Girl Scout leaders said they were blindsided by the move, and they are gearing up an aggressive campaign to recruit and retain girls as members.
Among the initiatives is creation of numerous new badges that girls can earn, focusing on outdoor activities and on science, engineering, technology and math. The organization is expanding corporate partnerships in both those areas, and developing a Girl Scout Network Page on LinkedIn to support career advancement for former Girl Scouts.
"Girl Scouts is the premier leadership development organization for girls," said Sylvia Acevedo, the Girl Scouts' CEO. "We are, and will remain, the first choice for girls and parents who want to provide their girls opportunities to build new skills ... and grow into happy, successful, civically engaged adults."
The Girl Scouts and the BSA are among several major youth organizations in the U.S. experiencing sharp drops in membership in recent years. Reasons include competition from sports leagues, a perception by some families that they are old-fashioned and busy family schedules.
The Boy Scouts began as a movement in England in the early 20th century. It took off in America almost immediately as a way to keep young men (as the Scout Oath puts it) "physically strong, mentally awake and morally straight." Tens of millions of American boys have taken that oath, and only the most determined attain scouting's highest rank: Eagle Scout, an elite group of American men, from billionaires like Michael Bloomberg to presidents like Gerald Ford.
Since the early 1970s, the Boy Scouts have lost two-thirds of their annual membership -- some four million boys. And that's a tradition they want to change while still preserving almost everything else.
The Girl Scouts say they have about 1.76 million girls and more than 780,000 adult members, down from just over 2 million youth members and about 800,000 adult members in 2014.
The overall impact of the BSA's policy change on Girl Scouts membership won't be known any time soon. But one regional leader, Fiona Cummings of Girl Scouts of Northern Illinois, believes the BSA's decision to admit girls is among the factors that have shrunk her council's youth membership by more than 500 girls so far this year.
She said relations with the Boy Scouts in her region used to be collaborative and now are "very chilly."
"How do you manage these strategic tensions?" she asked. "We both need to increase our membership numbers."
Surbaugh said BSA's national leadership respected the Girl Scouts' program and hoped both organizations could gain strength.
"If the best fit for your girl is the Girl Scouts, that's fantastic," he said. "If it's not them, it might be us."