Measurements

Linear

Tremendous range in size related to geographic cline (Appendix I). Size differences within a given area are related to sexual dimorphism and natural variation. Rangewide average sexual dimorphism for linear measurements is approximately 6% for bill, tarsus, and wingspan. Natural variation in size is in response to differential use of available habitats or niche partitioning. For example, smaller individuals are more apt to use multiple-lake territories, while the largest individuals occupy partial lake territories (Evers 2001).

Field studies demonstrate linear measurements including bill, tarsus, and wingspan (Appendix I) and egg size (BRI unpubl data) reflect geographic clines. Yodel frequency also follows similar patterns (Mager et al. 2007b).

Standard linear measurements of the bill include the length from nares to tip (culmen), bill depth and width at the middle of the nare, tarsus diameter above the vestigial toe, and wingspan (measured using one wing and then doubling). Overall North American measurements are well established (Table 2), including by state and province (Appendix I).

Mass

Body mass widely ranges from 2,230 g to 7,600 g (Table 2); range is not so broad by state or province (Appendix I). Sexual size dimorphism is pronounced and averages 26.6 +/- 4.6% (based on average body mass, n=18 states and provinces; DCE); males average larger than females. Because males often yodel during capture attempts, and only males yodel, sexing adults in the field is straightforward. Within a geographic area (i.e., state or province), overlap in the weights of males and females are usually small or non-existent; for a breeding pair it is exceptionally rare for the female to be heavier than the male (BRI unpubl. data). Variation across range is consistent with cline for mass. Individuals of interior breeding populations found in the upper Great Lakes and central Canada are smallest, and increase in size to the east and west (Anderson et al. 1970, Storer 1988; Appendix I). For example, body mass of adult male loons from the upper Great Lakes averages 33% less than their Maine counterparts (Appendix I).

Although the Yellow-billed Loon (Gavia adamsii) is often touted as the largest loon species, advances in field data collection, including live capture weights demonstrate average weight for male Common Loons breeding in Maine are greater than male Yellow-billed Loons (Table 2). Differences for linear measurements for males are 1-3% greater in Common Loons, except culmen length (Table 2). The Yellow-billed Loon bill averages 13% longer than the Common Loon. Differences between the species for females are mixed, although female Yellow-billed Loons have bills averaging 16% longer than Common Loons.