The main thing is to be consistent. If it's not too much trouble, try going back the next day and the day after that. But tell them as soon as possible about the problem. Try to talk to a person about it who knows a lot about birds. Even people labelled "animal department" who work there, tend to have their areas of expertise. At my store, brandon knows the most about fish, caitlin knows the most about reptiles, and i know the most about small animals and birds. But we all know a fair amount. Still. If you told either of them it looked like the bird was plucking, they'd pull "stop plucking" spray off the shelf and hose down the bird

So ask the employee about birds first, ask questions you already know the answers to, and see if they know the answers, or if they're honest enough to say that they don't know. If they seem like someone competent, then address the issue of cruelty, but be careful about calling it cruel. People can take it the wrong way and think you're blaming them, personally.
So, in conclusion, pick out the most competent person, able to do something, tell them about the issue, go back for a few days to see if its being resolved. If nothing is being righted, call some type of organization- even a bird rescue group would help out.
Like one of the posts above me said, DO NOT BUY the bird. I see it first hand. If we buy reptiles and sell a lot, we buy more. If we buy reptiles and they dont sell at all, we don't buy those anymore. For every bird you "save" from a pet store you're ruining the lives of two more.