The birds fell victim to plunging temperatures towards the end of March and were unable to feed properly as they prepared for their annual migration to Europe, Gerhard Verdoorn from BirdLife South Africa told the SAPA news agency.

"The tens of thousands of birds were falling down everywhere and just dying," Verdoorn said, adding that farmers in Limpopo had at first feared the birds were being poisoned.

The dead swallows were only a small proportion of the mass migration which began on March 28, said the expert.

Verdoorn suggested that humans could try and prevent what was becoming an annual tragedy by addressing the root causes of climate change.