AbstractAnimals that show aggression often risk injury and incur steep energetic costs. Thus, aggression should occur at such times and towards such opponents as to maximize fitness. We tested hypotheses predicting adaptive territorial aggression in the common loon, a species in which ease of observation of territory owners and floaters (prebreeders) seeking to evict them provide a rare window onto owner-floater competition. As predicted, older, more competitive floaters (4-year-olds and upwards) tended to intrude into territories that had produced chicks the previous year (and, hence, were of high quality). Older floaters also showed predicted increases in aggression and territorial yodeling, and a lower rate of submissive behaviors than younger floaters. Floaters of all ages intruded more often than neighboring territory owners, as predicted, but tended to avoid territories with chicks. For their part, owners yodeled more often and behaved more aggressively during chick-rearing, although yodels peaked in frequency 2 weeks before aggression, suggesting that males with young chicks yodel to discourage intrusions, but employ aggression to protect older chicks. Territory owners showed the predicted higher rates of aggression and yodeling towards older, more dangerous floaters than towards young, submissive ones. However, territorial pairs did not treat floaters more aggressively than neighbors, overall. Moreover, owners showed no spike in aggression nor yodeling following a year with chicks, perhaps to avoid providing social information to floaters that use chicks as social information to target territories for eviction.Significance statementFloaters are young nonbreeding individuals that compete with territory owners and are future breeders. Yet floaters are difficult to study because they are mostly unmarked, nomadic individuals. Owing to extensive efforts to capture juvenile common loons, we have established a large population of marked floaters in this species. Hence, loons offer a rare window to investigate efforts of floaters to settle on breeding territories. We found that older floaters (4 to 8 years), which are capable of evicting owners from their territories, target high-quality territories for their intrusions, show more aggression, and show less submissiveness during intrusions than do young floaters. We further discovered that territory owners are more aggressive towards older, more dangerous floaters. Our findings show that territory owners recognize the degree of threat posed by each floater and treat each floater differently on that basis.