The nucleus mesencephalicus lateralis, pars dorsalis in neotropical passerines

The volume of the nucleus mesencephalicus lateralis, pars dorsalis and the optic lobe were determined in 8 species of neotropical birds in the order Passeriformes: Pyrocephalus rubinus, Pitangus sulphuratus, Atticora fasciata, Molothrus bonariensis, Tangara nigrocincta, Ramphocelus dimidiatus, Euphonia xanthogaster and Spinus magellanicus. The ratio of the volume of the nucleus to the volume of the optic lobe was calculated for each species, and was found to be larger in all 8 species than in a series of nonpasserine species examined previously by Cobb. A possible relationship between the relative size of this nucleus and the auditory capabilities of birds is discussed.