Plasticity in the avian brain: From molecular to behavioral neurobiology

Birds diverged from mammals approximately 300 million years ago. The avian and mammalian telencephalon followed a pattern of convergent evolution, with similar solutions to functional brain organization. Here, we review the extensive literature on the various types of plasticity in the avian brain. Data collected with behavioral, lesion and electrophysiological techniques are discussed, as well as pharmacological and gene-expression studies, revealing the fine structure of molecular cascades at the basis of functional plasticity in a broad range of processes. Passive avoidance learning (onetrial- learning following experience of an aversive-tasting substance) and filial imprinting (the peculiar phenomenon occurring in precocial species whose newly hatched chicks become socially bonded to the first salient object encountered) will be discussed as examples of neuronal and behavioral plasticity. Brain asymmetries, often reported in these studies, will be also reviewed. Another example of avian brain plasticity considered is the difference in hippocampal/telencephalon volume ratio among scatter-hoarding and non-storing birds. Species relying on spatial memory to relocate their caches (hiding locations) have an enlarged hippocampus due to either seasonal pressures or different habitat demands stimulating neuronal recruitment. Plasticity associated with song production and perception in telencephalic nuclei in songbirds, and with spatial localization of sounds in the barn owl is also considered. Birds are amongst the vertebrates that display the most sophisticated and flexible learning abilities in the animal kingdom. Not surprisingly, in birds environment plays a crucial role on neuronal network plasticity, with experience-dependent changes acting at both structural and functional levels during ontogenesis. Overall, the study of plasticity in the diverse avian models proved to be a valuable tool for the understanding of brain mechanisms per se but also their evolution in the context of special ecological constraints, two mandatory steps in order to understand mammalian brains and this may be an useful aspect in the therapeutic field.