We've had these girls for 5 weeks. Presumably they were 2-3 days old before the feed store put them out for sale.I'm not sure at what age the babies' immune systems are formed enough for them to encounter wild bird poop, but in Black Canyon I let them free range about 4-8 weeks of age with no problems. Here I haven't let my babies free range just because some of them look like they'd fly too well and it's a different neighborhood--I wouldn't want to go door to door trying to catch one of them!
Free ranging probably isn't going to happen here. I'm probably a little paranoid because they're my first chickens but we also see hawks very regularly.
