They are safe. I give mine sunflower heads. For a flock of 8 I would give pieces no bigger than about 6" around. I know that it is impossible to break them perfectly. Just estimate when you break them up.
A couple pieces a week won't hurt them.
I grow a variety called lemon queen just for the...
How many birds and how big is the sunflower head?
I would think the biggest issue could be to many seeds=pigging out.
For bigger heads maybe break them into more reasonably sized chunks.