Fig. 5
Total speed of migration in spring and autumn against “lean” body mass. a Population-specific average total speed of migration for each season (spring: Δ, autumn: ♦) against the species’ “lean” body mass, as given in the Handbook of the Birds of the World. Body mass values are presented in species-specific colours (Table 1). The blue dotted line shows the predicted relationship between the total speed of migration and body mass as detailed in eq. (7) by Hedenström [20]. The red dotted line (spring) and orange dotted line (autumn) indicate the observed effect of body mass on daily migration speed for 102 species of North American migratory birds that use powered flight, after La Sorte et al. [21]. Please consider that the approach used by these authors likely underestimated individual-based migration speeds and that in my approach, high total speeds of migration are likely biased due to missing the first migratory fuelling period and by low total speeds of migration obtained by underestimating the total migration distance. b Total speed of migration per season (spring: n = 32; autumn: n = 34) presented in boxplots (boxes present 5, 25, 50, 75, and 95% percentiles and outliners as dots). The Y-axes show the log10-transformed values of the total speed of migration