Most leaves are safe for birds.
Cherry and Grape vines are banned and banished from my yard because Cherry is toxic to rabbits and Grape vine is deadly to goats.
Birds do not normally eat leaves and buttons and most quail and pheasant use them as shelter and nesting hides.
when the Maple trees start to bud in spring I go out and clip back some of the smaller trees and bring in the branches with buds attached to the rabbits, it boosts their ability to shake off winter blahs. plus its a tiny bit of a sweet treat.
I would imagine bringing in branches with buds will have the buttons picking them off and enjoying a treat.
I give mine maple and pine branches. I think it is funny to watch them when you change things around. They are nosie little things. They run around exploring there new world LOL
I would prob'ly stay away from the walnut family, too... Walnut, butternut, etc. They produce a chemical (juglone) that inhibits the growth of other plants. I have no idea if it does anything to birds, but I wouldn't take the chance.
And, speaking of maple buds in the spring, I found out last spring that basswood leaves are edible when they are newly flushed, and still small and tender. They taste a bit lemony.