Sternal Punctures in Common Loons (Gavia immer): Gender and Territorial Aggression

Abstract. Aggression between adult Common Loons (Gavia immer) occurs frequently during the breeding season. In the past, it was suspected that most instances of intraspecific aggression took place between adult males. However, recent reports from the field suggest that female adult loons also participate in these battles. Postmortem investigation has shown that the damage incurred during these battles can range from incidental to fatal injuries. Sternal punctures are the most lasting evidence of this trauma and can serve as long-term records. Nearly half (46%, n = 268/574) of the loons received at Tufts Wildlife Clinic for postmortem from 2007 to 2015 were found to have sternal punctures. We conducted a focused analysis of the sternal punctures of 102 of these adult loons (49 female and 53 male) and found no significant difference in the number, distribution, or severity of sternal punctures between male and female loons. This suggests that the frequency and intensity of aggressive physical contests are remarkably similar between sexes and that there is no gender difference in the severity of these battles. Female Common Loons participate in lethal combats as often as their male counterparts. These findings provide an opportunity to better understand and study the biological or behavioral drivers of this aggression that is prevalent among both male and female Common Loons.