Body Mass in Common Loons (Gavia immer) Strongly Associated with Migration Distance

During 25 field seasons between 1988 and 2012, Biodiversity Research Institute captured and uniquely color-banded 2,730 adult Common Loons (Gavia immer) on breeding territories in 11 States and seven Provinces throughout North America. Body mass was obtained from each individual; tarsus and bill measurements were obtained from more than half the birds banded. Clinal variation in body mass, tarsal width and bill length was observed. Body mass varied from 2,700 g to 7,600 g; loons from populations in the upper Great Lakes and central Canada were smallest, and size increased to the east and west. Examination of band return and satellite tracking data resulted in three migration distance groups: < 1,500 km; 1,500-3,500 km; and >= 3,500 km. Body mass was inversely related to migration distance. Males were significantly larger (> 20%) than associated females, and within-pair differences increased with decreasing migration distance (i.e., males from coastal States were proportionally larger than their mates compared to interior State pairs).