Abstract
Although more massive flight muscles along with larger wings, higher wingbeat frequencies and greater stroke amplitudes enhance force and power production in flapping flight, the extent to which these parameters may be correlated with other morphological features relevant to flight physiology and biomechanics remains unclear. Intraspecifically, we hypothesized that greater vertical load-lifting capacity would correlate with higher wingbeat frequencies and relatively more massive flight muscles, along with relatively bigger hearts, lungs, and stomachs to enhance metabolic capacity and energy supply, but also with smaller body size given the overall negative allometric dependence of maximum flight performance in volant taxa. To explore intraspecific correlates of flight performance, we assembled a large dataset that included 13 morphological and kinematic variables for a non-migratory passerine, the Eurasian tree sparrow (Passer montanus). We found that heavier flight muscles and larger wings, heavier stomachs and shorter bills were the most important correlates of maximum load-lifting capacity. Surprisingly, wingbeat frequency, wing stroke amplitude and masses of the heart, lungs and digestive organs (except for the stomach) were non-significant predictor variables relative to lifting capacity. The best-fit structural equation model (SEM) indicated that load-lifting capacity was positively correlated with flight muscle mass, wing area and stomach mass, but was negatively correlated with bill length. Characterization of individual variability in flight performance in a free-ranging passerine indicates the subtlety of interaction effects among morphological features, some of which differ from those that have been identified interspecifically for maximum flight performance in birds.
Highlights
Two cameras were used in this experiment; one high-speed video camera (JVC GCP100BAC; operated at 50 frames−1; see Supplemental Material) positioned laterally at a distance of 80 cm to the chamber was used to film the beads remaining on the chamber floor during maximum load-lifting flight and by subtraction to determine the total extra weight lifted by the bird
Variation in total lifted load was best explained by variability in bill length, stomach mass, gut length, wing area and flight muscle mass (Table 2)
Many external morphological and internal anatomical measurements are positively correlated with maximum load-lifting performance in Eurasian tree sparrows (Table 1), but the strongest correlates are a greater flight muscle mass, larger wings, heavier stomach and a shorter bill (Table 3), all of which are independent of sex
Summary
Birds exhibit a broad diversity of flight-related morphological and physiological characteristics (Hedenström, 2002; Lee et al, 2014; Puttick, Thomas & Benton, 2014; Altshuler et al, 2015; Butler, 2016), many of which reflect multiple trade-offs in flightHow to cite this article Wang Y, Yin Y, Ge S, Li M, Zhang Q, Li J, Wu Y, Li D, Dudley R. 2019. Larger species typically possess bigger wings and higher pectoral muscle mass, whereas wingbeat frequency declines with increasing body mass (Groom et al, 2018); higher wingbeat frequencies and greater stroke amplitudes yield increased force and power production (Chai & Dudley, 1995; Hedenström, 2002; Altshuler et al, 2015). Avian flight is an energy-demanding activity requiring powerful respiratory and cardiovascular systems to support the intense metabolism of the associated skeletal muscles (Hedenström, 2002; Lee et al, 2014; Altshuler et al, 2015; Butler, 2016; Nespolo et al, 2018). Respiratory, cardiovascular and nutritional systems represent important features for sustaining powered flight
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