I addressed the subject of slots on my website, Nestboxbuilder.com, as there is a misconception on why slot boxes work to deter sparrows in some areas (if they work at all):
“Nestboxes with a horizontal slot rather than a conventional round or oval entry were designed to be less attractive to House Sparrows. These boxes are usually very shallow and have a small nest cavity. In some areas House Sparrows do not favor this combination, while in other regions slot boxes have no effect in deterring them. It is generally agreed that the small, shallow cavity of a slot box has more to do with deterring House Sparrows than the slot itself.”
Two of the original slot box designs, the Troyer and the Hughes, have a very small floor: 3 1/8″ x 3 1/2″, for a total floor space of only 11 square inches. Because the fronts on these boxes angle outward as they go up, the higher the nest is built, the larger it becomes. But the bird doesn’t see this on initial examination – it sees only that tiny floor. Additionally, these two slot designs are very shallow – about 4 1/2″ from bottom of slot to top of floor. This “drop” on traditional NABS style boxes ranges from 5 1/2″ to 6″.
Regional habitats also play a role in whether birds will accept or reject a box, slot or otherwise. If competition for cavities is severe, birds will be forced to nest in whatever is available, including slot boxes with tiny floors and shallow drops. If there are plenty of cavities for the birds to choose from, sparrows, in particular, will look for the larger cavity.
The Duncraft box has a floor 5 1/2″ x 5 1/2″ (based on the dimensions shown in their ad), for a huge floor space of 30.25 square inches. Ironically, this large floor actually defeats the original premise of it being “sparrow resistant” (the Foster and Smith box appears slightly smaller, but it’s not small enough). And as Gin pointed out, a slot entry will actually make it easier for a sparrow to drag in all the trash this bird uses to build its nest.
So the truth is, simply putting a slot entry on a box doesn’t do a thing to deter sparrows. The slot, a small floor size and a relatively shallow drop must all be combined into the design to even begin to hope it’s unattractive to sparrows.
Hope this helps…
Fred
Waleska, GA
NestboxBuilder