Until you have almost been hit in the face with a dead bird dangling by a thread or string wrapped hanging on a pine tree limb, wrapped around their little leg...you may not understand the "harm" of donating your scraps to the nest-building cause. No, i have not left scraps out for the birds but, yes, i was almost struck in the face by a that dead bird .
I can only imagine the terror of a long and frightening predicament that poor bird suffered. Leave nature alone! They survive just fine without our percieved good deeds.
Sandy