A swarm of bees has killed 63 endangered African penguins on a beach outside Cape Town, the Southern African Foundation for the Conservation of Coastal Birds said on Sunday.

“After tests, we found bee stings around the penguins’ eyes,” said the foundation’s David Roberts, a clinical veterinarian. According to WTOP “This is a very rare occurrence. We do not expect it to happen often, it’s a fluke,” he told AFP by telephone, adding that dead bees were also found at the scene.

The protected birds, found on Friday, were from a colony at Simonstown, a small town near Cape Town. The area is a national park and the Cape honey bees are part of the ecosystem.

“The penguins… must not die just like that as they are already in danger of extinction. They are a protected species,” said Roberts. The South African National Parks said the birds were taken to the foundation for post-mortems and samples sent for disease and toxicology testing. “There were no external physical injuries found on any of the birds,” a parks statement said. The post-mortems showed all the penguins had multiple bee stings.