Multiple indices of body condition reveal no negative effect of urbanization in adult house sparrows

As urbanized areas expand and develop throughout the world, the importance of understanding their effects on wildlife increases. Living in cities may be stressful for animals but may also provide benefits at the same time, and the sum of these effects should manifest in the body condition of individuals. Studies addressing this phenomenon tend to evaluate one or few indices of body condition, each of which may be subject to various confounding effects and seasonal changes. In this study we used multiple approaches to assess the effects of urbanization on adult body condition in house sparrows (Passer domesticus), a passerine undergoing population declines in urban habitats. In line with earlier studies, we found that sparrows in more urbanized habitats have reduced body mass. However, birds had similar scaled mass index (body mass corrected for body size) along the urbanization gradient at all times of the year, contradicting the previous result on type-1 regression residuals. In the non-breeding season, urban and rural birds had similar levels of corticosterone, hematocrit, and heterophil:lymphocyte ratio. In the molting season, hematocrit indicated better condition in rural birds whereas H:L ratio showed the opposite; however, these trends were not consistent between age groups. Two condition-dependent plumage traits, male bib size and wing bar size, showed no systematic variation along the gradient of urbanization. These results suggest that the environmental conditions experienced by adult house sparrows are not more stressful in more urbanized habitats, and they also highlight the importance of considering multiple indices of body condition.

► We examine how individual body condition relates to urbanization in a declining species.
► The more urbanized the habitat, the less house sparrows weigh.
► Yet none out of six body condition indices shows consistent relationship with urbanization.
► Urban birds have smaller body size but not disproportionately lower weight for their size.
► No physiological or morphological sign of stress in urban birds.