Anatomy and Evolution of Avian Brain and Senses: What Endocasts Can Tell Us

Abstract

While a part of the evolutionary history of the bird brain remains unresolved, bird diversifcation appears to have been marked by an increase in visual abilities and a decrease in sense of smell. Both capacities are refected in the degree of development of the brain structures responsible for managing that particular sensory input. Similarly, more complex cognitive abilities would be accompanied by larger relative brain sizes in certain neornithine clades, such as parrots and songbirds. How can we not marvel at the intelligence exhibited by a crow carrying a walnut to a street so when a car crushes it, it becomes easier to access the nutritional content? How can we not be surprised when we see a grey parrot learning how to use a key to open a door?Birds exhibit a wide range of niche occupation and dissimilar behaviors. This diversity seems to be refected in the great disparity of brain structures. Although it is possible to identify widely varying degrees of encephalization, as well as modularity of functions, the functional interpretation of such differences is still speculative among birds. This places birds as an excellent subject for studying the correlation between brain, behavior and cognition. The last few decades have brought important breakthroughs in the understanding of several features of the avian brain, while at the same time it has highlighted the increasing necessity for more descriptive and functional information. The picture of the evolution of avian brain and senses presented here shows that further investigation is needed, from improved availability of open-source visualization software, to increases in the number of brain and inner ear models in open databases, as well as a wider sample of living and extant species belonging to different families, orders and lifestyle habits. Brain development in wild birds is also an unexplored feld that would represent a great breakthrough. Complementing morphological studies of 3D virtual brain models with histology and establishing how the surface characteristics of the endocast correlate with different aspects of brain tissues would represent a great advance that will allow the study of micro- and macroevolutionary patterns in brain evolution.