Last year, Violet Green Swallows (VGSs) were very interested in this box as it hung under the eaves of our shed. I was very disappointed when English Sparrows (aka House Sparrows) moved in and drove them away. The English Sparrows eluded our elimination efforts and nested in the box, but we eliminated the eggs. This year, I retrofitted the box with a new front. The VGSs returned and successfully raised at least two fledglings in the slot box.
The slot box design is a good alternative to round holes for people who would like to place bird boxes near houses for two reasons. First, it does not attract bluebirds which are extremely susceptible to cat predation around houses. Secondly, it discourages use by the non‐native and extremely aggressive English Sparrow.
Only swallows can comfortably use properly sized slots. VGSs and other swallows are not as susceptible to predation from cats as bluebirds; they eat insects in the air whereas bluebirds frequently catch insects on the ground. (Of course, the best protection for birds is to keep cats indoors, especially during the day when birds are active).
English Sparrows will have difficulty squeezing their round bodies into a properly sized slot. However, I have read that even this design is not fool‐proof against the wily English Sparrow. Always monitor any box you put up. English Sparrows are NOT a protected species; they are classified as non‐native pests by USDA and are legal to trap or shoot. English Sparrows will kill native birds, such as bluebirds and swallows. This European species will out-compete native birds for nesting spaces and will kill any bird that dares nest in a hole it claims or covets.