Why should nest boxes be off of the ground?

[Arielle]

I have a stack of nest boxes, two wide, three high. THe top two are the popular nest boxes. THe bottom two almost never have an egg, unless someone was crossing her legs and was desperate, then the others follow suit and I find a huge pile. IT is all or nothing in the bottom two, usually nothing. Saves getting down on my knees.

 

[BirdMan32]

Four feet high, two feet wide, two feet deep with 4 boxes. While the top ones are most popular, the one at the bottom (on the ground) gets it fair share as well.