Barn owl plumage is as diverse as the habitats they occupy. Breast color varies continuously from a dark, pheomelanin-based reddish-brown to nearly pure white. In addition, the number and size of eumelanin breast spots vary from heavily spotted to no spots at all. Plumage color and spottiness have a strong genetic component, are not inherited independent of each other, and signal different qualities of owls. Individuals of a single population can vary widely, and both sexes can display the same range of possible extents. The strength of these dimorphisms has been found to vary by region and latitude. One of our research goals was to explore and quantify barn owl plumage polymorphism in the winegrape-growing region of Napa Valley, CA, where agricultural producers have installed hundreds of nest boxes to facilitate rodent pest control. We developed standardized field methods to photograph barn owls captured at nest boxes and later processed the images using MATLAB script. Our results support previously identified patterns in sex, breast coloration, and plumage spottiness found in other regions. Specifically, females were on average darker and spottier, expressed as larger black spots, compared to males.