Population increase and aspects of colonization of the Tree Sparrow Passer montanus in the agricultural landscapes of southern Finland

Abstract

While many farmland bird species are declining in Western Europe, the Finnish Tree Sparrow Passer montanus population has increased significantly during recent decades. During 1984-2002, we studied the population changes, colonization characteristics and nest sites of the Tree Sparrow in a large area of agricultural landscape within southern Finland. The study focused mainly on the local landscape level. To explain the patterns of colonization, we took into account the conspesifics of the species itself, the abundance of resources, the land use types in the agricultural area, the effects of a potential competitor (the House Sparrow Passer domesticus) and the amount of human impact. The Tree Sparrow population increased exponentially during the study period. The species colonized places where House Sparrows were present, but we did not observe competition between the two species. Human impact had a positive effect on colonization. The Tree Sparrow showed a strong capacity for colonization in the agricultural environment. The land use of the agricultural area did not have significant effects on colonization at the spatial scale used. The Tree Sparrow showed more diversity in nest sites than the House Sparrow.