As you might have noticed, the pouch of flexible skin attached to the lower half of a pelican’s bill can hold a lot of water – close to 20 liters, or about five gallons. Just lifting its water-filled beak is a task in itself for a pelican. Seabirds such as gulls might make it more difficult by pestering pelicans – even sitting on their heads or backs – hoping to steal a fish.
When a fishing pelican’s bill enters the water, the pouch is contracted and the bill slightly opened. Muscles in the pouch pull the bird’s flexible lower jawbones outward to create a large oval opening. Then, when the bird closes its beak, the lower jaw returns to its original unbowed position, so that small fish and water are trapped inside the pouch.
To drain the water, a pelican lifts its head very slowly with the mandibles slightly parted. Excess water flows out the sides of the bill. It’s as simple as that. When most of the water is expelled, the bird tosses its head back and swallows the fish head first.
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