Plasticity of the avian song control system in response to localized environmental cues in an equatorial songbird.

A striking feature of the vertebrate brain is its plasticity. In high-latitude vertebrates, seasonal plasticity of the brain is driven by ubiquitous photoperiod cues and therefore is highly predictable and synchronous across extensive geographic ranges. A pronounced example of seasonal brain plasticity occurs in the nuclei that regulate song behavior in songbirds. These nuclei are larger in breeding than in nonbreeding birds. In the tropics, photoperiod varies little annually, and other environmental cues important for breeding can show considerable local geographic variability. We investigated whether localized patterns of seasonal breeding in tropical birds are associated with brain plasticity. We studied two populations of rufous-collared sparrows (Zonotrichia capensis) that breed, only 25 km apart, on the equator but out of phase with each other. We measured gonadal activity and the size of song nuclei (high vocal center, robust nucleus of the arcopallium, and area X) during each population's breeding and nonbreeding periods. Breeding males had larger song nuclei and greater gonadal activity than did nonbreeding birds. This plasticity was associated with local environmental cues, such that the two populations exhibit asynchronous changes in brain structure. These results demonstrate the sensitivity of the brain and its ability to use a variety of environmental cues to coordinate seasonal plasticity and reproduction.