Flight performance
When testing monarchs on a tethered flight mill (Fig. 2A), eastern monarchs flew longer (F1,54=8.81, P=0.004) and thereby realized greater flight distances than western monarchs (Fig. 2B, C; F1,54=4.56, P=0.037). In contrast, western monarchs flew with greater power than eastern monarchs (Fig. 2D; F1,54=6.00, P=0.018), which is expected in butterfly populations adapted to shorter flight distances (McKay, Ezenwa & Altizer, 2016). Female and male butterflies did not differ in flight duration (F1,51=0.052, P=0.82), flight distance (F1,51=0.10, P=0.75), or power (F1,51=0.007, P=0.93). These differences in flight performance between eastern and western migratory monarchs correspond with their drastically different migration distances.
We also found significant differences in wing morphology of eastern and western monarchs. Wing morphology measurements showed that eastern monarchs had larger wings than western monarchs (PC1: F1,54=11.0, P=0.002), confirming previous studies (Altizer & Davis, 2010; Freedman & Dingle, 2018). Wing morphology is an important determinant of migration in many winged animals (Altizer & Davis, 2010) and migratory monarchs have larger forewings than non-migratory monarchs (Dockx, 2007; Altizer & Davis, 2010; Dockx, 2012; Li et al. , 2016; Yang et al. , 2016; Flockhart et al. , 2017). In contrast, eastern and western monarchs did not differ in wing shape (PC2: F1,54=2.05, P=0.16). Additionally, wing size and shape did not significantly affect flight duration (PC1: F1,53=1.40, P=0.24; PC2: F1,52=0.34, P=0.56), flight distance (PC1: F1,53=1.73, P=0.19; PC2: F1,52=0.82, P=0.36) or flight power (PC1: F1,53=0.47, P=0.50; PC2: F1,52=1.68, P=0.20). We also found no differences in wing size (PC1: F1,53=0.042, P=0.84) and shape (PC2: F1,53=1.42, P=0.24) between females and males (within location).
Average speed did not vary significantly between eastern and western monarchs (F1,54=0.001, P=0.98) or between male and female butterflies (F1,51=0.20, P=0.66); moreover, wing size and wing shape did not affect average speed (PC1: F1,53=1.05, P=0.31; PC2: F1,52=1.81, P=0.19). Similarly, average weight loss during flight trials did not vary between eastern and western monarchs (F1,54=0.0007, P=0.98), and also was not affected by sex, wing size, and wing shape (Sex: F1,51=0.84, P=0.36; PC1: F1,53=0.002, P=0.97; PC2: F1,52=1.42, P=0.24).