lesserleigh

Hello.
I have a nice little backyard here in Philly, and I recently have discovered that I share it with someone, or should I say something, else. Only problem is I have no idea what! I only do container gardening for the time being...The first time I noticed was with my radishes. I noticed something had dug them up, and nibbled on what they could. I have also notice dug up areas in the bed for my sunflowers, and sometimes my tomatoes. Today was the saddest for me, b/c one of my baby tomatoes had been attacked! She was dropped from the vine and I found her discarded in the dirt. On her were little tiny teeth marks... or maybe beak marks... I don't know. That is why I am posting... this might be a longshot though...please help me identify what creature might have done this. I assumed squirrels, but I rarely see any in our backyard, which is blockaded by other buildings and their yards. I never see or here rats scurrying, but I do not doubt their presence. Would they attack a garden?? Would they attack a garden opposed to eating all the yummy discarded trash that riddles our city?? The one creature I do constantly see are birds. The occassional cardinals and some other small chirpy birds. Would they be interested in what I have going? What can I do do deter them??? I am so nervous that now that they know what i have back there, it is going to be war.
Please help.
Thanks