DOI: 10.5061/dryad.bp70s
title: Data from: Soaring across continents: decision-making of a soaring migrant under changing atmospheric conditions along an entire flyway

description: (1) Thermal soaring birds reduce flight-energy costs by alternatingly
 gaining altitude in thermals and gliding across the earth’s surface. To
 find out how soaring migrants adjust their flight behaviour to dynamic
 atmospheric conditions across entire migration routes, we combined optimal
 soaring migration theory with high-resolution GPS tracking data of
 migrating Honey Buzzards Pernis apivorus and wind data from a global
 numerical atmospheric model. (2) We compared measurements of gliding air
 speeds to predictions based on two distinct behavioural benchmarks for
 thermal soaring flight. The first being a time-optimal strategy whereby
 birds alter their gliding air speeds as a function of climb rates to
 maximize cross-country air speed over a full climb-glide cycle (Vopt). The
 second a risk-averse energy-efficient strategy at which birds alter their
 gliding air speed in response to tailwinds/headwinds to maximize the
 distance travelled in the intended direction during each glide phase
 (Vbgw). (3) Honey Buzzards were gliding on average 2.05 ms-1 slower than
 Vopt and 3.42 ms-1 faster than Vbgw while they increased air speeds with
 climb rates and reduced air speeds in tailwinds. They adopted flexible
 flight strategies gliding mostly near Vbgw under poor soaring conditions
 and closer to Vopt in good soaring conditions. (4) Honey Buzzards most
 adopted a time-optimal strategy when crossing the Sahara, and at the onset
 of spring migration, where and when they met with the best soaring
 conditions. The buzzards nevertheless glided slower than Vopt during most
 of their journeys, probably taking time to navigate, orientate and locate
 suitable thermals, especially in areas with poor thermal convection. (5)
 Linking novel tracking techniques with optimal migration models clarifies
 the way birds balance different trade-offs during migration.
description: Flight measurements,
 wind data and theoretical speeds for thermal-soaring Honey
 BuzzardsHoney Buzzards were
 tracked with high-resolution GPS-loggers, measuring 3D positions every 10
 seconds during two cycles of 30 minutes each on every day spent on
 migration during autumn 2012 and spring 2013. Based on running average of
 vertical speeds we distinguished climbing from gliding events. We then
 extracted the location, altitude and time at which birds started/ended
 each climb and each glide, and created one record for every
 climbing-gliding cycle that was completely covered by high-resolution
 measurements. Based on these data we calculated distances travelled, track
 direction, altitude gained/lost, duration, ground speeds, climb/sink rates
 from the start to the end of every climb, of every glide and across every
 full climbing-gliding cycle. We then determined wind conditions to the
 start of each glide by linearly interpolating wind estimates from the
 ECMWF deterministic forecast model (2.5x2.5 degrees, 3 hour resolution).
 By combining tracking data with wind data we could determine determine the
 heading and air speed achieved during every climb, glide and
 climbing-gliding cycle. Finally, we generated an aerodynamic model of a
 Honey Buzzard in thermal-soaring flight based following Pennycuick 2008
 (see also original paper) to compute the theoretical minimum-sink speed
 (Vms), optimal air speed (Vopt), best-gliding air speed (Vbg) and
 best-gliding air speed in head-/tailwinds (Vbgw) for a Honey Buzzard
 (assuming mass = 1kg, wing span = 1.3m, wing area = 0.26m-squared, other
 parametrizations see Vansteelant et al. 2016). By comparing real gliding
 air speeds (Va) with theoretical benchmarks for time-optimal (Vopt) and
 risk-averse (Vbgw) flight, we calculated a measure of risk-averison
 (RAFI). All calculations were implemented in R version 3.1. Detailed
 descriptions of all columns are provided in the readme.txt file. Note that
 all distance are measured in meters, time differences in seconds and
 speeds are noted in meters per second.Vansteelant-etal-JAB-Dryad.xlsx