TAMPA, Fla. (WFLA) — As powerful Hurricane Ian closed in on Florida Wednesday morning, people at John’s Pass Marina spotted a large flock of frigatebirds apparently fleeing the storm.

“We had a big flock of frigatebirds move past us,” Captain Dylan Hubbard, the owner of Hubbard’s Marina, told WFLA’s Brittany Mueller. “And those frigatebirds are normally hundreds of miles offshore.”

Frigatebirds are seabirds commonly found along the coasts of tropical oceans and hundred miles out to sea. The birds are able to ride for weeks on air currents, and can travel thousands of miles without getting tired. They are able to travel for up to 260 miles in a single day for up to 48 straight days or more, according to research published in the journal Science.

“To have them come in big concentrations like that—they’re normally kind of loners—is a surefire, ominous sign that bad weather is to come,” Hubbard added. “That’s actually how they used to tell hurricanes were coming back in the day before newscasts like yours.”

Erin Rae shared video on Twitter showing a flock of startled birds swirling over Clearwater as the storm moved into Florida.

“The birds appeared all the sudden swirling around in the air,” she said.

Humans have long depended on forecast models and trackers to tell them when the storm will arrive, but how, exactly do birds predict the weather?

According to the Georgia Department of Natural Resources Wildlife Resources Division, some birds appear to possess a warning system that will tell them when a storm is approaching.

Some research suggests low-frequency sound waves generated by the large storm will trigger the birds’ departure. Other research shows they likely take notice once the wind picks up.

Most seabirds will seek shelter, taking cover inside tree holes, under sheds or on the sides of houses. Others will fly ahead of the storm. If they get caught in the outer winds of the storm, they’ll head downwind until they reach the eye and keep flying inside the storm until it dissipates.

Although storms can be deadly to birds, most manage to survive the bad weather.

“Although storms can have short-term negative impacts on nesting shorebirds and seabirds, storms often create new nesting habitat. Please be aware that hurricanes and tropical storms may change where shorebirds and seabirds nest in coastal areas, and give them space for the best chance at nesting success,” the Florida Fish and Wildlife Conservation Center says.