It is the unique arrangement of the long bill with its voluminous distensible pouch that makes pelicans remarkable. The upper mandible is strong and stiff, while the lower mandible is loosely articulated and flexible, to enable considerable distension of the enormous skin pouch which hangs from it. Both the large bill and the massive pouch are evidently adaptations to the bird’s requirement of catching a great quantity of fish. In addition, the bill is very sensitive and can detect fish in murky water. The upper mandible acts as a lid for the pouch when it is used like a fishing net, and the pouch can hold as much as 13 litres (2.9 gallons) of water (including fish) after a catch.
Answered by Hein van Grouw, Department of Zoology, Natural History Museum, London.