If it is dark outside, yet light inside of the coop, all of the birdies will be in the coop. 

Any bird that isn't in the coop when the door closes will be outside for the night and may end up being a meal for a raccoon, skunk, fox, coyote, or other predator. 

If a bird is standing in the doorway when the door slides closed, there will be a "door close error" and I will receive a text message as well as an email.  The 12V car antenna isn't powerful enough to crush whatever is in the doorway. 