I've often thought that the falcons wait for what they think is a sure thing as they conserve energy for the hunt. This also makes sense to me as it gives their prey a false sense of security.
1/25/16: Egg Alert! We have our first egg of 2016! We have noticed that the mother has been leaving it exposed a good bit. We're hoping this is just delayed incubation, which the parents sometimes do with the first egg to ensure the eggs hatch closer together. If another egg is coming, it should be here in about three days.