Automated telemetry reveals age specific differences in flight duration and speed are driven by wind conditions in a migratory songbird.

Greg W Mitchell,D Ryan Norris,Philip D Taylor,Bradley K Woodworth

doi:10.1186/s40462-015-0046-5

Greg W Mitchell, D Ryan Norris + Show 2 more

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https://doi.org/10.1186/s40462-015-0046-5

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Abstract

BackgroundGiven that winds encountered on migration could theoretically double or half the energy expenditure of aerial migrants, there should be strong selection on behaviour in relation to wind conditions aloft. However, evidence suggests that juvenile songbirds are less choosy about wind conditions at departure relative to adults, potentially increasing energy expenditure during flight. To date, there has yet to be a direct comparison of flight efficiency between free-living adult and juvenile songbirds during migration in relation to wind conditions aloft, likely because of the challenges of following known aged individual songbirds during flight. We used an automated digital telemetry array to compare the flight efficiency of adult and juvenile Savannah sparrows (Passerculus sandwichensis) as they flew nearly 100 km during two successive stages of their fall migration; a departure flight from their breeding grounds out over the ocean and then a migratory flight along a coast. Using a multilevel path modelling framework, we evaluated the effects of age, flight stage, tailwind component, and crosswind component on flight duration and groundspeed.ResultsWe found that juveniles departed under wind conditions that were less supportive relative to adults and that this resulted in juveniles taking 1.4 times longer to complete the same flight trajectories as adults. We did not find an effect of age on flight duration or groundspeed after controlling for wind conditions aloft, suggesting that both age groups were flying at similar airspeeds. We also found that groundspeeds were 1.7 times faster along the coast than over the ocean given more favourable tailwinds along the coast and because birds appeared to be climbing in altitude over the ocean, diverting some energy from horizontal to vertical movement.ConclusionsOur results provide the first evidence that adult songbirds have considerably more efficient migratory flights than juveniles, and that this efficiency is driven by the selection of more supportive tailwind conditions aloft. We suggest that the tendency for juveniles to be less choosy about wind conditions at departure relative to adults could be adaptive if the benefits of having a more flexible departure schedule exceed the time and energy savings realized during flight with more supportive winds.Electronic supplementary materialThe online version of this article (doi:10.1186/s40462-015-0046-5) contains supplementary material, which is available to authorized users.

Highlights

Given that winds encountered on migration could theoretically double or half the energy expenditure of aerial migrants, there should be strong selection on behaviour in relation to wind conditions aloft
We suggest that the tendency for juveniles to be less choosy about wind conditions at departure relative to adults could be adaptive if the benefits of having a more flexible departure schedule exceed the time and energy savings realized during flight with more supportive winds
In our study we found that juveniles generally flew with less supportive winds than adults (Fig. 4a), after controlling for the effect of tailwind component, we did not find a direct effect of age on groundspeed, which would have indicated potential differences in airspeed

Summary

Introduction

Given that winds encountered on migration could theoretically double or half the energy expenditure of aerial migrants, there should be strong selection on behaviour in relation to wind conditions aloft. Billions of migrating birds, bats, and insects encounter winds during flight that are on the same order of magnitude as their airspeeds, creating enormous challenges and opportunities that could hypothetically double or half their energy expenditure during flight [1,2,3,4,5] This observation has resulted in winds figuring prominently in optimal migration theory given their potential effects on flight duration, range, and speed, stopover duration, and orientation (e.g., [3, 6,7,8,9,10]). Mitchell et al [17] found that juvenile Savannah sparrows (Passerculus sandwichensis) were much less choosy about wind conditions aloft relative to adults during migratory departure from the breeding grounds This suggests that juveniles were not always departing with supportive winds, there is no direct evidence that this translated to slower migration speeds or longer flight durations during migration. We build upon Mitchell et al [17] by quantifying the costs and benefits in terms of flight efficiency associated with different departure decisions made by adults and juveniles during autumn migration

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