Pointed Wings, Low Wingloading and Calm Air Reduce Migratory Flight Costs in Songbirds

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

Comparing Aerodynamic Efficiency in Birds and Bats Suggests Better Flight Performance in Birds

Flight is one of the energetically most costly activities in the animal kingdom, suggesting that natural selection should work to optimize flight performance. The similar size and flight speed of birds and bats may therefore suggest convergent aerodynamic performance; alternatively, flight performance could be restricted by phylogenetic constraints. We test which of these scenarios fit to two measures of aerodynamic flight efficiency in two passerine bird species and two New World leaf-nosed bat species. Using time-resolved particle image velocimetry measurements of the wake of the animals flying in a wind tunnel, we derived the span efficiency, a metric for the efficiency of generating lift, and the lift-to-drag ratio, a metric for mechanical energetic flight efficiency. We show that the birds significantly outperform the bats in both metrics, which we ascribe to variation in aerodynamic function of body and wing upstroke: Bird bodies generated relatively more lift than bat bodies, resulting in a more uniform spanwise lift distribution and higher span efficiency. A likely explanation would be that the bat ears and nose leaf, associated with echolocation, disturb the flow over the body. During the upstroke, the birds retract their wings to make them aerodynamically inactive, while the membranous bat wings generate thrust and negative lift. Despite the differences in performance, the wake morphology of both birds and bats resemble the optimal wake for their respective lift-to-drag ratio regimes. This suggests that evolution has optimized performance relative to the respective conditions of birds and bats, but that maximum performance is possibly limited by phylogenetic constraints. Although ecological differences between birds and bats are subjected to many conspiring variables, the different aerodynamic flight efficiency for the bird and bat species studied here may help explain why birds typically fly faster, migrate more frequently and migrate longer distances than bats

The Role of Wind-Tunnel Studies in Integrative Research on Migration Biology

Wind tunnels allow researchers to investigate animals' flight under controlled conditions, and provide easy access to the animals during flight. These increasingly popular devices can benefit integrative migration biology by allowing us to explore the links between aerodynamic theory and migration as well as the links between flight behavior and physiology. Currently, wind tunnels are being used to investigate many different migratory phenomena, including the relationship between metabolic power and flight speed and carry-over effects between different seasons. Although biotelemetry is also becoming increasingly common, it is unlikely that it will be able to completely supplant wind tunnels because of the difficulty of measuring or varying parameters such as flight speed or temperature in the wild. Wind tunnels and swim tunnels will therefore continue to be important tools we can use for studying integrative migration biology

Partial regression plots for the cruise phase general linear model.

<p>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.</p

Frequency histograms for wind speed and pressure vertical velocity.

<p>The top graphs show the atmospheric conditions from the North American Regional Reanalysis Model <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002154#pone.0002154-Mesinger1" target="_blank">[21]</a> 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.</p

Vortex wake, downwash distribution, aerodynamic performance and wingbeat kinematics in slow-flying pied flycatchers.

Many small passerines regularly fly slowly when catching prey, flying in cluttered environments or landing on a perch or nest. While flying slowly, passerines generate most of the flight forces during the downstroke, and have a 'feathered upstroke' during which they make their wing inactive by retracting it close to the body and by spreading the primary wing feathers. How this flight mode relates aerodynamically to the cruising flight and so-called 'normal hovering' as used in hummingbirds is not yet known. Here, we present time-resolved fluid dynamics data in combination with wingbeat kinematics data for three pied flycatchers flying across a range of speeds from near hovering to their calculated minimum power speed. Flycatchers are adapted to low speed flight, which they habitually use when catching insects on the wing. From the wake dynamics data, we constructed average wingbeat wakes and determined the time-resolved flight forces, the time-resolved downwash distributions and the resulting lift-to-drag ratios, span efficiencies and flap efficiencies. During the downstroke, slow-flying flycatchers generate a single-vortex loop wake, which is much more similar to that generated by birds at cruising flight speeds than it is to the double loop vortex wake in hovering hummingbirds. This wake structure results in a relatively high downwash behind the body, which can be explained by the relatively active tail in flycatchers. As a result of this, slow-flying flycatchers have a span efficiency which is similar to that of the birds in cruising flight and which can be assumed to be higher than in hovering hummingbirds. During the upstroke, the wings of slowly flying flycatchers generated no significant forces, but the body-tail configuration added 23 per cent to weight support. This is strikingly similar to the 25 per cent weight support generated by the wing upstroke in hovering hummingbirds. Thus, for slow-flying passerines, the upstroke cannot be regarded as inactive, and the tail may be of importance for flight efficiency and possibly manoeuvrability

Determining approximate flight altitude.

<p>Here we show how we determined the flight altitude of one of our Swainson's Thrushes. We used the bird's groundspeed and track direction as well as data on the winds aloft at the various pressure surface levels to predict the bird's heading, which could then be compared to its actual heading of 60°. It is clear from the table that the bird was flying near the 975 mb pressure surface level, so we used data from the 975 mb pressure surface level in our analysis of the effects of atmospheric conditions on heart rate. We performed this analysis for each of the 15 Swainson's Thrushes in this study to determine their flight altitudes.</p

Frequency histogram for average heart rate.

<p>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.</p

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

<p>The top graphs show the wingtip shapes and wingloadings of 93 Swainson's Thrushes captured in Champaign-Urbana, IL in May 2003–2005 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0002154#pone.0002154-Bowlin2" target="_blank">[25]</a> 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.</p