Many new neurons are added to the adult avian brain. Most of them die 3-5 weeks after they are born (Nature (Lond.) 335 (1988) 353; J. Comp. Neurol 411 (1999) 487). Those that survive replace, numerically, older ones that have died (Neuron 25 (2000) 481). It has been suggested that the new neurons enhance the brain's ability to acquire new long-term memories (review in Sci. Am. 260 (1989) 74). If so, perhaps an increase in social complexity affects the survival of new neurons in a social species. To test this hypothesis, we treated adult zebra finches (Taeniopygia guttata) with [3H]-thymidine immediately before introducing them into one of three different social environments that differed in complexity and killed them 40 days later. There was a significant difference between experimental groups in the number of [3H]-labeled neurons in neostriatum caudale (NC), high vocal center (HVC) and Area X, three forebrain regions that are involved in vocal communication. In these regions, birds placed in a large heterosexual group had more new neurons than birds kept singly or as male-female pairs. Regulation of new neuron survival by extent of circuit use may be a general mechanism for ensuring that neuronal replacement is closely attuned to environmental change.