Browsing by Author "Shepard, Emily L.C."
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- Data packageData from: Thermal soaring in tropicbirds suggests that diverse seabirds may use this strategy to reduce flight costsGarde, Baptiste; Fell, Adam; Krishnan, Krishnamoorthy; Jones, Carl G.; Gunner, Richard; Tatayah, Vikash; Cole, Nik C.; Lempidakis, Emmanouil; Shepard, Emily L.C.Thermal soaring can offer substantial reductions in flight cost but it is often assumed to be confined to a relatively narrow group of fliers (those with low wing loading relative to their body mass). Using high-frequency movement data, including magnetometry and GPS, we identified thermal soaring in a seabird previously thought to use only flapping flight; red-tailed tropicbirds (Phaethon rubricauda). We tracked 55 individuals breeding on Round Island, Mauritius, and examined the environmental conditions that predicted thermal soaring in 76 trips (ranging from 0.8 to 43 h, mean= 5.9 h). Tropicbirds used thermal soaring and gliding flight for 13% of their flight time on average (range 0 - 34%), in association with both commuting and prey-searching/ pursuits. The use of thermal soaring showed strong variation between trips, but birds were more likely to soar when flying with tailwinds. This enables them to reduce their flight costs without a substantial increase in trip duration, which is pertinent in the breeding season when they are constrained by time and the need to return to a central place. Birds may therefore be able to increase the amount of thermal soaring outside the breeding season. Overall, we suggest that thermal soaring may be more widespread than previously thought, given that birds without specific morphological adaptations for this behaviour can soar for extended periods, and the bio-logging approaches best-placed to detect thermal soaring (high-frequency GPS/ magnetometry) tend to be used in the breeding season, when thermal soaring may be less likely.
- Data packageData from: Vultures respond to challenges of near-ground thermal soaring by varying bank angle(2018-11-27) Williams, Hannah J.; Shepard, Emily L.C.; Duriez, OlivierMany large birds rely on thermal soaring flight to travel cross-country. As such, they are under selective pressure to minimise the time spent gaining altitude in thermal updrafts. Birds should be able to maximise their climb rates by maintaining a position close to the thermal core through careful selection of bank angle and airspeed, however, there have been few direct measurements of either parameter. Here we apply a novel methodology to quantify the bank angles selected by soaring birds using on-board magnetometers. We couple these data with airspeed measurements to parameterise the soaring envelope of two species of Gyps vulture, from which it is possible to predict “optimal” bank angles. Our results show that these large birds respond to the challenges of gaining altitude in the initial phase of the climb, where thermal updrafts are weak and narrow, by adopting relatively high, and conserved, bank angles (25-35°). The angle of bank decreased with increasing altitude, in a manner that was broadly consistent with a strategy of maximising the rate of climb. However, the lift coefficients estimated in our study were lower than those predicted by theoretical models and wind-tunnel studies. Overall, our results highlight how the relevant currency for soaring performance changes within individual climbs; when thermal radius is limiting, birds vary bank angle and maintain a constant airspeed, but speed increases later in the climb in order to respond to decreasing air density.