Linda Violett’s 2-Hole Mansion was originally designed for Western (and Mountain) Bluebirds, which are slightly larger than Easterns, plus Westerns usually have one or two more chicks in a clutch. This partially explains the larger floor size of the 2hm (5 1/2″ x 5 3/4″, or 31.625 square inches).
The box is also very deep: the drop from bottom of entries to top of floor is 8 1/2″, compared to the 5 1/2″ or 6″ drop on conventional NABS-style boxes.
Some other reasons for this large, deep design are: room for the chicks to separate in hot weather; more space to exercise wings during development (Linda calls this “flutter space”); too deep for avian predators to reach the nest; “attic” space to keep accumulating hot air away from the nest; and others. You can read Linda’s design considerations concerning the floor space here.
The Defensible Box
Ostensibly, two entries provide the bird with an escape route in case of attack, so the Bluebird can take the fight outside, where it will usually defeat an attacking sparrow. We’ve all heard that. Whether Bluebirds will actually do this in every case might not be automatic, however. My opinion is that it’s a learned behavior, and dependent upon circumstances and upon the individual bird. But field tests conducted by Linda, Dick Purvis and others have proven that overall, over time, it works. Christine Boran, on her site The Woolwine House Bluebird Trail, conducted a positive, six-year test on 2 holers.
Do you need a Linda Violett 2-Hole Mansion to provide two entries? No. For Eastern Bluebirds, the new Xbox (designed by Tom Comfort) will easily accommodate 2 holes, as will any box with at least a 5 1/2″ wide front.
As far as a Van Ert working in this box, the plate on the Van Ert partially blocks both holes, leaving a crescent shaped opening of about 5/8″ on both holes. To escape, the sparrow would need to wedge itself inside this crescent, then force the plate spring to open the plate further. Could it do this? In a panic it probably could. However, the box wasn’t designed with trapping in mind. In fact, if Linda got wind of someone doing this (I asked her) she would scold you with unmerciful wrath (she did me) :-).
Whether a 2-holer is right for you, your habitat and your birds can only be answered by putting these style boxes up. I do know that there are far more 2 holers in use throughout the entire country than one might imagine. If you would like to build this box, or just take a look at the plan, it’s on the Plans Page of my site.