I realize that it's difficult to process the idea that people you like, with whom you have a friendly relationship, are cruelly neglecting their animals but that's the fact here.
By feeding their birds for them you are enabling their behavior.
The bottom line is that they HAVE NO RIGHT to,
A. Allow their birds to leave their property in search of food,
B. Expect you to feed their birds for them.
By expecting you to take care of their birds for them and ignoring the fact that you've told them their birds are on your property and eating your feed they have proven that they're not good neighbors and not friends. Rather, they are taking advantage of you.