1.Urbanized habitats differ from natural ones in several ecological features, including climate,
food availability, strength of predation and competition. Although the effects of urbanization on
avian community composition are well known, there is much less information about how individual
birds are affected by these human-generated habitat differences.
2.In this study we investigated the relationships between the morphological characteristics and the
degree of habitat urbanization in house sparrows, Passer domesticus (Linne 1758) . We collected
data for more than 1000 non-breeding adult birds in Hungary between 1997 and 2006, from seven
sites including farmlands, suburban areas and city centres.
3.We found that the body mass, tarsus length and body condition of free-living sparrows differed
among the sites: birds in more urbanized habitats were consistently smaller and in worse condition
than birds in more rural habitats. A composite measure of habitat urbanization (based on building
density, road density and vegetation cover) explained over 75% of variance between sites in the
studied traits, after we controlled for the effects of sex, year, season and time of capture.
4.The difference in body mass between rural and urban sparrows was significant when birds were
kept in aviaries under identical conditions, with constant ad libitum food availability. It is therefore
unlikely that the reduced body size and condition of urban sparrows are a consequence of reduced
access to food for adults (e.g. due to strong competition), or their short-term responses to high food
predictability (e.g. by strategic mass regulation).
5.Wesuggest that habitat differences in nestling development or adaptive divergence of sparrow
populations due to distinct environmental conditions (such as differing predation pressure) may
account for the differences along the urbanization gradient.