Flying through gaps: how does a bird deal with the problem and what costs are there?

Abstract

Animals flying in the wild often show remarkable abilities to negotiate obstacles and narrow openings in complex environments. Impressive as these abilities are, this must result in costs in terms of impaired flight performance. In this study, I used a budgerigar as a model for studying these costs. The bird was filmed in stereo when flying through a wide range of gap widths from well above wingspan down to a mere 1/4 of wingspan. Three-dimensional flight trajectories were acquired and speed, wingbeat frequency and accelerations/decelerations were calculated. The bird used two different wing postures to get through the gaps and could use very small safety margins (down to 6 mm on either side) but preferred to use larger when gap width allowed. When gaps were smaller than wingspan, flight speed was reduced with reducing gap width down to half for the smallest and wingbeat frequency was increased. I conclude that flying through gaps potentially comes with multiple types of cost to a bird of which the main may be: (i) reduced flight speed increases the flight duration and hence the energy consumption to get from point A to B , (ii) the underlying U-shaped speed to power relationship means further cost from reduced flight speed, and associated with it (iii) elevated wingbeat frequency includes a third direct cost.

Flight performance, activity and behaviour of breeding pied flycatchers in the wild, revealed with accelerometers and machine learning

“…Changing speed like this frequently must come with added energy expenditure as well as requiring skilful manoeuvring while flying in a complex and dense forest environment (cf. Henningsson, 2021).…”

Crash 2 Squash: An Autonomous Drone for the Traversal of Narrow Passageways

“…These needs have seen insects and birds leverage their soft bodies [ 22 ] with streamlined shapes [ 23 ] to ease alignment and squeeze into tight spaces while surviving the inevitable collisions. These animals also fold their wings to reduce their size, [ 24,25 ] and use legs and claws to generate propulsion when flapping is precluded. [ 26 ] Soft morphing bodies and multimodal locomotion are combined with sharp perception and motion coordination into effective gap negotiation strategies.…”

Birds both avoid and control collisions by harnessing visually guided force vectoring

“…Foraging birds, bats and insects frequently navigate cluttered habitats, negotiating obstacles on the wing. The severity of a collision depends largely on an animal's momentum (mass times velocity) during impact [1], so it is critical for larger animals like birds and bats to use rapid changes in posture [2,3] or reliable manoeuvring strategies to avoid injury. Arboreal birds, in particular, are adept at negotiating intervening branches as they perform short flights (less than 1 m) up to 30 times per minute [4] in search of food.…”