The National Zoo said Thursday that after 63 years it is severing its relationship with its longtime nonprofit partner, Friends of the National Zoo.

The action is being taken as the zoo faces an estimated $15 million budget shortfall due to the coronavirus, which has kept the Smithsonian landmark in Washington closed for much of the past year.

FONZ, which has also been financially slammed by the virus, will have to vacate its offices at the zoo, on Connecticut Avenue, and its members will be offered memberships in a similar program run by the zoo.

“It’s a sad moment in our history,” Steve Monfort, the zoo’s director, said. “Friends of the National Zoo has been with us for 63 years. . . . It’s a very tough decision. . . . There’s nothing negative to say about FONZ.”

FONZ was launched in 1958 with a $50 donation from the zoo’s retired director William M. Mann, a renowned zoologist and explorer who had run the zoo for 31 years, according to the zoo’s website and Mann’s obituary.

He became the first FONZ member. About 18,000 households now have memberships, which brought discounts and other perks.

FONZ also hosted events and ran guest services and concessions, among other things, the zoo says on its website.

Over the years, it helped fund field studies of elephants in Sri Lanka and the release of golden lion tamarins in Brazil, and helped secure the 3,000 acres of land that is now the Smithsonian Conservation Biology Institute in Front Royal, Va.

It helped raise money for giant pandas and for the first ZooLights display, and it recruited thousands of volunteers, according to the zoo’s website.

It saw the arrival and departure of iconic animals, as well as births and deaths, for more than six decades.

“The covid-19 pandemic has had an enormous financial impact on both FONZ and the zoo,” said Lynn Mento, the executive director of FONZ. “It required us to rethink our long-standing relationship. Both sides had to think carefully, as revenues started to decline, how we can make a go of this together.”

She added: “And we simply were not able to set a path forward. So we decided it was the right time for a seamless transition of responsibilities from FONZ over to the zoo.”

Mento called the decision “hugely sad.”

“It’s an enormous loss,” she said. “For the fabulous FONZ team, it’s a personal loss. But for our members and visitors and our education families and our event attendees, it’s a real loss for the community.”

The formal separation will take place over the next 90 days.

Monfort said that “both FONZ and the zoo are heavily reliant on nonfederal revenues that are generated through being open to the public.”

And the zoo has often been closed to the public in recent months. It first shut down in March because of the virus, reopened in July and closed again in November.

“The longer and longer it went, the more evident . . . the extent of our financial losses became,” Monfort said.

There were layoffs at the zoo over the summer. FONZ had to cut its staff, too, Mento said. FONZ was down to 26 salaried employees, from 70.

But further cuts in expenses were needed for the zoo to survive. “I have to prioritize that over anything else,” Monfort said. “It was terrible. It’s a Sophie’s choice, really.”

While FONZ has been a loyal and beneficial fundraising partner, he said, the zoo pays FONZ for programs and work it does. In addition, much of the FONZ fundraising, as well as the volunteer program, have been shifted over to the Smithsonian and the zoo, Monfort said.

Still, at times, the flow of funds from the zoo to FONZ was greater than the funds coming from FONZ to the zoo, Monfort said. “The cash flow wasn’t always positive from FONZ to the zoo,” he said.

Mento says that is misleading.

“The Zoo pays FONZ to perform in-park services, such as operating guest shuttles, renting strollers, and even emptying the trash, that FONZ performed more efficiently than either the Zoo or for-profit vendors could,” she said in an email.

“If these ‘fee-for-service’ payments for work that they request of us are excluded, FONZ has provided millions of dollars to the Zoo to support conservation research, education, and outreach programs,” she said.

Larry Hanauer, chair of the FONZ board of directors, said in an email: “FONZ offered to continue providing these and other services in an arrangement that would have generated more than $7 million revenue annually . . . but we could not reach an agreement.”

The two had been trying since July, Monfort said.

“We had no . . . preconceived notion that we would end up . . . dissolving the partnership,” Monfort said. “It wasn’t something that was planned. It unveiled itself to us really as we started cranking on the numbers.”

Smithsonian leadership got involved. “We wanted to be sure we tried everything we could,” he said.

“At the end of the day, the decision was made, mutually,” to part ways, he said. “We value what they did, and we’ve really respected their passion for our mission. . . . These are great people. This is something that is heartbreaking for us.”

Mento said that while the future is uncertain, “the FONZ mission to save endangered animals and inspire the next generation of conservationists . . . in some form will continue.”