Experimental manipulation of chest spotting alters territorial aggression in urban and rural song sparrows

In many taxa, melanin-based coloration is a signal of dominance or fighting ability and is associated with concentrations of hormones that may mediate aggressive behavior. Previous studies found that experimental manipulation of melanin-based signals can result in manipulated individuals receiving more social challenges in some but not all species. These differences could arise from mismatches between the signal, behavior, and hormone concentrations. In the present study, we experimentally manipulated the chest spotting of urban and rural male song sparrows (Melospiza melodia) following an assessment of their territorial aggression and initial concentrations of corticosterone and testosterone and then assessed their behavior and hormone concentrations 2 weeks later. We found that males generally displayed less territorial aggression in the second trial, consistent with our previous findings. Males in the enlarged treatment decreased aggression to a greater degree than those in the reduced treatment. The effect of the plumage manipulation was similar across the rural and urban habitats. Despite the changes in behavior we detected, we found no effects of the manipulation on concentrations of testosterone or corticosterone. Our results show that melanin-based spotting in male song sparrows is a signal of territorial aggression but the physiological mechanisms that mediate the relationships between chest spotting and behavior remain to be identified. Significance statement Many bird species use their plumage to signal their dominance status, fighting ability, or motivation during interactions with other individuals to resolve conflicts without a fight. Here, we asked whether chest spotting is a signal in territorial interactions among male song sparrows. We experimentally increased or reduced the extent of spotting in males and measured the change in their aggression. We found that reduced-spotting males showed a more moderate seasonal decrease of aggression compared to males with enlarged spotting reduced aggression, possibly because the former experienced more intrusions later on in the breeding season while the latter experienced fewer intrusions. These results are consistent with chest spotting size in song sparrows functioning as a signal of territory holding potential of the bearer.