Flight responses by a migratory soaring raptor to changing meteorological conditions

Abstract

).Soaring birds that undertake long-distance migration should develop strategies to minimize the energetic costs of endurance flight. This is relevant because condition upon completion of migration has direct consequences for fecundity, fitness and thus, demography. Therefore, strong evolutionary pressures are expected for energy minimization tactics linked to weather and topography. Importantly, the minute-by-minute mechanisms birds use to subsidize migration in variable weather are largely unknown, in large part because of the technological limitations in studying detailed long-distance bird flight. Here, we show golden eagle (Aquila chrysaetos) migratory response to changing meteorological conditions as monitored by high-resolution telemetry. In contrast to expectations, responses to meteorological variability were stereotyped across the 10 individuals studied. Eagles reacted to increased wind speed by using more orographic lift and less thermal lift. Concomitantly, as use of thermals decreased, variation in flight speed and altitude also decreased. These results demonstrate how soaring migrant birds can minimize energetic expenditures, they show the context for avian decisions and choices of specific instantaneous flight mechanisms and they have important implications for design of bird-friendly wind energy.

Use of multiple modes of flight subsidy by a soaring terrestrial bird, the golden eagleAquila chrysaetos, when on migration

“…Tested GPS horizontal precision was less than or equal to 2.5 m, and vertical precision was less than or equal to 22.5 m [22]. Between 39.58 and 42.58 north latitude and during daylight hours, the GPS collected data on bird movements at 30-60 s intervals (this is as rapidly as the technology would allow; figure 1).…”

“…Interface 12: 20150530 flight they used. Previous work indicates that eagles demonstrate a flight altitude response to changes in topography [36] and to variation in wind speed [22]. We also know that flight speed of eagles using a thermal-glide strategy is faster than that of eagles using exclusively orographic updraft [19], that eagles preferentially migrate when weather conditions favour availability of that thermal updraft [21], and that adult and pre-adult eagles face different temporal and energetic pressures when migrating [21].…”

Can accelerometry be used to distinguish between flight types in soaring birds?

“…The distribution and strength of these updraughts vary in space and time [20], which is likely to have profound consequences for the flight paths selected by these animals as well as their time and energy budgets [21]. Therefore, the ability to further classify soaring flight types according to the source of lift, as well as periods of gliding between them, would contribute greatly to our understanding of the factors driving space use for these birds [22].…”

Where eagles soar: Fine‐resolution tracking reveals the spatiotemporal use of differential soaring modes in a large raptor

“…For example, when wind speeds increase, golden eagles Aquila chrysaetos shift flight strategies from thermal soaring to orographic soaring to reduce the energetic costs of migration (Lanzone et al., 2012). The altitude gained via either uplift type is a form of potential energy that reduces the need for powered flight providing calorific savings (Pennycuick, 2008).…”

“…Therefore, the preferential use of areas that maximize net energy gain is expected. Although there is good theoretical understanding of uplift availability and its use for soaring (Bohrer et al., 2012; Pennycuick, 2008), empirical studies using modern technology to investigate whether birds utilize uplift in line with these expectations are only now emerging (Katzner et al., 2015; Lanzone et al., 2012; Péron et al., 2017; Shepard, Lambertucci, Vallmitjana, & Wilson, 2011; Sherub, Bohrer, Wikelski, & Weinzierl, 2016). …”