Abstract In this paper 16 distinct, heritable colour aberrations (mutations) in the House Sparrow are described, based on specimens found in museum collections, records of individuals seen in the wild and from bird breeders keeping aberrant coloured sparrows in captivity. Based on the frequency found in the museum specimens Brown is the most common mutation in the House Sparrow, followed by Ino and Albino. Besides the mutations there is also a, presumably, non-heritable aberration called Progressive Greying described. Progressive Greying is in fact by far the most common colour aberration found in the species but was, in the past, always assigned as ‘Partial Albino’ without its real nature being understood. This paper will give some insight in the nature of Progressive Greying.

Abstract The occupation of nest-boxes by House- and Tree Sparrow in Warsaw was investigated in 2005-2009 and in 2012. Riparian forests, younger and older parks in downtown, and housing estates were included in the study as 4 types of habitats corresponding to the urbanization gradient of Warsaw. 1035 inspections of nest-boxes suitable for both species (type A) were carried out during the breeding period and 345 nest-boxes of other types were inspected after the breeding period. In order to determine the importance of nest-boxes for both species on different plots, obtained data were analyzed using Nest-box Importance Coefficient (NIC). This coefficient describes species-specific rate of occupation of nest-boxes as well as the contribution of the pairs nesting in them. Tree Sparrow occupied a total of 33% of A-type nest-boxes, its densities were positively correlated with the number of nest-boxes, and seasonal differences in occupation rate were low for this species. The NIC and the rate of nest-box occupation for Tree Sparrow decreased along an urbanization gradient. House Sparrow used nest-boxes very rarely, only in older parks and some housing estates. Total rate of nest-box occupation for House Sparrow in studied plots was 4%, and NIC was relatively low. However, locally, installation of nest-boxes limited House Sparrow decline caused by reduced availability of its typical nest sites. Both species used only A-type nest boxes. The rate of nest-box occupation by House Sparrow decreased sharply since 1980s, and opposite trend was observed for Tree Sparrow. These alterations are consistent with the general changes in both species populations in Warsaw in recent decades: decrease in House Sparrow and increase in Tree Sparrow number. The presented results suggest that loss of nest sites may not be the main reason of decrease in House Sparrow population in Warsaw. Additionally, House Sparrow decline leads to increase in nest sites (including nest-boxes) available for Tree Sparrow, what may contribute to the expansion of the latter species.