Why do bats always go out to hunt at night but not in the day?
My guesses are:
1) because of the cool night temperatures.
Bats are mammals and are warm blooded.
Flying requires a lot of energy and generates a lot of excess body heat.
So flying in the cooler temperatures at night helps the bats shed excess
body heat in the cooler night air.
This thought is re-enforced by their habit of roosting in dark cool caves
during the day.
2) Another reason is because a lot of insects come out at night, like
mosquitoes, fireflies, moths, and gnats.
3) Another reason is that the night air is cooler and has a higher density
than the hot air during the day, making it easier to derive lift by flapping
wings in cool, denser air.
Dr. Stewart has done some real thinking, and is right all down the line.
keep in mind that bats have developed echolocation which allows them to navigate
at night, probably better than can be seen with their vision during the day. It
is safer to hunt at night for predators and competition for food are all but
eliminated; except of course with other bats.
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Update: June 2012