Chicks have a lower survival rate because they're more vulnerable while they're small & weak. They're easier prey for hawks and their smaller size makes them more tempting for predators like opossums & raccoons, even cats & snakes. They can be more vulnerable to diseases too. That's why some folks prefer buying adult hens, to let someone else have the hassle & worry of protecting chicks for 20 weeks before getting even one egg from them in return.
Keep in mind that hens lay the most the first 2-3 years of their laying lives. So when you're buying hens you want very young ones, about 6 months old, right at "point of lay". Which means they're pullets just beginning their laying careers. Some unethical chicken sellers will sell you their older hens who are laying less often. There are ways to check the age of a hen, something about the color left in their legs or something, does anyone here know more?
I've never bought grown chickens, just raised my flock from chicks, but I think their friendliness is part nature & part nurture. Some birds will always act wary, no matter how much you handle them as chicks. But most adults will learn to appreciate their owners once they realize who is handing out the treats!
