Breeding parameter changes in two syntopic urban Sparrow species with contrasting population trends

The number of House Sparrows (Passer domesticus) in urban populations has decreased considerably in recent decades in many parts of Europe, particularly in its western part. In Warsaw (Poland), between the 1970s/1980s and the 2000s a decline of almost 50% was recorded. During the same period, Tree Sparrows (Passer montanus), a species also known to have decreased in western Europe, increased significantly in Warsaw. This paper addresses the changes in breeding parameters of both species between the 1980s and the 2000s, i.e., in the period when their numbers changed significantly. Both species have advanced their breeding periods. Despite that the advancement was more significant in the House Sparrow, it did not appear to influence the number of breeding attempts per year, which increased in the Tree Sparrow. In addition, clutch size increased in the Tree Sparrow but not in the House Sparrow. In both species, the level of egg losses did not change, while the nestlings losses decreased, resulting in higher average brood productivity. The annual breeding productivity increased in both species due to fewer breeding losses, and in the Tree Sparrow due to a higher number of broods per season. The results suggest that food availability for sparrow nestlings (likely arthropod resources) in the urban habitats of Warsaw are sufficient to ensure both sparrow species a high level of breeding success. This contrasts with the conclusions of studies of urban House Sparrow populations in western Europe. The strong improvement in Tree Sparrow breeding productivity may be a factor that has determined the growth of its population in recent times.