Frogs will eat anything that can fit in their mouths. One type will eat fruit, although all are carnivores, basically..from plankton to fruit to ducks to snakes. If something is poisonous to a frog's system, the frog will turn it's stomach out to get rid of the matter and wipe it off with it's foot . So, yes, frogs will eat birds.
Reading a lot about them through the years; also The Exploratorium.