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

Melissa S Bowlin,Martin Wikelski,Jeffery Kelly

doi:10.1371/journal.pone.0002154

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.

Highlights

During a 42 day, 4800 km journey from Panama to Canada, an average Swainson’s thrush (Catharus ustulatus), a typical passerine migrant, beats its wings approximately 3.2 million times and uses about 1300 kJ of energy on flight alone: energy that would keep it alive almost fifteen days if it did not migrate [1,2]
Low wingloading, or body mass divided by wing area, is thought by some researchers to decrease the cost of transport for migrants [6,7], though this pressure is opposed by the need to carry adequate fuel supplies [8]. Such hypotheses remain untested on free-flying birds [9], but see [10] because previous observational methods revealed nothing about the physiology of individual migrants during natural, uninterrupted migratory flight
Our data provide the first direct test of two predictions regarding the relationships between morphology and energy expenditure in naturally migrating, free-flying songbirds: Swainson’s Thrushes with (i) more pointed wingtips and (ii) lower wingloading had lower heart rates and lower energy expenditure [20] than those with more rounded wingtips or higher wingloading

Summary

Introduction

During a 42 day, 4800 km journey from Panama to Canada, an average Swainson’s thrush (Catharus ustulatus), a typical passerine migrant, beats its wings approximately 3.2 million times and uses about 1300 kJ of energy on flight alone: energy that would keep it alive almost fifteen days if it did not migrate [1,2]. Low wingloading, or body mass divided by wing area, is thought by some researchers to decrease the cost of transport for migrants [6,7], though this pressure is opposed by the need to carry adequate fuel supplies [8] Such hypotheses remain untested on free-flying birds [9], but see [10] because previous observational methods revealed nothing about the physiology of individual migrants during natural, uninterrupted migratory flight. Wingtip pointedness (P,0.001), wingloading (P = 0.001), and average wind speed during the flights (P = 0.025) were significantly associated with heart rate; we found a trend for pressure vertical velocity (P = 0.075), a measurement of large-scale atmospheric stability, to predict heart rate (Figure 3). If the variable only explained an additional 1% of the variation, the probability of detecting a significant result was 0.11 [22]

Discussion

Findings

Materials and Methods