Ceylon cinnamon is safe in small amounts and even mildly beneficial — but use Ceylon, not cassia: cassia (the common supermarket cinnamon) is high in coumarin, which is hard on the liver in quantity.
Keeper's noteFor an occasional pinch on warm mash, supermarket cinnamon is fine. If you use it regularly, buy Ceylon cinnamon specifically. Never cinnamon-flavored or sugared products.
Why cinnamon is good for parrots
There are two cinnamons. Ceylon ('true' cinnamon) is low in coumarin and safe as an occasional pinch. Cassia — what most supermarket 'cinnamon' actually is — contains much higher coumarin, which can stress the liver if fed regularly. A rare light sprinkle of either is unlikely to harm, but for any routine use, choose Ceylon.
How to serve it
Ceylon cinnamon for any regular use; a small pinch only.
An occasional light sprinkle of cassia is unlikely to harm.