Why do my ducks nibble at my hands?

[Alice Monroe]

I have three ducks. When I put my hand out they let me pet them, but they also like to nibble at my fingers. They don't seem to do it aggressively or anything, but is there is reason for it?

 

[SunHwaKwon]

You or someone has been hand feeding them treats is my guess. That's why I hate using food as a taming tool.