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Why is the lower bill of a skimmer longer than its upper bill?

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Illustration of a crab plover (Dromas ardeola), showing a slender wading bird with long legs, a long pointed black bill, white head and underparts, and contrasting black wings and back. — Charadriiform Crab plover (Dromas ardeola)

The lower bill of a skimmer is longer than its upper bill to facilitate its unique method of foraging. These birds fly over the surface of calm water with the long, knifelike lower mandible submerged to cut through the water.12

When the lower mandible encounters prey, such as a fish or crustacean, the bird's head doubles under its body and the upper mandible snaps down to clamp the prey shut. The lower mandible is one-third longer than the upper, and its sharp upper edge is designed to fit into a groove in the upper mandible when the bill is closed.123