Varied diets have allowed bats to thrive and survive once they reach new lands. There is even evidence of coevolution between bats and pollinating plants. For example, the durian fruit is very popular in Malaysia. Twenty years ago it was discovered, after a massive die-off of the fruit trees, that they were pollinated by nearby cave-dwelling bats. Quarrying was threatening to wipe them out and, consequently, the durian trees as well. Fear of the loss of the fruit led to the suppression of the quarrying in that area. (Pye 1999)
Though most of the larger bats are frugivorous (fruit eaters) many small bats are carnivorous in a variety of ways. Some hunt by flying among their prey while others, like those mentioned above, hover near trees or other surfaces and pick off creepy crawly bugs. These latter types, the Hawaiian hoary bat included, are referred to as gleaners. Those that swoop are most likely insectivorous, but some species eat fish or frogs or even other bats. The Phyllostomus hastatus of Trinidad is omnivorous and enjoys fruit, insects, and even vertebrate prey when it can get its feet on them. Then, of course, are the three species of vampire bats. They live on the fresh blood of larger mammals. With their sharp incisors they bite or scrape the skin until the blood flows. They produce an anticoagulant to stop the blood from clotting and must often feed for an extended period of time. This is because blood is a relatively diluted food and it takes large amounts to get the nutrition that the bats need. Vampire bats’ stomachs and kidneys rapidly separate the water from the rest of the blood. In doing so, these bats can become engorged and unable to fly. After feeding they often hop away! Because of their peculiar feeding habits vampire bats can be carriers of blood-borne diseases, especially rabies. This can be passed on to other animals, livestock, or even humans. But these bats rarely attack humans and they don’t live in the San Francisco Bay area so please don’t allow this to prejudice you against the other 800 species of bats.
Video of one method of Bat Feeding: Vampire Bats