I found a budgie in my backyard?

I was sitting at the edge of my deck today and a budgie comes flying onto my leg. it scared me half to death. its a boy and it looks like its been previously owned by someone because it has a circular tag with numbers around its ankle. i took it inside and started it feeding it some of my cockateil's food and it was extremely hungry. ive been asking around if anyone owns it and have posted ads. should i keep it? what should i do. i just find this whole situation really odd hahaha