I was listening to James Macdonald (July 16, 2010). He was visiting Moody Church in Chicago and Dr. Erwin Lutzer was showing them around a beautiful new addition to their facility.
As they were touring about on the rooftop, the group kepot hearing this strange whistling like sound.
Inquisitive, Macdonald asked Erwin about the strange sound. Lutzer replied, "Oh the first sound you heard was a Falcon. The second sound you heard was the Falcon attacking a Pigeon and Killing it.
This was a tape that was repeatedly played exactly every two minutes in order to keep the pigeons from nesting on the roof.
YOu see a pigeon apparently has only a two minute memory. And if every two minutes they were reminded that Falcon's were near and what Falcon's could do to them...they would never nest there. More than two minutes they would forget....but every two minutes the pigeons were reminded.