Figure 1. Side view of a pigeon brain showing the major brain subdivisions. The diencephalon lies between and is hidden by the cerebrum and the optic lobe.

Figure 2. Frontal views through the cerebrum of a turtle (a) and a pigeon (b), with the major regions identified; areas that are homologous for reptiles and birds are shown by the same colours. Hy, hypothalamus; S, septum.

Figure 3. Schematic diagrams of frontal views through the cerebrum, diencephalon and midbrain of the pigeon showing the cell groups that make up the major visual circuit (a) and the major auditory circuit (b) in birds. BG, basal ganglia; Ento, entopallium; EW, nucleus of Edinger–Westphal; Hp, hippocampus; M, mesopallium; Hy, hypothalamus; IC, inferior colliculus; N, nidopallium; Ov, nucleus ovoidalis; Rt, nucleus rotundus; SN, substantia nigra.

Figure 4. Frontal view through the cerebrum of a pigeon, with the major regions identified and the slab-like zones making up the Wulst (hyperpallium apicale, hyperpallium intercalatum and hyperpallium densocellulare) and dorsal ventricular ridge (mesopallium and the nidopallium) indicated by different colours.

Figure 5. Side view of a songbird brain showing the song control cell groups of the forebrain and their interconnections. The green arrow indicates the serially connected structures forming the forebrain motor circuit for song control, while the red arrows show the connections of the forebrain song learning circuit. Aud CTX, auditory cortex (also known as field L); Cb, cerebellum; DLM, dorsolateral medial nucleus of the thalamus; HVC, higher vocal centre; LMAN, lateral magnocellular anterior nidopallium; nXII, hypoglossal nucleus; RA, robust nucleus of the arcopallium.