Urban and colorful male house finches are less aggressive

Rapid human urbanization can have strong and varied impacts on the behavior and fitness of wild animals. The "credit-card hypothesis" predicts that low predation and high food predictability in cities lead to the presence of many weak competitors in urban populations. However, no experimental studies to date have found support for this hypothesis. Here, we studied the relationship between urbanization and aggressiveness in males of a widespread North American songbird (the house finch, Haemorhous mexicanus) while taking into account the degree of sexual signal elaboration (plumage coloration), which is known to impact competitive outcomes. In paired laboratory experiments, we found that colorful urban males were less aggressive than drab urban males, whereas there was no significant difference in aggressiveness between colorful and drab rural males. Moreover, we found that colorful urban males were less aggressive than colorful rural males, whereas there was no significant difference in aggressiveness between drab urban and drab rural males. In 4-bird trials (i.e., trials with colorful and drab males from both urban and rural environments), we found that colorful urban males were consistently less aggressive than all others. Taken together, these results support the credit-card hypothesis and the idea that plumage color is an important predictor of social status in urban environments. Finally, in a model-presentation study, we found that urban males with lower body condition avoided drab male models. Urban settings, along with the social and foraging conditions they create, may exert novel selection pressures that shape the competitiveness and status signaling systems of animals.