Intraspecific variation in avian pectoral muscle mass:: constraints on maintaining manoeuvrability with increasing body mass

1. Within a single year, long-distance migrants undergo a minimum of four cycles of fuel storage and depletion because their migrations have at least one stopover. Each cycle includes an almost twofold change in body mass (m(b)). Pervasive predation threats beg the question whether escape flight abilities keep up with such large changes in m(b). 2. We derive aerodynamic predictions how pectoral muscle mass (m(pm)) should change with m(b) to maintain constant relative flight power. 3. We tested these predictions with data on red knot Calidris canutus, a long-distance migrating wader that breeds in arctic tundra and winters in temperate and tropical coastal areas. We focused on the subspecies C. c. islandica. 4. m(pm) varied with m(b) in a piecewise manner. In islandica knots with m(b) <= 148 g, the slope (1.06) was indistinguishable from the prediction (1.25). In heavy knots (m(b) > 148 g) the slope was significantly lower (0.63), yielding a m(pm) 0.81 times lower than predicted at pre-departure weights (210 g). 5. Manoeuvrability tests showed that above 160 g, knots were increasingly unable to make a 90 degrees angle turn. This is consistent with m(pm) being increasingly smaller than predicted. 6. Relatively low m(pm) enables savings on mass and hence flight costs, and savings on overall energy expenditure. We predict that reduced escape flight ability at high m(b) will be compensated by behavioural strategies to minimize predation risk.