Image copyright National Weather Service Image caption Radar in Florida detected birds (in green and yellow) flying 3,000 feet in the air

A huge swarm of birds migrating is nothing unusual - flocks can fly thousands of miles undetected.

But in a rare event on Monday, US National Weather Service radar captured birds migrating from South America over Florida.

Atmospheric conditions caused the systems to detect the birds, which took hours to fly over the Key West station.

The group measured at least 90 miles (145km) in radius, according to meteorologists.

In 2017 a swarm of migrating Printed Lady butterflies was detected by radar over Colorado, but the phenomenon is rare.

A GIF produced by the Weather Service shows the migrating birds in green and yellow, and rain depicted in blue.

Skip Twitter post by @NWSKeyWest 📡 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 Report

Millions of birds make arduous journeys of thousands of miles every year to migrate with the seasons to breed and seek food supplies.

In the Americas, birds fly north from South America and the Caribbean to the US, starting in the early months of the year, before returning.

An animation produced by Cornwell University shows 118 migratory species in the Americas, some beginning their journeys north in February, with the largest movement in March and April.

One of the birds that flies over Florida after spending the winter in the Caribbean and Central America is the tiny northern parula, with a wingspan of just 6-7 inches (16-18cm).

Image copyright Getty Images Image caption The northern parula is one of the birds that flies over Florida for its summer migration

The pectoral sandpiper also flies over Florida, making a round trip of nearly 19,000 miles. It nests in Alaska and northern Canada, before wintering in southern South America.

The exact reasons how and why birds choose to migrate at a certain time are not fully understood.

"Some birds might be hanging out on the Yucatan or in Cuba waiting for tailwinds to pick them up," Gina Kent, a conservation scientist with the Avian Research and Conservation institute, told the Tampa Bay newspaper.

"On a good wind, more birds that might not otherwise be ready could jump on it."

The group that passed over Key West started in Cuba the day before, passed over the Keys and landed in Florida just before sunrise on Monday, said Weather Service meteorologist Kate Lenninger.

Specific weather conditions in Florida caused the radar to capture the birds' presence.

"There was kind of a stable layer of air above us that was deflecting the radar beam closer to the surface," Lenninger added. "So, we were able to pick up more low level objects."

Image copyright Getty Images Image caption 118 species of birds are known to migrate north-south in the Americas

Weather radars use electromagnetic pulses to measure the location and intensity of precipitation, including rain.

Radar measures the time it takes those pulses to return from an object, which works out its location, the UK Meteorological Office explained.

"It has been known for many years that other objects can also create a return such as flocks of birds."

Bird populations in the US and Canada are at risk, with the number declining by three billion or 29% in the last 50 years, according to two studies.

Climate change may also be putting migrating birds at risk.

Golden eagles may be unable to change the time of their annual migration to match the changing arrival of spring.

In the UK, changes in the timing of migrations may also lead to mismatches between when chicks need to be fed and the availability of food.