Twofold seasonal variation in the supposedly constant, species‐specific, ratio of upstroke to downstroke flight muscles in red knotsCalidris canutus

Abstract

We show that in a long‐distance migrant shorebird species with outspoken seasonal changes in body mass and composition, the red knotCalidris canutus, the ratio between the masses of the small flight muscle (musculus supracoracoideus, powering twists and active upstrokes of the wings) and the larger flight muscle (musculus pectoralis, for the downstrokes) is far from constant. During an annual cycle thesupracoracoideus/pectoralisratio varied more than twofold between values of 0.058 (±0.005 SE) in early winter period and of 0.124 (±0.05 SE) on the High Arctic tundra breeding grounds. The ratios thus spanned a range from those typical of soaring raptors and seabirds to those of fast and agile fliers and birds with rapid take‐offs. The overall average ratio was 0.102 (±0.001 SE, for non‐starved knots, and 0.103±0.001 including starved knots) and did not differ between males and females. As predicted from the known functions ofsupracoracoideusandpectoralis, the ratio was a negative function of body mass. However, after arrival on the breeding grounds (0.124) and during winter starvation (0.135) particularly high ratios were reached: these may be times when wing‐manoeuvrability (in flight display and during the evasive ‘rodent run’ away from predators at the nest) and an ability for rapid take‐off and active up‐strokes (from –near– the nest, and in times of depletion of flight muscle mass during winter starvation) may be at premium. The particularly low ratio of 0.06 in early winter is puzzling. Many aspects of avian phenotypes have recently been shown to be intraindividually variable. To a twofold seasonal variation in flight muscle mass (Dietz et al. 2007), we can now add the twofold variation in the ratio between the muscles for the upstroke and the downstroke.