The red-winged blackbirds are frequent visitors but there are also plenty of others including doves and finch styled birds.
As for the post job - it is a black pipe (1/4" I think) from Home Depot with a flange screwed on the top that allows for the house to be mounted. There is also an ~18" extension screwed to the bottom which allows for mounting into the ground (surrounded by a poured cement block). Bottom line - the house can hold as many birds as can manage to land on it at once.