the radius of the flock measured at least 90 miles out from the center — but the actual size of the migration could have been much bigger.

The migration is typical this time of year as more than 118 species of birds start returning to North America after wintering in Central and South America or the Caribbean.

📡 Key West radar has had a busy night, but not because of weather! The most impressive display of migratory birds so far this year occurred overnight. This product shows biological targets in green/yellow flying north over the Keys. Showers/rain are depicted in darker blues. 🐦 pic.twitter.com/V2PJfucxJA — NWS Key West (@NWSKeyWest) February 17, 2020

The birds often migrate at night — in flocks that ranging from hundreds of thousands to more than a million — because the night sky helps them find their way. The stars and moon aid their navigation, and a more stable atmosphere at night helps the birds keep a steady course. The migrations often go undetected, but atmospheric conditions Monday allowed weather radar to capture the massive movement of birds that took hours to move over Key West.

“There was kind of a stable layer of air above us that was deflecting the radar beam closer to the surface," said Weather Service meteorologist Kate Lenninger of the Key West office. “So, we were able to pick up more low level objects.”

Migrations typically start around this time of year. A Cornell University study found that many species migrate in a clockwise pattern, flying out over the Atlantic Ocean as they head south, then looping back up further inland, crossing the Florida Keys and the Gulf of Mexico. Most North American songbirds, shorebirds and raptor species will cross the gulf on their way back home to raise the next generation of foundlings, said Gina Kent, a conservation scientist with the Avian Research and Conservation institute.

Migration often starts with songbirds like sparrows, wrens and warblers and raptors like the swallow-tailed kite. But sometimes it has more to do with opportunity.

A swallow-tailed kite is among the species that may have flown over Key West Monday.

“Some birds might be hanging out on the Yucatan or in Cuba waiting for tailwinds to pick them up,” Kent said. “On a good wind, more birds that might not otherwise be ready could jump on it.”

Lenninger said the group that passed over Key West early Monday started in Cuba the day before, passed over the Keys and landed in Florida just before sunrise Monday. The Weather Service’s radar in Miami also caught some of the birds. The migration patterns are typical and expected this time of year, she said, and they sometimes capture flocks on radar as the birds make their way south during fall as well.

A yellow-rumped warbler perches on a pine tree off the Osprey Trail at Honeymoon Island State Park in Dunedin. [ CLIFFORD, DOUGLAS R. | St. Petersburg Times ]

Early migration can help the survival of some songbird species threatened by earlier springs. Spring has sprung earlier in recent years, said University of Florida researcher Stephen Mayor, causing some songbirds to arrive in their new habitats too late to take advantage of the early boost in seeds and insects that comes with springtime plant growth.

But earlier migrations can also be perilous. Kent said early birds crossing the gulf risk hitting late-season cold fronts with strong headwinds. If birds reach those winds, for them it’s like hitting a wall: It leaves them with nowhere to go. Kent said she’s tracked kites that have run into cold fronts over the Gulf of Mexico and were left circling aloft for up to three days before dying.

At Tampa Bay’s Weather Service facility in Ruskin, meteorologist John McMichael said their radar hasn’t detected any migrations this season. Occasionally, distinct patterns of migrating birds do show up on radar. But he said they’re not nearly the size of the massive flock seen over Key West.