Foraging at the edge of the world: low-altitude, high-speed manoeuvering in barn swallows

Abstract

One contribution of 17 to a theme issue 'Moving in a moving medium: new perspectives on flight'. While prior studies of swallow manoeuvering have focused on slow-speed flight and obstacle avoidance in still air, swallows survive by foraging at high speeds in windy environments. Recent advances in field-portable, high-speed video systems, coupled with precise anemometry, permit measures of high-speed aerial performance of birds in a natural state. We undertook the present study to test: (i) the manner in which barn swallows (Hirundo rustica) may exploit wind dynamics and ground effect while foraging and (ii) the relative importance of flapping versus gliding for accomplishing high-speed manoeuvers. Using multi-camera videography synchronized with wind-velocity measurements, we tracked coursing manoeuvers in pursuit of prey. Wind speed averaged 1.3-2.0 m s 21 across the atmospheric boundary layer, exhibiting a shear gradient greater than expected, with instantaneous speeds of 0.02-6.1 m s 21 . While barn swallows tended to flap throughout turns, they exhibited reduced wingbeat frequency, relying on glides and partial bounds during maximal manoeuvers. Further, the birds capitalized on the near-earth wind speed gradient to gain kinetic and potential energy during both flapping and gliding turns; providing evidence that such behaviour is not limited to large, fixed-wing soaring seabirds and that exploitation of wind gradients by small aerial insectivores may be a significant aspect of their aeroecology.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Flight performance in the altricial zebra finch: Developmental effects and reproductive consequences

“…In volant birds, flight performance contributes to many ecologically relevant functions, including mate displays (Clark, ; Clark & Feo, ; Usherwood, ), prey capture (Helms, Godfrey, Ames, & Bridge, ; Warrick, Hedrick, Biewener, Crandell, & Tobalske, ), predator escapes (Kullberg, Fransson, & Jakobsson, ; Kullberg, Jakobsson, & Fransson, ; van den Hout, Mathot, Maas, & Piersma, ), and arrival time to migration grounds (Bowlin & Winkler, ; Matyjasiak, ). Thus, flight performance can potentially affect both reproductive success (via mate choice; reviewed in Barske, Schlinger, Wikelski, & Fusani, ; Byers, Hebets, & Podos, ) and survival (Møller, ).…”

Moving in a moving medium: new perspectives on flight

“…A final and exciting example of the exploitation of smallscale aerial heterogeneity is presented by Warrick et al [39]. This study coupled high-speed video with measurements of near-ground wind speeds.…”

Gliding for a free lunch: biomechanics of foraging flight in common swifts (Apus apus)

“…Application of video measurement methods has already revealed flight speeds well beyond those achieved in wind tunnels by quantifying mating or display behaviour in common swifts (Henningsson et al, 2010) and hummingbirds (Clark, 2009), along with high-speed turns and pursuits in cliff swallows (Shelton et al, 2014). Field trajectory recordings also revealed energy extraction from the ground to air wind speed gradient by foraging barn swallows (Warrick et al, 2016). In the recordings of the present study, the common swifts were also engaged in foraging, permitting comparison with the foraging flight dynamics of two evolutionarily distinct coursing aerial insectivores (swallows and swifts) as well as comparison of natural and wind tunnel flight in swifts.…”

“…Common swifts typically forage higher in the atmosphere where vertical wind shear should be less common but we hypothesize that they take advantage of other environmental energy sources such as thermals to reduce the cost of foraging. Lastly, barn swallow flight manoeuvres during foraging are typically low energy and the birds appear to depend on a fast approach to unsuspecting prey rather than a sharp turn to catch evasive prey (Warrick et al, 2016). We expect that common swifts operate similarly because wing loading and thus flight speed tends to increase with body size.…”