adapt-a-bill

Yet more cryptic prions

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Prions are the most abundant birds in the Southern Ocean, occurring in countless millions. One reason for their success is the range of bill types they have evolved on the same basic body plan, which allows them to exploit a diversity of planktonic prey.

The group known as 'whalebirds' have a curtain of palatal lamellae along the sides of the birds' upper mandibles. Indeed, 'prion' comes from the Greek word for a saw, which refers to these saw-like structures. Whale birds use their large, muscular tongues to pump mouthfuls of water through the lamellae to strain tiny zooplankton.

Broad-billed Prions have 120 to 130 lamellae along each side of the upper mandible. Their large gular pouch can be distended, just like a baleen whale’s buccal cavity, to accommodate about four millilitres of water. This might not sound like much, but it represents about two per cent of their body mass, which is like a person holding 1.5 litres of water in their mouth. The water is pumped through the sides of the bill, trapping all animals larger than the 0.16-millimetre gap between successive lamellae.

Northern Fairy Prions (above, off Mercury Island, New Zealand) have more angular, well patterned heads than Subantarctic Fairy Prions (above right, off the Prince Edward Islands).