Pointed wings, low wingloading and calm air reduce migratory flight costs in songbirds

Abstract

Migratory bird, bat and insect species tend to have more pointed wings than non-migrants. Pointed wings and low wingloading, or body mass divided by wing area, are thought to reduce energy consumption during long-distance flight, but these hypotheses have never been directly tested. Furthermore, it is not clear how the atmospheric conditions migrants encounter while aloft affect their energy use; without such information, we cannot accurately predict migratory species' response(s) to climate change. Here, we measured the heart rates of 15 free-flying Swainson's Thrushes (Catharus ustulatus) during migratory flight. Heart rate, and therefore rate of energy expenditure, was positively associated with individual variation in wingtip roundedness and wingloading throughout the flights. During the cruise phase of the flights, heart rate was also positively associated with wind speed but not wind direction, and negatively but not significantly associated with large-scale atmospheric stability. High winds and low atmospheric stability are both indicative of the presence of turbulent eddies, suggesting that birds may be using more energy when atmospheric turbulence is high. We therefore suggest that pointed wingtips, low wingloading and avoidance of high winds and turbulence reduce flight costs for small birds during migration, and that climate change may have the strongest effects on migrants' in-flight energy use if it affects the frequency and/or severity of high winds and atmospheric instability.

Figure 1. Wingtip shape in Swainson's Thrushes.

To the left is a relatively pointed wing (C₂ = 0.139) and to the right is a relatively round wing (C₂ = 0.493). Grid squares in both pictures are 6.35 mm on each side.

Figure 2. Frequency histogram for average heart rate.

The graph shows the average heart rates (mean±s.e.m., 13.35±0.18 Hz) of 15 free-flying Swainson's Thrushes migrating over the central United States in spring after initial ascent and prior to final descent. Note the large amount of variation. The histogram for the initial ascent phase (not shown) is virtually identical except the mean is higher: 14.43±0.16 Hz.

Figure 3. Partial regression plots for the cruise phase general linear model.

The dependent variable in the analysis was average heart rate (Hz) during the cruise phase of 14 migratory flights. Heart rate (and thus energy expenditure) increased with increasing wingtip roundedness (C₂), wingloading (gm⁻²), and wind speed aloft (ms⁻¹), independent of wind direction. It decreased with increasing pressure vertical velocity (increasing atmospheric stability, Pas⁻¹), although not significantly. Axes are unstandardized residuals.

Figure 4. Frequency histograms for wingtip shape (C₂, lower values indicate more pointed wings [3]) and wingloading.

The top graphs show the wingtip shapes and wingloadings of 93 Swainson's Thrushes captured in Champaign-Urbana, IL in May 2003–2005 while the bottom graphs show the same data for the 15 Swainson's Thrushes followed in this study. Average wingloading is slightly higher in this study than in the larger sample primarily because of the added mass of the transmitter.

Figure 5. Frequency histograms for wind speed and pressure vertical velocity.

The top graphs show the atmospheric conditions from the North American Regional Reanalysis Model for 1–31 May, 2003–2005, at the 925 mb level at 0300 UTC (2100 local time) over the site where our thrushes were captured and released. The bottom graphs show the atmospheric conditions encountered by the 15 Swainson's thrushes in this study.