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When Chinese Blue Breasted Quail are about 6 weeks old they will start getting adult feathers indicative of male and female birds. The older they get the more obvious these indicators get. With each new mutation and new generation of combined genetics there are becoming more and more changes to the "rules" of how to tell the difference between males and females. Though these are guidelines to sexing your quail there is always an exception.
White colored birds can be hard to sex by color alone. White birds are easiest sexed by watching their individual mating behaviors. Birds of color will have a few differences between males and females.
One of the indicators is the bib. Males in certain mutations have a white line or bib on their throat. Some bibs like in red breasted males are a thin, single white line. Of course with new mutations and combinations of mutations some of these markings are being lost in the genetics. Other males like wild colored males will have a double white line on their throat. Note that not all white marks on the throat are indicative of a male. Females such as the Blue Faced can have a blotch of white on their throat and face. It is normally not as defined as a bib but is white and causes confusion to many owners.
Another indicator is red or blue feathering. Females never show these colors. Blue is solely a mark of a male and seen in Blue Faced, Red Breasted, and Wilds. Red, not to be confused with Cinnamon colors, is a deep brick red color. This red is seen on the vent area of all dark colored males. Silver males will have a pinkish red color as seen below. If you see red on the vent it is a male. Vent color sexing is a sure bet and fairly easy.

This Silver Red Breasted Male has the pinkish color present even up to his breast. This color can be seen in most all versions of Silver males although in some it maybe just a few feathers closer to the vent. On occasion Silver males will not show the pink vent feathers.

Note the deep brick red color on this Red Breasted males underside. In some males the red color will only be lower and closer to the vent area. All dark colored males have this red and seeing red feathers of this color indicates a male. Be careful not to confuse this red with the color of a Cinnamon bird. Also note the gray blue of this males chest. This color is only present in males although all males will not show this color if it is seen the bird is a male.