Broody hens will get angry when you try to disturb them while sitting on eggs. You can test for broody hens by placing a dummy egg in a nesting box and seeing which hen settles. Even a golf ball will do the trick. Hens lay their eggs roughly one a day and they will not settle on the eggs consistently until a decent sized clutch is laid. Newly laid fertilised eggs will carry on developing even if they are not kept at a steady warm temperature. The embryos grow faster or more slowly depending on the conditions. The vital period when steady warmth is needed for the embryo to develop is from about 12 days after laying. During this time the hen will sit on the eggs and will rotate them from time to time. To help the process go smoothly you should protect your broody hen from the others so she is not attacked. The nesting box needs to be well ventilated and shaded. Eggs are porous to allow oxygen in and carbon dioxide out so make sure the nesting material is clean. Eggs have protection against nasty bacteria but a dirty nesting box could overwhelm this natural protection or harm a newly hatched chick. New born chicks are also vulnerable to attack from other hens so consider removing them at hatching until they are robust enough to join the flock.