Brianna Amingwa and Lamar Gore stopped in mid-conversation when their ears caught a trill above the marshland at John Heinz National Wildlife Refuge near the Philadelphia International Airport.

They quickly settled on the source: a warbling vireo, a tiny songbird with a big-throated sound that rises and dips as if asking the same question over and over.

While birding is a booming outdoor activity, Amingwa and Gore, both African-American, fit into a much smaller slice of it that they hope grows.

The experience of black birders came to the forefront last month after a white woman called police on Christian Cooper, a black man who had asked her to leash her dog in New York City’s Central Park. Cooper was birding at the time.

While unrelated to his bird-watching, the experience motivated other black birders to unite and create an online community.

In response, a group of black scientists organized the first Black Birder Week, held May 31 to June 5. Within days, the group had 30,000 followers. Organizers encouraged participants to use the hashtag #BlackBirders Week to upload photos of birds they had spotted.

Amingwa, 27, an environmental education supervisor at Heinz Refuge, was one of the organizers of the national movement.

She said the goals were to make people aware there are black people actively involved in birding, as well as nature and science, and to start a conversation about what it’s like for “nonwhite folks to go birding.”

About 25 black birders across the country put together the weeklong social media event using the Twitter handle @BlackAFinSTEM.

“I was shocked how quickly it grew from the first tweet,” Amingwa said. “There were folks from all over the world putting up tweets.”

Black people who support environmental issues and enjoy the outdoors have a fraught history with being outside stemming from the Jim Crow-era when they could be harassed, or worse, if they ventured into woods or remote trails.

Amingwa, who has been birding since 2012, noted that there have always been black farmers, environmentalists and hikers.

“That’s a story that hasn’t been told,” she said.

Reactions on trails toward black people can be overtly racist, but are mostly subtle, Amingwa and Gore say. They can range from a look of surprise at seeing a black person in an outdoor environment to deliberate actions to make them feel uncomfortable.

Corina Newsome, 27, was also one of the organizers of Black Birding Week. She is an ornithologist and graduate student at Georgia Southern University studying avian ecology. She once took an ornithology course that required her to recognize 200 birds by sight and 75 by sound.

But her parents worried for her when she ventured outdoors alone.

“That thing that happened to Christian is so familiar,” Newsome said. “We know things can escalate quickly and people assume you are doing something wrong because they are not used to seeing black people.”

Newsome said people can mistake birding as “creepy” if someone is walking around a neighborhood with binoculars.

“Adding blackness on top of that can make us feel particularly vulnerable,” she said.

Though some smaller birding groups have not always been welcoming, Newsome said the National Audubon Society has been. A black birder livestream on the Audubon Society’s Facebook page drew 200,000 viewers, she said.

For now, Newsome said the Black Birder Week organizers are discussing the possibility of forming a nonprofit and creating a permanent group.