Fig. 1.

Cellular scaling rules for brains of songbirds and parrots compared with those for mammals. (A) Avian and mammalian brains depicted at the same scale. Numbers under each brain represent brain mass (in grams) and total number of brain neurons (in millions). Notice that brains of songbirds (goldcrest, starling, and rook) and parrots (cockatoo) contain more than twice as many neurons as rodent (mouse and rat) and primate (marmoset and galago) brains of similar size. (Scale bar: 10 mm.) (B) Brain mass plotted as a function of total number of neurons. Note that allometric lines for songbirds (green line) and parrots (red line) do not differ from each other, but they do differ from allometric lines for mammals (for statistics, see SI Results). (C) Brain mass plotted as a function of total number of nonneuronal cells. (D) Brain mass plotted as a function of body mass. (E) Total number of brain neurons plotted as a function of body mass. Allometric lines for the taxa examined are significantly different (for statistics, see SI Results). Each point represents the average values for one species. Data points representing noncorvid songbirds are light green, and data points representing corvid songbirds are dark green. The fitted lines represent reduced major axis (RMA) regressions and are shown only for correlations that are significant [coefficient of determination (r²) ranges between 0.831 and 0.997; P ≤ 0.021 in all cases]. Because nonneuronal scaling rules are very similar across the clades analyzed, the regression lines are omitted in C. Data for mammals are from published reports (for details, see Methods). CL, pigeon (Columba livia); DN, emu (Dromaius novaehollandiae); GG, red junglefowl (Gallus gallus); TA, barn owl (Tyto alba).