Several other species of birds might take up residence in your nestboxes.
Tree Swallows (and other swallows) and House Wrens are native to British Columbia. Nestbox monitors usually welcome Tree Swallows and sometimes House Wrens, although wrens can evict or even kill bluebirds. If you don't want wrens, remove their nests before they lay eggs. Once they lay their eggs, you must leave them be.
House Sparrows and European Starlins are not native to North America and are considered invasive species. Therefore nests, eggs, young and adults may be legally disposed of or dispatched.
Unwanted species can be discouraged by box placement. House Wrens like boxes that have bushes or trees nearby. House Sparrows are most common in urban locations, so put your boxes in rural, open countryside. Starlings can be kept out if you keep the entrance hole to the specified size; too large and starlings, or other species, such as cowbirds, might enter.
above: Tree Swallow (photo by Harold Sellers)
below: House Wren (photo by Harold Sellers)
left, upper: European Starling
left, lower: House Sparrow
above: Black-capped Chickadee
photos courtesy of allaboutbirds.org