|Greater bulldog bat|
One of the more unlikey food sources, however, is fish. Yet, when you think about it, fish is a really common food source for birds - there are a huge number of bird species that feed primarily on fish, including seagulls, kingfishers, and ospreys, among many others. Clearly, there's no inherent reason why a flying mammal couldn't eat fish, yet very few of them do.
As a recent review makes clear, however, "very few" is not "none".