Systemic testosterone treatment induces adult female canaries to develop male-like song. This same treatment induces a doubling in size of the forebrain nucleus robustus archistriatalis (RA), known to be involved in song control, and a 51% increase in the number of synapses formed on RA neurons. In central RA, the number of synaptic vesicles per synapse increases as do several measures of synaptic size. Housing in spring-like conditions is also associated with larger synapses and more vesicles per synapse than housing in fall-like conditions. We suggest that formation of new synapses on existing neurons leading to or associated with modifications in synaptic morphology is important for acquisition of a new behavior. We also suggest that maximal behavioral and anatomical effects are associated with testosterone given under spring-like conditions.