The impact of urbanization on body size of Barn Swallows Hirundo rustica gutturalis

Urbanization implies a dramatic impact on ecosystems, which may lead to
drastic phenotypic differences between urban and non-urban individuals.
For instance, urbanization is associated with increased metabolic costs,
which may constrain body size, but urbanization also leads to habitat
fragmentation, which may favour increases in body mass when for
instance it correlates with dispersal capacity. However, this
apparent contradiction has rarely been studied. This is particularly
evident in China where the urbanization process is currently occurring at
an unprecedented scale. Moreover, no study has addressed this issue across
large geographical areas encompassing locations in different climates. In
this regard, Barn Swallows (Hirundo rustica) are a suitable model to study
the impact of urbanization on wild animals because they are a widely
distributed species tightly associated with humans. Here, we collected
body mass and wing length data for 359 breeding individuals of Barn
Swallow (H. r. gutturalis) from 128 sites showing different levels of
urbanization around the whole China. Using a set of linear mixed-effects
models, we assessed how urbanization and geography influenced body size
measured using body mass, wing length and their regression residuals.
Interestingly, we found that the impact of urbanization was sex-dependent,
negatively affecting males’ body mass, its regression residuals, and
females’ wing length. We also found that northern and western individuals
were larger, regarding both body mass and wing length, than southern and
eastern individuals. Females were heavier than males, yet males had
slightly longer wings than females. Overall, our results showed that body
mass of males was particularly sensitive trait to urbanization, latitude
and longitude, while it only showed a weak response to latitude in
females. Conversely, while wing length showed a similar geographical
pattern, it was only affected by urbanization in the case of females.
Further research is needed to determine if these phenotypic differences
are associated with negative effects of urbanization or potential
selective advantages.