I've never caught the Rensselaer falcons in the process of changing places, but seeing each parent sitting in the same position really highlights the difference in their sizes.
Wikipedia says that females are larger than males, so Mama Falcon is probably the one on the top left, with the more pronounced markings on her wings. Her tail reaches almost to the outer ring scraped in the gravel.
Papa Falcon is below, with shorter wings and a tail that reaches nearly half way to the outer ring. He is a more uniform color.
If still images of a bird on a box are a little too boring, check out "2009 Utica Peregrine Falcons part 1" on YouTube (the Utica Peregrines have their own videographer).