Sub-lethal exposure to lead is associated with heightened aggression in an urban songbird

Many urban areas have elevated soil lead concentrations due to prior large-scale use of lead in products such as paint and automobile gasoline. This presents a potential problem for the growing numbers of wildlife living in urbanized areas as lead exposure is known to affect multiple physiological systems, including the nervous system, in vertebrate species. In humans and laboratory animals, low-level lead exposure is associated with neurological impairment, but less is known about how lead may affect the behavior of urban wildlife. We focused on the Northern Mockingbird Mimus polyglottos, a common, omnivorous North American songbird, to gain insights into how lead may affect the physiology and behavior of urban wildlife. We predicted that birds living in neighborhoods with high soil lead concentrations would (a) exhibit elevated lead concentrations in their blood and feathers, (b) exhibit lower body condition, (c) exhibit less diverse and consistent vocal repertoires, and (d) behave more aggressively during simulated conspecific territorial intrusions compared to birds living in neighborhoods with lower soil lead concentrations. Controlling for other habitat differences, we found that birds from areas of high soil lead had elevated lead concentrations in blood and feathers, but found no differences in body condition or vocal repertoires. However, birds from high lead areas responded more aggressively during simulated intrusions. These findings indicate that sub-lethal lead exposure may be common among wildlife living in urban areas, and that this exposure is associated with increased aggression. Better understanding of the extent of the relationship between lead exposure and aggression and the consequences this could have for survival and reproduction of wild animals are clear priorities for future work in this and other urban ecosystems.