City Birds Are Angrier Than Country Birds

In Alfred Hitchcock's 1963 opus The Birds, two outsiders arrive in a seaside town in northern California with a cage of love birds in tow. Their arrival in the Bodega Bay triggers birds of all types to begin attacking the residents in the area, but Hitchcock ultimately leaves the reason for this strange avian behavior a mystery.

Luckily, new research published in Biology Letters which found that birds that live in areas with high human populations (urban/suburban areas) are more territorially aggressive than birds in rural areas might be able to shed some light on this Hitchcockian mystery.

"A possible reason for this is that these birds have less space but better resources to defend," said Scott Davies, a postdoctoral associate in biological sciences at Virginia Tech. "Living near humans provides better food and shelter, but it also means more competition for these limited resources."

To reach this conclusion, Davies and his colleagues measured the territorial aggression of 35 male song sparrows on the Virginia Tech and Radford University campuses (which served as a stand in for an urban area due to their high levels of human impact) and 38 male song sparrows in the country.

In order to measure a bird's aggression, the researchers simulated an intrusion into that bird's territory by playing a recording of a male song sparrow and watching the bird's reaction. For birds living on campus, the researchers noted higher levels of aggressive behavior, such as: a tendency to remain near the speaker, the vigorous flapping of their wings, loud singing, and finally the singing of a "soft song," a garbled sound produced by birds which is often indicative of an impending attack. Although country birds also exhibited this behavior, the researchers noted that it was carried out with less vehemence.

The researchers are not the first to notice different levels of territorial aggression from rural and urban song birds—other recent studies have also noticed this link and similarly attributed the increased aggression to the defense of resource rich territories. Still, Davies and his colleagues' work is important for understanding how human population growth affects wildlife, a question that will become increasingly important as the global population adds 2 billion more people to the planet through 2050.

"Predicting the impact that human population growth will have on wildlife requires studying the species that adjust and persist in human impacted habitats," said Kendra Sewall, an associate professor of biology at Virigina Tech. "Suburban sprawl is a primary form of human habitat change and though many species can survive in our backyards, their behavior and physiology may change to cope with shifts in resources and with new disturbances."