This sign on the Capital Crescent Trail now warns users about owl attacks. (Montgomery Parks)

Runners who use the Capital Crescent Trail in Bethesda might want to stick to daylight hours, because after dark, there’s an owl on the prowl.

Three runners have reported being attacked by a large bird, which one of them identified as an owl, while using the trail after nightfall or before dawn in the past two weeks, said Bill Hamilton, principal natural resources specialist for Montgomery Parks.

All of the runners suffered minor scratches when the bird swooped down from behind their heads. What was most upsetting to them was the initial fright, Hamilton said — when they felt something whack them from behind and worried that it might be human, not avian.

A fourth runner said that he, too, was attacked, but he did not report it to authorities.

Del Wilber said he was running about 7 p.m. “I felt something hit me in the back of the head,” said Wilber, a former Washington Post reporter. He fell to the ground and later found three or four scratch marks on his head that seemed to have come from talons. “Another guy was like, ‘Looks like a bird got you.’ ”

Hamilton asked his staff to put up signs on the trail saying, “CAUTION! An aggressive owl lives here!”

The signs remind those on the trail that it really is open only from sunrise to sunset. When it’s dark out, it’s for the birds.

“We have to share these wild lands,” Hamilton said. “One reason parks are open from sunrise to sunset is so predatory animals have a chance to hunt.”

He speculated that the culprit might be a juvenile owl that will outgrow its habit of attacking runners if it is left alone.

Juveniles are learning to stake out their territory and can be very possessive, Hamilton said. And they are still honing their skills at attacking prey, so a runner’s pony­tail might make for good target practice.

Because it is nocturnal, this owl doesn’t hoot in daylight hours, but it does tweet. A parody Twitter account, @moco owl, began tweeting Tuesday after the trail signs went up.

“You think that it’s easy out here, catching mice with these bikers and strollers scaring them away?” the “owl” asked on Twitter.

It even interacted with the @NIH Bear account, started last year when a bear in a tree at the nearby National Institutes of Health campus captured researchers’ and bear-lovers’ attention.

Pointing out a major water main break in Bethesda on Tuesday afternoon, the owl asked, “Hmmm — maybe there will be some good fish there. What do you think @NIH_Bear?”

Dana Hedgpeth contributed to this report.