Describing the natural history of endemic and endangered species is useful for developing effective conservation plans. Two important pieces of natural history are the timing of reproduction and morphometrics. In this study, we estimate the reproductive season of Antioquia Brushfinch (Atlapetes blancae) based on a large database of observations and captures. We also provide the most extensive morphometric database of males, females, and juvenile individuals for this species. Taking advantage of ecological modeling techniques, we estimated that the reproductive season of A. blancae most likely spans from February through August with peak reproductive activity during May. The reproductive activity is highly correlated to day length (r=0.68 95% CI = 0.2 – 0.9; t=2.9, df=10, p = 0.01) and precipitation during the first rainy season of the year (r= 0.88 95% CI = 0.3 – 1; t=3.9, df=4, p=0.02). Furthermore, it is also highly correlated with the reproductive activity of many other species in the genus Atlapetes (r=0.81 95% CI = 0.4 – 0.9; t=4.4, df=10, p = 0.001). We also found that juveniles are smaller than adults and females are smaller than males providing evidence for both, age and sex size dimorphism. This study is the first attempt to provide important missing information about A. blancae that could help with understanding population viability. With this study, we now can focus conservation actions on periods of reproduction which are important bottlenecks of bird populations.