The toucan bird has a long and thick beak. The length of a toucan’s beak accounts for 1/3 of the bird’s total length, but thanks to its density being approximately 0.1 g/cm
3, the mass is only one-twentieth of the total bird. Similar to the toucan, the hornbill’s beak occupies 1/4 of the bird’s total length and the density is about 0.3 g/cm
3 [39][98][99]. Toucan and hornbill beaks are 3D cellular structures made of foam and covered with a hard surface
[11]. This structure achieves the perfect fusion of low density and sufficient rigidity.
Figure 7a, d show toucan and hornbill beaks schematically
[11]. The hornbill’s beak has a unique casque formed from a cornified keratin layer
[11][12][39]. The hard surface (keratin shell) of toucan and hornbill beaks consists of multiple layers of keratin scales (
Figure 7e)
[39]. The keratin scales are hexagonal, are glued together and overlap each other. Each keratin scale is about 50 µm in diameter, 1 µm in thickness and the total shell thickness is approximately 0.5 mm
[39]. The intermediate filaments (fibers) are embedded in the keratin matrix and create a difference in orientation from one layer to the next. So, keratin is a protein-based fiber-reinforced composite material and the mineralization of calcium increases its hardness
[12][39]. The viscoplastic response of the glue was shown to cause the keratin shell to exhibit a failure mode that was strain-rate dependent. When stretching at a strain rate of 5×10
−5 /s, failure occurred due to the pullout of the scales. When at 1.5 × 10
−5 /s, failure took place due to fracture of the scales
[39].
Figure 7c,d shows the optical and SEM micrographs of toucan and hornbill beaks. The beak bone trabecula consists of an elliptical or cylindrical rod and the pores of the beak bone trabecula are sealed off by thin membranes
[12][39]. Thus, it can be considered as a closed-cell foam, as defined by Gibson and Ashby
[100]. This closed-cell foam is composed of fibers with higher calcium content, whose Young’s modulus is twice that of the keratin shell. Meanwhile, the membranes have a composition similar to the keratin shell
[39]. The compressive response of the foam was successfully modeled by the Gibson–Ashby constitutive equation for closed-cell foam. It was found that because the density of hornbill foam is three times that of a hornbill’s beak, its strength is correspondingly higher
[99][101]. The mechanical behavior of a complete beak is dominated by hard surfaces and foam. When the bird’s beak is loaded, most of the load is borne by exterior keratin, while the foam increases the energy absorption rate, stabilizes the deformation of the beak and prevents catastrophic damage. In addition, the hollow core makes the beak exhibit a high bending resistance
[12][39].