Structural scale of the beak
The beaks of an adult Red-Bellied Woodpecker (Melanerpes carolinus), which is a medium sized bird living in the southern United States, were studied for structure-property relationships. The woodpeckers obtained had body lengths of 24~25 cm. The upper and lower beaks were separated from the body as shown in Fig. 1(b), and all tests were carried out at ambient conditions.
The full length of the beak is about 4 cm, and the cross-section of the woodpecker beak is composed of the outer rhamphotheca layer, the middle foam layer, and the inner bony layer as the schematic of Fig. 1c illustrates. The upper beak has a cavity in the center of the beak, which decreases the weight while still keeping the bending resistance fairly high. The curvature of the tip of the upper beak was measured at 19.07 mm-1, and that of the lower beak was approximately 12.01 mm-1. The density of the upper beak was 1.1 g/cm3, and that of the lower beak was 1.348 g/cm3 measured using the Archimedes method . The lower density of the upper beak is due to the cavity and more porous area at the root part of the upper beak  .