Is there a bribe so seductive that it will tame a wild bird?

I feed the sparrows at my work. They definitely know who I am, whether I shave or wear a hat or whatever. They will come to my feet. But there are alot of other people around all the time so they are skiddish.

If it is cold and about 8 a.m. they will come right to my hand to get food from me. So, I know they are up to it if the feeling is right.

Now what I think will help is a bribe. If I just had something that was too tempting then maybe I could have them eating out of my hand.

Any ideas? Sparrow crack?