The internal bone structure of the male zebra finch demonstrates optimization by combining low weight with strength. (A) Ventral and (B) dorsal halves of a clipped bone surface rendering of non-contrasted μCT scan of the male syrinx, revealing the inside surface of the syrinx and cross-sectional views of the bronchial rings. The bronchial half-rings are hollow, laterally flattened, thin-walled bones fortified with trabeculae. The holes (asterisks) in the tympanum indicate lower X-ray attenuation values due to very thin walls or lower-density bone. The boxed inset shows a detailed view of bronchial half-rings B1 and B2 with trabeculae in bronchial half-ring B1. (C) Medial view of a semi-transparent volume rendering of the left hemisyrinx. Trabeculae can be seen as bright bars or dots, when seen on-axis, due to high density bone tissue (dashed circles). (D) Medial view of right hemisyrinx. Lateral flattening (ellipses) of bronchial half-rings increases their resistance to bending in the horizontal plane (dotted line and shaded plane) and therefore increases the maximal perpendicular force () that can be applied before mechanical failure occurs due to breaking [75, 76]. The trabeculae prevent failure of the bones due to buckling . Abbreviations as listed in Table 1.