The helmeted hornbill casque is reinforced by a bundle of exceptionally thick, rod-like trabeculae

Among hornbill birds, the critically endangered helmeted hornbill (Rhinoplax vigil) is notable for its casque (a bulbous beak protrusion) being filled with trabeculae and fronted by a very thick keratin layer. Casque function is debated but appears central to aerial jousting, where birds (typically males) collide casques at high speeds in a mid-flight display that is audible for more than 100 m. We characterized the structural relationship between the skull and casque anatomy using X-ray microtomography and quantitative trabecular network analysis to examine how the casque sustains extreme impact. The casque comprises a keratin veneer (rhamphotheca, ∼8× thicker than beak keratin), which slots over the internal bony casque like a tight-fitting sheath. The bony casque's central cavity contains a network of trabeculae-heavily aligned and predominantly rod-like, among the thickest described in vertebrates-forming a massive rostrocaudal strut spanning the casque's length, bridging rostral (impact), and caudal (braincase) surfaces. Quantitative network characterizations indicate no differences between male and female trabecular architectures. This suggests that females may also joust or that casques play other roles. Our results argue that the casque's impact loading demands and shapes a high-safety-factor construction that involves extreme trabecular morphologies among vertebrates, architectures that also have the potential for informing the design of collision-resistant materials.