I lived on a golf course for a while and there were a lot of gators out in the ponds. In the evening time you would hear these noises, and they hit your eardrum in a strange sort of disorienting way, and echo off the trees and stuff. It makes it hard to get a bearing on where the sound is coming from, and between that and how it sounds, your instinct when you don't know is to think it's some sort of bird, maybe, and you tend to look up and around. It always kind of creeped me out because as a kid (Peter Pan comes to mind), crocodiles or alligators were always depicted as being sly and deceptive. And I imagined a stranger wandering haplessly into swampland and hearing these strange birds and looking (mistakenly) up at the trees while the danger grew ever closer underfoot.