Seasonal growth of song control nuclei precedes seasonal reproductive development in wild adult song sparrows

In seasonally breeding adult songbirds, the brain regions that control song undergo dramatic seasonal morphological changes. During late winter and ea rly spring, increasing day length triggers an increase in circulating testo sterone that ultimately causes several song nuclei to grow in volume. The t iming of this growth relative to the seasonal development of the reproducti ve system is not known. This question was investigated in two populations o f wild song sparrows (Melospiza melodia morphna). Both populations live at the same latitude (46 degreesN), but breed at different altitudes. One popu lation resides on the Pacific coast in Washington, and the other resides in the foothills of the Cascade Mountains. Both populations experienced the s ame photoperiodic conditions, but the timing of seasonal reproductive devel opment differed between populations. Coastal birds initiated gonadal recrud escence approximately 2 weeks earlier than montane birds. Despite this temp oral difference in reproductive development, there were no differences betw een these groups in the seasonal growth of two song control nuclei, HVc and RA. During late February, both groups had low circulatory levels of testos terone (mean for coastal birds was 1.01 +/- 0.37 ng/ml; mean for montane bi rds was 1.41 +/- 0.26 ng/ml) and fully recrudesced song nuclei (for example , mean HVc volume in coastal birds was 1.77 +/- 0.08 mm(3); mean HVc volume in montane birds was 1.76 +/- 0.09). Also at this time, both populations w ere in the earliest stages of seasonal reproductive development as judged b y the degree of gonadal recrudescence (mean gonad volume was less than 10% of typical breeding size in both populations). It is concluded that seasona l song system growth is completed before seasonal reproductive development in response to submaximal levels of circulating testosterone. (C) 2001 Acad emic Press.