If it is 2 feet wide (and if my 20" diameter guess is about right), then the bird would be flying at ~26,426 feet high.
If 1 feet wide, then ~10,313 feet high.
if 3 inches wide, then ~2,578 feet high.

My guess would be some kind of insect in front of the sun...likely a gnat.

Formula:
distance = (size/2) * 1/tan(apparent size/2)