That's the advice I used when letting my hen take her chicks into the "big fenced back-yard" (about 1/4 acre with grass, trees, flower-beds, etc...) They are about 10-12 weeks old. My hen is also hyper-vigilant, and extremely aggressive when she perceives a "threat" to her brood. My poor Aussie/Heeler believes this is his "flock" and tries to help her, but she often times shoos him off. lolA good rule of thumb to follow is they can free-range once they can fly high enough to get away from ground predators and are large enough that a hawk won't take them.