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

African penguins, which inhabit the coast and islands of southern Africa, are on the International Union for Conservation of Nature’s red list, meaning they face a high risk of extinction.

The death of these penguins is a very rare occurrence. “After tests, we found bee stings around the penguins’ eyes,” said the foundation’s David Roberts, a clinical veterinarian. And dead bees were also found on 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 park statement said. The post-mortems showed all the penguins had multiple bee stings.