2"
That would be for chickadees and titmice. Too small for bluebirds.
A bird house of this size has a surface area of 856 square feet.
Great Question! Birds are very picky. Bird houses should have a hole in them the size for the bird to barely fit into. Small birds like very small bird houses with very small holes. Larger birds like bird houses which have medium size holes, but much room inside. Some birds, like the black martin like to have a long approach path to their house, so their bird house must be on a very tall pole high above everything or they will not use it. Some birds just take what they can get. If you have a bird feeder and you get mostly little birds, hang a couple little bird houses in a nearby tree. If you want larger birds put their larger house in your back yard, hanging many size houses or too many bird houses in one tree I have found tends to leave empty houses. Birds are very social, but they are not like people, they do not like overcrowded homes.
I have found a good site for you. Go to www.purplemartin.org. They give you tips and dimensions. The hole should be a semicircular hole with the flat side on the bottom. The best way for you to get the dimensions is to see the web site. They have pictures.
A hole the size of a quarter is plenty big enough to allow entrance to a bat.
It will make an aircraft the size of a house and make its radar signature the size of a hummingbird or even a bird.
A nest, or a bird house.
A bird house is called a nest.
ummm...a bird house...or a bird cage...maybe a bird nest
It means that the baby bird inside got out.
The entrance hole on bluebirds should be 1.5 inches.