English: Ventral and dorsal views of the brains of a, Emu (diurnal, flightless); b, Kiwi (nocturnal, flightless); c, Barn Owl (nocturnal, flying), and d, Pigeon (diurnal, flying). OT: optic tectum; ON: optic nerve ; OB; olfactory bulb (which actually consists of a cortical-like sheet in the adult kiwi – see Fig. 6b); V: vallecula. Note the reduced diameter of the optic nerve in Kiwi compared with that in the three other species (see text for actual measurements). In the dorsal view of Kiwi, note the caudal extension of the large telencephalic hemispheres, which completely hide the underlying midbrain. Note also in Kiwi that there is no obvious bulge on the dorsum of the hemisphere that identifies the Wulst in species such as Barn Owl and Emu. Scale bars: Emu, 1 cm; Kiwi, Barn Owl and Pigeon: 0.5 cm.