Millions of butterflies are flitting across the sky in a northeasterly direction as the annual migration of the brown-veined white butterfly gets into full swing in South Africa.

Johan van der Walt, writing a blog for Wildlife South Africa, said the migration took place during mid-summer. Depending on climatic conditions such as rain or drought, their numbers differed each year.  

"Every year in mid-summer [December or January] they gather in their millions when they migrate in a northeasterly direction," he wrote.

"In reality, the flight is emigration and not really a migration, as they only fly one way and then die at the end of their journey.”

Following and preying on them are insect-eating birds and dragonflies out for an easy attack on the slow-moving butterflies.

“It originates in the arid regions of the Karoo and Kalahari. These populations owe their strength to the main food plant of the caterpillars, the shepherd’s tree. The core populations are maintained by the females laying eggs on shepherd’s trees before they move off to migrate.

“The mass of white butterflies probably plays an important role in pollination, but this is still poorly understood. In fact, there is much that we still do not know about this widespread butterfly.”

Van der Walt said after crossing the Northern Cape, parts of the Free State and the North West on their journey northeast, the butterflies fly mainly through Gauteng and parts of Mpumalanga and Limpopo.

"These butterflies start flying, and laying eggs, from a specific area in the southwest Kalahari [in the Northern Cape]. As they fly in a northeasterly direction, more and more join the flight. They also pause to lay eggs along the way."

He added that little research had been done on where they fly to, but they have been spotted flying above the ocean near the Mozambican coast.