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


RESULTS
We 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.


CONCLUSIONS
Our 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.
Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1 Canada ; Wildlife Research Division, National Wildlife Research Center, Environment Canada, Ottawa, ON K1H 0H3 Canada.
Greg W
GW
Mitchell
Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1 Canada ; Department of Biology, Acadia University, Wolfville, NS B4P 2R6 Canada.
Bradley K
BK
Woodworth
Department of Biology, Acadia University, Wolfville, NS B4P 2R6 Canada ; Bird Studies Canada, Port Rowan, ON N0E 1M0 Canada.
Philip D
PD
Taylor
Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1 Canada.
D Ryan
DR
Norris
Aeroecology; Airspeed; Altitude; Automated telemetry; Crosswinds; Flight costs; Groundspeed; Migration; Songbirds; Tailwinds; Wind support