Allometric relationships and the evolution of the avian brain

A relatively larger brain is thought to have relatively more neurons and thus higher processing capacity. However, to what extent brain region volumes, and numbers and sizes of neurons vary with brain size remains uncertain. Here, I provide quantitative measurements on the cerebellum, telencephalon, and oculomotor nuclei across different bird species. Within the cerebellum, different neuronal populations increase in number and size at different rates relative to brain size, but there is little variation across clades. Using evolutionary path analysis, I show that the relationship between telencephalon and cerebellum size is a function of migration. Last, the oculomotor nuclei differ in volume across clades, but these differences are not driven by neuron numbers. I conclude that grade shifts in brain size are not always driven by changes in neuron numbers. I suggest that neuron size better explains changes in the size of brainstem motor nuclei than neuron number