Nest-box monitors often put out boxes for particular bird species. How successful are they at attracting the intended occupants? The Birdhouse Network compiled data from 1997 through 2001 to measure box-occupancy rates - the percentage of available boxes in which birds attempted to nest, as determined by evidence that at least one egg was laid.

Figure 2. Box-Occupancy Rates, 1997-2001 (among occupied boxes only, the percentage occupied by different species, determined by the presence of at least one egg).

We calculated box-occupancy rates based on the number of nest attempts for a given species and the number of boxes provided. Occupancy rates were 59 percent for small boxes (hole diameter less than two inches), 48 percent for medium boxes (hole two to three inches), and 59 percent for large boxes (hole greater than three inches). We broke the data down to show the percentage of occupied boxes used by different species (Figure 2).

Small boxes, designed to attract songbirds, were mostly occupied by Eastern Bluebirds, Tree Swallows, and House Wrens. In the West, Tree Swallows settled in a majority of boxes (24 percent of occupied boxes), followed by Western and then Mountain bluebirds. Other notable western species included Violet-green Swallows and Ash-throated Flycatchers. Boxes with medium-sized holes - including those placed for Purple Martins - attracted Eastern Bluebirds and Tree Swallows in more than 50 percent of occupied boxes, and martins in only 22 percent of occupied boxes. Large boxes, designed to attract birds such as American Kestrels and Wood Ducks, were occupied by small birds more often than by the large birds for which they were intended. Eastern Bluebirds, Tree Swallows, and House Wrens settled in 60 percent of occupied boxes, whereas kestrels and Wood Ducks occupied only 22 percent and 16 percent, respectively. Other species using large boxes included House Sparrows and European Starlings.