I found a Cactus Wren baby... What do I do?

He/She can not fly, so I put him in a box, because there is a large population of stray cats in my neighborhood. The mom and dad of the bird begun to get stressed out or protective or something, and I felt awful! But I figured I could feed the bird something until one of my parents gets home to return it to it's nest, because I can't find a ladder, and the nest is inside a saguaro cactus. What do I feed him/her? Is there a better temporary place I can keep it in, better than a box? Thanks, and please don't attack me for taking him in.