Compared to their country brethren, city birds behave more aggressively toward their neighbors. But contrary to (mostly anthropomorphic) expectations, overcrowding isn’t to blame, according to researchers comparing rural and urban populations of song sparrows.
Previous reports have suggested that populations of song sparrows (Melospiza melodia) living in cities show higher levels of territorial aggression. To investigate why this would be the case, Virginia Tech’s Sarah Foltz and colleagues examined the effects of a few environmental variables, including density and the availability of vegetation for building nests. The team measured the sparrows’ responses to recordings of another male’s song, Science explains, paying particular attention to how often males approached or attacked the speakers.
As it turns out, tempers didn’t flare because of cramped living quarters in dense cities. So, they conducted a different experiment to see if differences in the availability of food between the two habitats was what drove the aggression. Sure enough, rural birds became more aggressive when they were provided with food supplements. This means that readily available food played a major role in determining territorial aggression in song sparrows, they report in Behavioral Ecology this week.
Why would male sparrows respond more aggressively in the city where there’s typically more food? Sparrows defend food-rich, high-quality territories more aggressively, Science explains, though it isn’t clear if this strategy is an offensive or a defensive one. Urban sparrows might be more aggressive because bountiful territories are very valuable, or because their plentiful territory attracts more thieving, pirating sparrows.