Urbanization and blood parasite infections affect the body condition of wild birds.

Human landscape transformation, especially urbanization, strongly affects ecosystems worldwide. Both urban stressors and parasites have negative effects on organism health, however the potential synergy between those factors has been poorly investigated. We analysed the body condition (i.e. body mass after controlling for wing chord) of 2043 house sparrows (adults and yearlings) captured in 45 localities along an urbanization gradient in relation to Plasmodium, Haemoproteus and Leucocytozoon infection status. Body condition was negatively related to urbanization level and to urbanized land coverage but only in yearling birds from urban habitats. In addition, bird body condition tended to increase in rural habitats, significantly in the case of yearlings. Infected individuals by Plasmodium or Haemoproteus had higher body condition than un-infected birds, but this pattern could be due to a selective disappearance of infected individuals with lower body condition as suggested by the reduced variance in body condition in infected birds in urban habitats. These results provide support for a negative impact of urbanization on bird body condition, while Plasmodium and Haemoproteus may exert selection against individuals with lower body condition living in urban habitats, especially during earlier life stages, underlining the synergistic effects that urbanization and parasites may have on wild birds.