Get off my lawn: increased aggression in urban song sparrows is related to resource availability

Urban animals often show differences in aggression relative to their rural counterparts, but the ultimate and proximate origins of these differences are poorly understood. Here, we compared urban and rural song sparrows (Melospiza melodia), a species for which higher levels of aggression in urban populations have previously been reported. First, we confirmed elevated territorial aggression levels in urban birds relative to rural birds over multiple years. To begin to identify the environmental variables contributing to these differences, we related aggression to features of the social and physical environment, specifically population density and the availability of suitable nesting vegetation. Population distribution and the availability of suitable nest vegetation were not correlated with territorial aggression levels. Subsequently, we conducted a food supplementation experiment to determine whether potential differences in the relative availability of food between the 2 habitats might drive differences in aggression. Food supplementation increased territorial aggression significantly, particularly in rural birds. Thus, it appears that the availability of food could play a role in determining territorial aggression in song sparrows. The specific combination of these features found in urban areas may cause the increased levels of territorial aggression seen in these populations.