Feathers may be a bird’s most recognizable feature, but some birds have an extra bit of a crown – or actually, a casque. But what is this feature, what is it made of, and what purpose does a casque serve?

Casque Definition

(noun) A large, hard growth on the head or the top mandible of the bill of some bird species, including cassowaries, hornbills, and pelicans. While it may appear that the casque is a bony growth, dissection and testing have shown these structures may be solid or hollow and are either made of keratin – the same material as feathers, hair, and fingernails – or thick skin over a sponge-like structure similar to cartilage. Some casques are rigid while others may be mildly flexible or pliable, and their texture can range from a stiff, horny feel to a leathery surface.

The exact purpose of the casque can vary. In many species, the overall purpose is unknown or may fluctuate depending on the season or the bird’s actions. The casque could be used for…

• Courtship displays or to show good health and breeding readiness, particularly if the casque is only temporary and may be shed after the breeding season ends.
• Communication as a resonating chamber to amplify or modify sounds and songs, or else to identify an individual bird or species based on casque color, shape, or size.
• Protection from skull impacts or predator attention by shielding the more vulnerable parts of a bird’s head from direct contact with branches, rocks, or talons.
• Strengthening the bird’s bill by providing additional structural integrity along the length, particularly for species that use their bills for digging or prying.
• Body temperature regulation by adjusting blood flow through the casque to provide better cooling on hot days or restricting and insulating blood flow on cool days.

The size, shape, and color of casques can vary widely, and even in the same species there may be variation between genders or ages. This can make the casque a valuable field mark for birders seeking to identify different species or who want to identify subspecies or genders of birds with casques.

Familiar birds that have prominent casques include cassowaries and hornbills as well as the helmeted guineafowl and helmet vanga. The American white pelican has a temporary casque halfway down its bill during the breeding season, and the rhinoceros auklet has a small casque near the base of its bill when it is in its breeding plumage.

Pronunciation

KASK – Rhymes with task, flask, ask, mask, and cask

Also Known As

Horn (though it’s not actually a horn)