Physiological and morphological correlates of blood parasite infection in urban and non-urban house sparrow populations

Abstract

In the last decade, house sparrow populations have shown a general decline, especially in cities. Avian malaria has been recently suggested as one of the potential causes of this decline, and its detrimental effects could be exacerbated in urban habitats. It was initially thought that avian malaria parasites would not have large negative effects on wild birds because of their long co-evolution with their hosts. However, it is now well-documented that they can have detrimental effects at both the primo- and chronical infection stages. In this study, we examined avian malaria infection and its physiological and morphological consequences in four populations of wild house sparrows (2 urban and 2 rural). We did not find any relationship between the proportions of infected individuals and the urbanisation score calculated for our populations. However, we observed that the proportion of infected individuals increased during the course of the season, and that juveniles were less infected than adults. We did not detect a strong effect of malaria infection on physiological, morphological and condition indexes. Complex parasite dynamics and the presence of confounding factors could have masked the potential effects of infection. Thus, longitudinal and experimental studies are needed to understand the evolutionary ecology of this very common, but still poorly understood, wild bird parasite.

Fig 1. Location of the four house sparrow populations sampled in this study.

Brown and green dots represent urban and rural sites, respectively.

Fig 2

(A) Proportion of infected individuals in relation to population and urbanisation score, and (B) infection status (0 = uninfected, 1 = infected) in relation to capture date, in adults and juvenile house sparrows. (A) Dots with error bars indicate the means (± SE). Black dots correspond to the adults and grey dots to the juveniles. (B) Dots represent the infection status of the individuals. The black and grey solid lines with standard errors (grey areas) represent the model predictions for adults and juveniles, respectively.

Fig 3. (A) Haematocrit, (B) baseline corticosterone level, (C) stress-induced corticosterone level, (D) body mass, (E) tarsus length, (F) wing length and (G) body condition in relation to infection status (0 = uninfected, 1 = infected) and age.

Dots represent the raw data. Grey boxplots represent adults and white boxplots represent juveniles.