Hm... as far as I know there is no trick, like with mammals the moment the sperm hits the ovum the sex is fixed. No sex shifting by heat or hormone, pheromone like with so many other animals. So lower/right temperatures may help you to an higher hatching rate but not to more females. I know that there are rooster/bull/bock years where everyone you talk to has more male baby animals in the stable, but that may be so b/c other factors like weather, nutition etc. will effect the mobility of the sperm in mammals or ovulation in birds. Birds determent the sex by the WZ-system not the XY-system and in the WZ-system the female is heterozygot (WZ) while the male is homozygot(ZZ). And often the male year will only be one b/ca people will tell you if they have too many males but not that they have too many females,b/c for most livestock there is no such thing than too many females.