How come vampire bat manage to drink enough blood before its host throws it off?

it strongly holds on the host's back
it sucks blood rapidly
it is unnoticed by the host
it attacks in swarms
Vampire bats hunt during nights, seeking for sleeping animals. It doesn't land directly on a host, but instead lands nearby and approaches it on the ground. They make a small cut using razor sharp teeth and slowly lick the blood. A single feeding can last for 20 minutes.
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