 FACT: In order to help save the day, the hero and heroine come  across the idea of using shredded aluminum cans in order to disrupt  the bats "radar". They get this idea after a stoner teenager points  out that he avoided a ticket because the cans on his dashboard  screwed up the troopers radar. The problem is, bats dont navigate  using Radar (i.e. radio or light waves bouncing off of objects);  they use Sonar (sound waves bouncing off of objects), which wouldnt  be adversely affected by metal. While the confetti storm could  conceivably have been confusing to the bats, it wouldnt have  "blinded" them the way the movie implied (considering the fact that  bats are used to flying in swarms or through leafy wooded areas, so  its not like a bunch of fluttering objects would completely  disoriented them).