The incident is the first time a predator has broken into the exhibit in its 50 year history

The zoo’s flamingos live in a 9,750-square-foot outdoor yard with a heated pool and barn. Smithsonian’s National Zoo and Conservation Biology Institute

A wild fox snuck into the outdoor flamingo habitat at the Smithsonian’s National Zoo and Conservation Biology Institute on Sunday night and killed 25 of its American flamingos—about a third of the 74-bird flock, per a zoo statement. A Northern pintail duck was also found dead, and three additional flamingos were injured.

“This is a heartbreaking loss for us and everyone who cares about our animals,” Brandie Smith, director of the zoo, says in the statement. “The barrier we used passed inspection and is used by other accredited zoos across the country. Our focus now is on the well-being of the remaining flock and fortifying our habitats.”

Zoo staff discovered the bodies early yesterday morning, along with a fox, which escaped the yard. They moved the remaining flamingos indoors and the ducks to a “covered, secure outdoor space,” per the statement. Veterinarians are treating the injuries of the three wounded birds.

The zoo’s flamingos live primarily in a 9,750-square-foot yard surrounded by heavy-duty metal mesh. The mesh was replaced in 2017 and passed an accredation inspection by the Association of Zoos and Aquariums. Staff regularly conduct exhibit-integrity inspections, and found nothing of concern during the last inspection at 2:30 p.m. on May 1, per the statement.

The next morning, workers found the dead birds and a new, softball-sized hole in the mesh. This incident is the first time a predator has broken through the barrier since the exhibit was installed over 50 years ago, the zoo says.

Following the attack, workers fortified the metal mesh, set up live traps and installed digital, motion-sensor cameras to monitor activity overnight.

Both American flamingos and Northern pintail ducks are listed as species of “least concern” on the International Union for Conservation of Nature’s Red List of Threatened Species. Though flamingos face threats such as habitat loss and nest disturbance, the IUCN states their population trends are increasing. Northern pintail duck populations are decreasing because of wetland habitat loss, predation disease and petroleum pollution, per the IUCN.

Foxes are omnivores, normally feeding on small rodents, fish, birds or fish. Smith told the Washington Post‘s Dana Hedgepeth that the incident was “normal” fox behavior and she does not expect the animal has rabies.

Foxes have killed captive flamingos in zoos before. In 2014, a fox decapitated 15 Chilean flamingos at the Frankfurt Zoo in Germany, per the Associated Press. In 1996, six flamingos owned by Queen Elizabeth II were slaughtered by a fox, and a seventh died later, reported the Associated Press.

Foxes are frequently spotted in Washington, D.C. Last month, officials euthanized a rabid fox responsible for biting nine people on Capitol Hill.

The National Zoo’s oldest flamingo Betty, a 67-year-old Carribean flamingo, died of natural causes in January this year. Flamingos in human care usually live an average of 26 years.