Is there such a cage or has anybody got any ideas for such a cage?

I'm trying to encourage blue tits and and robins and other small birds into our garden but we have magpies pigeons and crows stealing the food. I'm thinking whether I could make an outdoor bird cage where the blue tits robins and other small birds can get through the bars where I'll be hanging peanuts, seeds and fat balls, but the crows and the magpies and the squirrels can't get in and the cats.  Is there such a cage or has anybody got any ideas for such a cage?