Cellular homologies of birdsong control circuits Bradley M. Colquitt1,2*, Devin P. Merullo3*, Genevieve Konopka3, Todd F. Roberts3, Michael S. Brainard1,2 1Howard Hughes Medical Institute, Chevy Chase, MD, 20815, USA; 2Departments of Physiology and Psychiatry, University of California-San Francisco, San Francisco, CA, 94158, USA; 3Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, 75390, USA * contributed equally The evolution of the six-layered neocortex is often credited with an increased capacity for complex behaviors and cognition in humans and other mammals. Birds, however, despite lacking a layered neocortex, display complex and non-instinctual behaviors including vocal learning, tool use, and problem-solving. It is unclear to what extent these advanced behavioral repertoires are supported by shared or distinct neural circuits. Here in songbirds, we use single-cell RNA-sequencing to characterize the molecular identities of cells in the song motor pathway, a pallial circuit with function and connectivity that has been likened to the mammalian neocortex. We find that each song region contains distinct sets of glutamatergic excitatory projection neurons but a similar set of GABAergic inhibitory interneurons, similar to patterns of neuronal diversity in mammals and reptiles. Song motor pathway glutamatergic neurons have gene expression patterns similar to those described in neocortical projection neurons, but at the level of transcription factor expression, display stronger similarity to neurons in the mammalian ventral pallium. We observed multiple GABAergic neuron classes that are conserved across birds, non-avian reptiles, and mammals, yet the most abundant class strongly resembles a cell-class that in mammals is not found in the neocortex but is present in non-neocortical pallial regions. Thus, our results indicate that avian song control circuits and the mammalian neocortex are located in different pallial regions yet contain overlapping cellular types and connectivity patterns. This suggests that shared constraints associated with complex behavioral repertoires such as vocal learning can result in the emergence of functionally similar neural circuits within evolutionarily distinct brain regions.