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“Why does it have two beaks?”

“Why does it have two beaks?” A common question, or some iteration thereof, that can be heard almost daily at the viewing window of our rhinoceros hornbill habitat. Often, our guests have seen the rhinoceros hornbills and think they are a giant toucan with a “thing on its face.” And though they do resemble toucans with their large colorful beaks, unfortunately neither are related beyond being birds. Toucans are native to Central and South America, while hornbill species are native to sub-Saharan African and Asia.

As for the “two beaks.” First, it is important to know that the beak is the exterior layer that covers the upper and lower jaw of the bird; it is what we see as their mouth. The hard structure found on the rhinoceros hornbill is called a casque. All 54 species of hornbills have some form of a casque including our southern ground hornbills, though theirs is more of a protruding ridge rather than a well-defined structure.

Casques are extensions of the upper mandible or skull, typically composed of keratin, the same stuff as our nails. And though our hornbill species are currently the only bird species at Zoo Atlanta that are in possession of this anatomical structure, they are not alone. Other species include cassowaries, certain fowl, and several species of curassow.

Focusing on the rhinoceros hornbills, there are several interesting things to point out about the casque. The first is the size difference. Males, who tend to be slightly larger in body size than females, also follow the trend of having larger casques. So, one way to tell our male, Bob, from our female, Nancy, is to look at this structure.

Another interesting thing is that chicks do not hatch with casques. They do not have fully developed casques until they are close to maturity. Even more interesting, is perhaps the coloration of the casque. As the chick is growing, the casque (and beak for that matter) are white. The bright orangish-yellow color that we see vividly on Bob and Nancy are the results of the buildup of the preening oil secreted from their uropygial gland, that pimple-like structure at the base of their tail. This is the same oil that birds would use to help condition their feathers during preening. Sometimes, it can take up to six years for the casque to be fully developed and cosmetically colored!

Now that we know what it is, let’s talk about why. The most prominent explanation for the rhinoceros hornbill casque is sound amplification. Since the casque is a primarily hollow structure, it allows for resonance to occur, helping to amplify and carry the calls of the species. This is especially important when living in a noisy forest and you are trying to reach your partner over all the other sounds. Early in the mornings, we can sometimes hear Bob calling to Nancy all the way down by the petting zoo.

For other species, the casque can be used for heavy pounding behaviors – whether as a hammer when hollowing out a nesting cavity, or even as a form of weapon against other individuals in sparring behaviors.

Currently, the rhinoceros hornbill’s status in their wild natural habitat is considered vulnerable. Though this species has been hunted for food and trade, with their casque and tail feathers sought after for traditional ceremonies, the larger threat to them is the loss of habitat due to deforestation for agricultural and timber needs.

Cynthia B.
Keeper II, Birds

Connect With Your Wild Side #onlyzooatl