Pullets have been hatched from pointed eggs, males from round. You see, each hen tends to lay eggs of one shape fairly consistently. One of the things he pointed out is that incubator temperature affects the percentage of male-to-female chicks. So what seems to be at work here is survival of the embryo—more females at lower temperature; more males at higher temperature. Life would be easy for we poultrymen if we could know how to tell the sex of baby chicks by egg shape, but we simply cannot. But as our experience grows, we can often see the differences by three or four weeks of age. In mammals, it seems to have something to do with conditions favorable to male producing vs. By eight weeks of age males of most breeds will begin to produce long, pointed saddle, hackle, and sickle feathers; females of all breeds tend to have broad, round feathers in these same sections. Some other heritage chicken breeds that can often be sexed at hatch include Barred Plymouth Rocks that produce chicks that are black with spots of yellow or white. For the record, chicken egg colors also have no bearing on the sex of the baby chick inside of it, too. By day 10, the cockerels will have caught up on feather development, so check early. Male chicks from these crosses will have whitish, grey, or pale brown down. Cockerels also will begin to develop combs at an early age—males being distinguishable by three to four weeks in breeds with large single combs; at six weeks for males with a pea comb. This is not to say that what would be a male chick can convert to female, remember, the hen determines the sex. In chicks from sexlink crossbreeds or other hybrid chicken breeds , we can often use down color as a method for how to tell the sex of baby chicks.