Forget plywood. Stay with cedar, white oak, Red wood something like that.
I'm with Steven on this. I've made a lot of bird feeders and just using pine, some finishing nails and screws, stain and polyurethane they last for years. They only take a couple hours to make so when they reach the end, they are easy to replace. I even use that same style.I ll offer a different path. It s how I treat birdhouses. Especially of the design your Mom has now. Something like that takes an hour, to an afternoon to make. So I use whatever I like, or have on hand, and just build another after the last one goes lights out.