Dozens of endangered African penguins have been killed by a swarm of bees at Simonstown, a town near Cape Town.
The two species usually coexist well but the penguins may have disturbed the bees' nest, provoking the attack, experts said.
“After tests, we found bee stings around the penguins' eyes,” said David Roberts, a veterinarian at the Southern African Foundation for the Conservation of Coastal Birds.
“This is a very rare occurrence. We do not expect it to happen often — it's a fluke.
“There were also dead bees on the scene,” he told AFP news agency.
South African National Parks said it suspected the 63 penguins died between Thursday afternoon and Friday morning. Dead Cape honey bees were also found at the scene - they die after stinging.
The parks authority said the birds were taken to the foundation for postmortems and samples sent for disease and toxicology testing.
“There were no external physical injuries found on any of the birds,” a parks statement said.
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 Centre for Biological Diversity said the population of African penguins has fallen by 90 per cent since the start of the 20th century.
Penguins usually live to about 20 years old and grow to a height of around 70cm. Their breeding season takes place between May and August, when they excavate burrows to nest in.
The International Union for Conservation of Nature said the penguins' population was in decline due to commercial fishing activities and “environmental fluctuation".