Country birds are bigger than city slickers
Few species do well across a gradient of urbanization from cities to remote farms. Among species that can thrive across these diverse habitats, are there differences between populations found in the city and those in the country? Do urban House Sparrows, for instance, differ from their country cousins?
Recent research along the urbanization gradient in Hungary has shed light on differences between city and country birds. Researchers discovered that House Sparrows in the city were smaller, on average, than individuals of the same species that lived in nearby rural areas. Differences were detected in body mass (weight), the length of the tarsus (a bone in the lower leg), and overall body condition. City-living House Sparrows were consistently smaller and in worse condition than their country cousins. Birds in the most urban settings were 5% lighter, on average, than birds in the most rural areas. These differences remained even after the researchers controlled for the potential effects of year, season, and sex.
One possible explanation for the variation in size is a difference in diet between the city and country sparrows. To control for the influence of diet, the researchers brought adult urban and rural birds into captivity, providing both groups with unlimited amounts of the same food. Despite the identical conditions experienced in captivity, differences in body size and condition remained after several months.
So why are city birds smaller than the birds from rural areas? One possibility is that birds raised in cities do not receive proper nutrition, leading to slower growth rates in the nest and smaller overall body size as adults. Previous research does suggest that nestlings in urban environments tend to be fed lower amounts of food or lower quality foods. Another possibility is that being small in the city helps birds survive. If smaller birds are better suited for the urban environment, we’d expect the size of city sparrows to decline from one generation to the next. Further studies, possibly where birds are raised in captivity under city and country diets, may help determine the importance of diet versus adaptation to the local environment in contributing to the size differences between city and country birds.
Source: Liker, A. et al. 2008. Lean birds in the city: body size and condition of house sparrows along the urbanization gradient. Journal of Animal Ecology 77:789-795.