Sex identification is an important research topic in avian ecology because easy and accurate sex identification is useful for studies of evolution, behavior, and population dynamics. We used field photographs to measure the markings on the tail feathers of adult Grey-faced Buzzards (Butastur indicus) breeding in Japan to elucidate the attributes that allow individual sex determination. We labeled the dark, horizontal bands B1, B2, and B3, starting from the tip of the tail feathers and proceeding toward the base; we designated the light, horizontal band between B1 and B2 as W1, and the light, horizontal band between B2 and B3 as W2. To clarify the difference between the tail feathers of the sexes, we measured the ratio of W2 to W1 (W2/W1) of the right tail feather 3, right tail feather 4, left tail feather 3, and left tail feather 4, and calculated the average of these four values (average W2/ W1). We tested these ratios using the Brunner-Munzel test to determine whether they discriminated the sexes. We found that the four individual feather ratios differed significantly between males and females. In addition, males and females could be reliably distinguished using the average W2/W1. This method can be widely used for fast, non-invasive sexing of Grey-faced Buzzards because it does not require sophisticated techniques.