Two great oceans converge at the southern tip of Africa, and at this time of year, so do humpback whales! Some 60 of the white-flippered leviathans were recently spotted near Cape Town as they cruised along their summer migration route.

The beautiful aerial footage was captured this week by YouTuber Kieran Donnelly. Humpbacks are some of the most commonly seen whales in the area as they move down south-west Africa biannually – but an aggregation this big is a rare sight! "We were about 1,900 metres offshore," Donnelly notes in the video description.  

Humpback whale activity tends to follow a split timeline: they feed in summer and breed in winter. The seasons differ on either side of the equator, which means populations in the northern and southern hemispheres never meet: while one heads away from the equator to gorge on krill, the other moves towards it to give birth in warm, tropical waters. 

Many commenters have described this week's sighting as a whale "feast" – and with the arrival of summer in South Africa, the timing does seem right for it. But southern-hemisphere humpbacks don't typically feed here. The whales are lumbering slowly away from their breeding grounds off Angola, towards Antarctica's cold, nutrient-rich waters. Once there, they'll feast at the edge of the pack ice for months. What we're likely seeing in Donnelly's video is a rest or socialisation stop en route.

You'll notice that many of the whales in this clip are slapping their large flukes against the water as they dive. This "lob-tailing" can be heard several hundred metres beneath the surface, and scientists speculate the manoeuvre is a form of non-vocal communication. 

Juveniles are typically the first to arrive at summer feeding grounds, followed by mature males and finally mother-calf pairs. Though they weigh up to one ton at birth, humpback whale calves are particularly vulnerable to predators. They must put on weight before making the long journey south, which could explain why we don't see any youngsters in Donnelly's video. Humpback milk is some 60 percent fat, and the giant babies suckle 200 litres of the nutritious substance per day to make their hefty weight goals.

It's important to note that unauthorised crafts cannot legally approach whales within 300 metres, so if you're lucky enough to spot the migration in action, make sure you keep your distance. 

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Top header image: Kieran Donnelly/screengrab