Some folks believe that since starter feed is formulated to be a complete & ideal diet for chicks, that treats should be limited in order for the chicks to fill up more on their feed. The same could be said for laying hens, that the more treats they eat the less layer feed they consume, and thus produce less eggs.
I rarely buy produce solely for the birds' consumption, but they do get fruit & veggies when it's available, in the form of things unfit for human consumption. Such as watermelon rinds, apple cores, limp lettuce, bruised fruit, etc. They also get bread crusts, leftover pasta, stale bread, and other such stuff.
I do keep some cracked corn on hand for bribes as well as occasional treats. It's invaluable for coaxing runaway chickens back into their pens.
In college, when I studied Pavlov's famous experiments in Classical Conditioning, I learned that the most difficult response to extinguish was that learned in response to random, infrequent rewards. He had dogs that were trained to expect food at the sound of a bell. He measured their expectation by the salivation they'd begin at the sound of the bell, before they saw or ate the food. If they got food every time they heard the bell, it was easier to cancel that response by ringing the bell and not giving them food. But the ones who got food only at random times after hearing the bell were the ones who were the most difficult to get to stop expecting food the next time they heard the bell. Sort of like gamblers, willing to continue hoping that the payoff will come the next time.
So I think it's better to give pure treats to my chickens at random intervals, not every single time I go out to see them. They'll quickly learn to associate the sound of your footsteps, or the opening of a gate latch, or the slam of a screen door with the possibility of food. And they'll come running to you. But some times I just talk to them nicely, and other times I'll bring them a treat, they never know when & they'll always come running.
I know that part of the pleasure of keeping animals is providing foods they enjoy eating. I'd take a moderate approach & not give them things more than once a week, or when available. Especially for young chicks, who, like children, need to eat their healthy food in order to grow up strong and not fill up on snacks.
