Wind tunnel as a tool in bird migration research

Abstract

Wind tunnels, in which birds fly against an artificially generated air flow, have since long been used to evaluate aerodynamic properties of steady bird flight. A new generation of wind tunnels has also allowed the many processes associated with migratory flights to be studied in captivity. We review how wind tunnel studies of aerodynamics and migratory performance together have helped advancing our understanding of bird migration. Current migration theory is based on the power‐speed relationship of flight as well as flight range equations, both of which can be evaluated using birds flying in wind tunnels. In addition, and depending on wind tunnel properties, performance during gliding and climbing flight, and effects of air pressure, humidity and turbulence on bird flight has been measured. Long‐distance migrant species have been flown repeatedly for up to 16 h non‐stop, allowing detailed studies of the energy expenditure, fuel composition, protein turnover, water balance, immunocompetence and stress associated with sustained migratory flights. In addition, wind tunnels allow the fuelling periods between migratory flights to be studied from new angles. We end our review by suggesting several important topics for future wind tunnel studies, ranging from on of the key questions remaining, the efficiency at which chemical power in converted to mechanical power, to new useful avenues, such as improving and calibrating the techniques used for tracking of individual birds in the wild.

Overall Dynamic Body Acceleration in Straw-Colored Fruit Bats Increases in Headwinds but Not With Airspeed

“…We found no relationship between wingbeat frequency and airspeed, and ODBA decreased slightly with faster airspeeds. To fly faster in wind tunnels bats generally do not change wingbeat frequency, but rely on amplitude and kinematic adjustments (Hedenstrom et al, 2007;Hubel et al, 2010Hubel et al, , 2012Hubel et al, , 2016Riskin et al, 2010Riskin et al, , 2012Iriarte-Diaz et al, 2011Hedenstrom and Johansson, 2015;Hedenström and Lindström, 2017). Eidolon helvum in a wind tunnel fly with wingbeat frequencies of 4.5-5.7 bps (Carpenter, 1986), and video of free-flying E. helvum emerging from their roosts showed bats flew at 4.4 ± 0.43 bps (Lindhe Norberg and Norberg, 2012).…”

Reduced metabolism supports hypoxic flight in the high-flying bar-headed goose (Anser indicus)

“…Given that birds underwent considerable training, including outdoor flights, and that wind tunnel flights were short even in normoxia, it would appear that the birds were reluctant to fly for long once instrumented in the conditions of the wind tunnel. Flow turbulence in the tunnel, the presence of the experimenters and the presence of the mask and tubing all will have increased flight costs and may have contributed to this (Hedenström and Lindström, 2017). Although wing-beat frequencies of our birds were higher than those of bar-headed geese in the wild (Bishop et al, 2015), values were similar between normoxic vs. hypoxic and instrumented vs. uninstrumented flights (Supplementary file 4; Whale, 2012).…”

Thomas Alerstam – migration ecologist extraordinaire

“…Thomas realised early on that flight mechanics was a key that could unlock many doors of migration studies, perhaps influenced by his collaboration with C. J. Pennycuick about migratory flight strategies of common cranes Grus grus across southern Sweden, using a combination of radar tracks and observations from a small aircraft (Pennycuick et al 1979). Later, it was through deliberations with C. J. Pennycuick that Thomas conceived the idea to build a modern purpose-designed wind tunnel for animal flight studies (Pennycuick et al 1997, Hedenström andLindström 2017), where flight mechanics and energetics, not least at varying speeds, could be investigated under controlled conditions. To this day it is 'common knowledge' that the peregrine falcon Falco peregrinus is the fastest bird on earth, often referred to as stooping at 300 km h -1 or more when hunting.…”