I have never used Frontline, but the things I have heard from others who have used it are:
do not use Frontline Plus as it is toxic to chickens (don't know if this is true, only that that is what I have been told)
do not eat the eggs for at least two weeks--one friend related that after Frontline treatment her birds fresh eggs were exploding
I know some people who have claimed it worked miracles in getting rid of mites and lice, and that with regular treatment they never again appear; I know others who have seen no difference than when using other products.
My best advice is whatever products you choose, you still have to keep a weather eye on your flock, and all its individual members. The bugs often do not infest the entire flock, but will primarily congregate on one or two members, causing weight loss and anemia. And yes, it can kill them. With young birds and chics they can kill rapidly.