Effects on rural House Sparrow and Tree Sparrow populations by experimental nest-site addition

Abstract

While rural House Sparrows (Passer domesticus) have declined in Sweden, the population of the related Tree Sparrow (P. montanus) has remained stable, suggesting that they are affected by different population regulating mechanisms. Here we investigated if the two species differ in degree of nest-site limitation. After conducting an initial survey of both species on farmsteads in southern Sweden, we supplemented a subset of farms with additional nest-boxes. The farmsteads were resurveyed 3–4 years later to compare any change in population sizes for the two species between farms with and without added nest-sites. We also considered the availability of other nest-sites by counting the number of tile-clad buildings on the farmsteads. We did not find any effect of adding nest-boxes on House Sparrow population sizes, but a significant increase in the population estimate of Tree Sparrows. The number of Tree Sparrows was related to the availability of tile-clad buildings, whereas no such relationship was found for House Sparrows. At low and intermediate population sizes, House Sparrow nest-box utilization declined with the number of tile-clad buildings, suggesting preference for nest-sites under tiles when available. Our results suggest that while Tree Sparrow populations in the southern Swedish farmland are nest-site limited, House Sparrow populations are mainly limited by other mechanisms. Our results suggest that nest-site limitation is unlikely to explain the stronger population decline in rural House Sparrows compared to Tree Sparrows and that nest-site addition is unlikely to be an efficient remedy for the rural House Sparrow decline.