Arriving like dark clouds, thousands of European starlings swoop and soar through the skies of Israel each winter, with stunning moves that could rival any aerobatics team.
The birds, migrating from Russia and eastern Europe, are airborne ballerinas — forming shapes like a falling leaf, a rising dove, a giant whale swimming across the sky — as they prepare to roost for the night.
Scientists are not exactly sure how they stay in perfect formation, but bird expert Zachary Slavin with the National Audubon Society says it could be as simple as keeping an eye on their nearest neighbors.
"So just like a school of fish, there is not a leader of the flock," he said, "but rather each individual bird is watching the birds nearest to it, keeping a set distance from them, and so they are all traveling together and reacting together."
"Part of what makes them move so fluidly and in such tight groups is their really fast reaction time," he added, "so they can react really quickly to the movement of the birds around them, and when all of them do that, the message moves through the flock really quickly."
Benefits of flocks
According to Slavin, the large flocks occur throughout the fall and winter, starting when starlings finish breeding in the late summer and continuing through their migration until they return to their breeding grounds in the late winter and early spring.
Being in a large flock helps them to find and exploit food sources that can be scarce in the winter, he says.
"Many pairs of eyes means better chances of finding food sources that they can share with the other members of their flock,” Slavin said. “Their large numbers allow them to outcompete other animals that may be vying for the same food source."
A flock also provides protection.
"When they're under attack from a predator like a falcon or a hawk, flying in these huge flocks with a really unpredictable motion and lots of individual birds makes it really hard for a predator to track individuals and catch individual birds," he said.
Where flocking occurs
While flocks of this size are not very common, Slavin says they do occur throughout the starlings' wintering and migration ranges, which stretch from northern Africa across southern Europe and western Asia.
They can also occur in places where they have been introduced, including in the United States.
About 60 to 100 of the iridescent black birds were released in New York's Central Park in the late 1800s as part of an effort by a group that wanted North America to have all the birds mentioned in William Shakespeare's plays.
In Act 1, Scene III of Henry IV Part I, Hotspur fantasizes about teaching a starling to say "Mortimer" — one of the king's enemies. "Nay, I'll have a starling shall be taught to speak nothing but Mortimer, and give it to him to keep his anger still in motion."
Not just the king is angry
Since starlings thrive in a variety of habitats, their numbers in the United States have grown to between 100 million and 200 million today.
That's part of their success, says Slavin, but also the reason they are now considered a menace to native bird species.
"One of the challenges has been that they are cavity nesters so they need a natural cavity, natural hole in the tree or a nest box to raise their young in," he said.
"They are outcompeting a lot of our native species, including things like chickadees and bluebirds and woodpeckers, for these cavities because they are larger and they more aggressive and they will actually remove those birds from cavities that they are trying to claim," Slavin said.
But it's easy to overlook that kind of behavior when flocks of starlings take to the sky in their mesmerizing dance.