Yup, in chickens the chromosomes are Z and W. Roosters have two the same, like female mammals, and hens have two different ones, like male mammals, so it's the hen's genetic input that determines the sex of the chicks. Each egg the hen lays is another coin toss, but going by what some people on here have said, there are some birds that constantly produce ratios of male/female chicks that are quite far off the 50:50 ratio you would expect...
But if you're just going by one season's worth of hatching, you can't say for sure that you've got birds like that yet. There's every chance that next year they'll produce three female chicks for every male one, and the ratios will even up.
There's nothing you can do to alter the ratio of male/female chicks in the eggs your hens lay, but by incubating at different temperatures, you can alter slightly the proportion of male/female eggs that actually hatch. I can't remember which, but at low incubator temperatures one of the genders is quicker to die and at high incubator temperatures the other gender is quicker to die. So by raising or lowering your temperature away from 99.5 will manipulate the male/female ratio somewhat, but only by killing off developing embryos. Of course, any consistently large deviation from 99.5 has the risk of badly affecting any chicks that DO go on to hatch, so this is more a theoretical explanation than a practical way to go about hatching chicks. I wouldn't ever try it myself anyway...
Just thought: If you consistently hatch more males than female, check yout incubator temps, as that could possibly be a cause of it!
