Cumulus are the characteristic fluffy white clouds of summer. They form when warm updraughts carry moist air so high that it cools and condenses out into a cloud. Because of the air current that forms it, the inside of a cumulus cloud is turbulent. You would not want to fly into one – unless you were a frigate bird.
Frigate birds spend much of their time out over the open ocean. Unlike many other sea birds they do not descend to the surface and swim. Instead, they remain airborne for months on end, and cumulus clouds help them achieve this.
A multinational team of researchers has described, in a recent paper in Science, how they followed great frigate birds (Fregata minor) with miniature tracking devices. They watched the birds deliberately seek out cumulus then allow themselves to be carried up by the thermals and into the cloud.
Frigate birds would regularly soar right through the turbulent cloud to gain altitude, and were tracked up to the freezing height of 4,000 metres – a cold place to find a tropical bird. This behaviour had never been seen before.
The updraughts that produce the cumulus are caused by upwellings of warm water in the ocean. Such upwellings bring plankton from the depths, which attract shoals of fish. Not only does cumulus give frigate birds a convenient lift, it also marks the best place to find food. From the birds’ point of view, cumulus really is a cloud with a silver lining.
