Although the rate of decline of House Sparrow numbers has reduced since 1994, the start of BBS, prior to that, between 1976 and 1994, there had been an enormous drop of about 70% detected by the Common Bird Census (see BTO webpage). The causes of the decline in House Sparrows are stated to be a decrease in survival and a decrease in productivity.
There is little data on the population level of Swifts prior to 1994, apart from the Atlases of 1968-72 and 1988-91, which show a contraction. It could also be the case that environmental factors affecting food availability are a contributory factor in the decline of Swifts.
|A House Sparrow in an internal Swift box in Fulbourn|
This may be more damaging for House Sparrows, as they prefer to nest in close association with their congeners, whereas Swifts will nest either alone or in close association with each other, depending upon the distribution of cavities.
|Many more House Sparrows than Swifts occupy these|
Zeist boxes at Edgecombe flats, Cambridge
Sparrow terraces, comprising 3 adjacent tit-like nest boxes are commonly erected for House Sparrows, but occupancy rates are low. They host more Great Tits and Blue Tits than House Sparrows.
|Swift nest on top of House Sparrow nest at Ely Maltings|
There can be a risk that Swifts get themselves entangled, especially if string or twine is brought into the nest, so removing this at the end of the season might be a good idea.