Overview of birds' beak
Avian beaks are structural biocomposite materials. Generally, structural biological materials comprise a brittle mineral and ductile protein interacting in a complex structure that is organized in a hierarchical manner [1-3]. The macrostructure of bird’s beak is mainly composed of bone and a β-keratin sheath covering the bone, which is called the rhamphotheca [4, 5]. A rhamphotheca grows continuously like fingernails and has a multifunctional role including protecting the beak from ambient moisture and carrying a mechanical load with its viscoelasticity during pecking [6, 7]. At the microscale, the rhamphotheca comprises keratin scales with a diameter of approximately 50 μm with the core part of the beak being a closed-cell trabecular-like bone [8-11]. At the nanoscale level, keratin scales comprise β–keratin filaments and have a small gap between them. For the multiscale mechanical properties, the Young’s modulus of the rhamphotheca was reported as ~1 GPa for toucans and hornbills with its anisotropy depending on the geometry of the keratin scales. Under compression, the stress plateau occurs at 0.3 MPa for the toucan beak and at 2 MPa for the hornbill beak. At the microscale, the reported value of microhardness for the rhamphotheca was ~200 MPa for the toucan, hornbill, and European starling . Contrary to the rhamphotheca layer, the core part of the beak (considered to be bone) had a wide range of microhardness values with respect to the species; 0.27 GPa for toucan beak and 0.39 GPa for hornbill beaks. Seki et al. also reported the nanohardness of the toucan beaks were 0.5 GPa and 0.55 GPa for the rhamphotheca and the foam trabeculae, respectively. The nanohardness of the hornbill beaks were 0.85 GPa and 0.94 GPa for the rhamphotheca and the trabeculae, respectively (Table 1).
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