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Aerodynamics of manoeuvring flight in animals

When animals fly in nature they face many challenges. Winds are not steady, obstacles are everywhere, a predator might be just around the corner and the prey that you need to capture is elusive. These challenges are constantly being thrown at the animals as soon as they take to the wings. The animals need to detect these, in most cases, unpredictable perturbations and process the sensory information in order to react and execute appropriate compensatory behaviours to stabilise or to avoid collision. To date, we know very little about how animals accomplish this, so to fully understand animal flight we therefore need to understand the fundamental processes involved in manoeuvring flight

A bat flying towards a rod with food with its wings spread against a dark background.
Brown long-eared bat manoeuvring while pursuing a prey in the Lund wind tunnel. Photo: A. Hedenström.

Project aim

This project has so far been focused on understanding fundamental manoeuvring behaviours and for the coming years we will widen the scope through three main approaches:

  1. Sensory-motor integration. Determining the links between sensory system input, motor control (kinematics) and aerodynamic output in flying animals during manoeuvres.
  2. Unsteady conditions. Identifying abilities and limitations of different evolutionary solutions (birds, bats and insects) for dealing with unpredictable disturbances in their air environment.
  3. Performance in the wild. Determining the manoeuvring performance related to challenges faced by animals during different flight behaviours in their day-to-day life.

Through this project, we aim to reach an understanding of what it really takes to do what the flying animals do in their everyday life.

The three main tracks:

Sensory-motor integration in manoeuvring flight

The aim for this subproject is to investigate how animals, between changing conditions, alter their assessment of the environment, modulate their kinematics and what the aerodynamic consequences are. We will study this through wind tunnel experiments and use bats as model species since their echolocation behaviour allows us to determine when and how they sample their environment.
We will focus on the effect on sensory-motor integration of three main aspects of the flight of the animals:

  • Overall flight speed
  • The speed of the evasive manoeuvre of a prey
  • Size of a prey

Manoeuvring flight under unsteady conditions

In this subproject, we study how animals cope with unpredictable perturbations – wind fluctuations and obstacles – similar to what they constantly experience in nature. We will use selected species from each of the three main groups of flying animals (birds, bats and insects) for wind tunnel experiments. This is a fundamentally different scenario from voluntary manoeuvres such as pursuing a prey, since perturbations force a response from the animal without allowing for forward planning. The comparison between such forced manoeuvres and voluntary prey pursuit (which we have been studying previously) will be a key element of this sub-project.

The main aspects to investigate are:

  • The effect of turbulence and gusts of winds on manoeuvring performance.
  • How the animals perform forced manoeuvres due to obstacles and the difference between forced and voluntary manoeuvres.
  • Comparison of birds, bats and insects to determine the differences in strategies and execution of compensatory manoeuvres.

Manoeuvring performance in the wild

We will study manoeuvring flight performance of birds in the field using miniature accelerometers to make on-bird measurements of accelerations that birds are experiencing in different natural flight behaviours such as food searching, transport and prey pursuit flights. Animal manoeuvring in the wild is largely unknown and the main aspects we will focus on are:

  • Performance envelope of birds in the wild.
  • Manoeuvring requirements and challenges during different flight behaviours.

An illustration with coloured patterns showing a bat wake vortex in a box.

The wake behind a bat during a leftward shallow turn. Vortices, particularly the wingtip vortices, can be seen and they reveal the subtle asymmetries generated by the bat for initiating and terminating the manoeuvre. Colours represent vorticity, red showing counter-clockwise rotation and blue clockwise.

Achievements so far

We have so far performed manoeuvring experiments on brown long-eared bats, pied flycatchers, silver-washed fritillary butterflies, and a pilot on tobacco hawkmoths. The experiments performed so far have been focused on characterizing fundamental manoeuvers and are designed to be as similar as possible between the different animal groups in order to make it possible for direct comparisons between them. We carry out experiments in the Lund wind tunnel where we allow the animals to follow a sideways moving prey. Just as the animal is about to catch the prey, the prey is moved and the animal follows by performing a controlled lateral movement. The manoeuver is recorded both with high-speed cameras for analysis of the animal's motion and with our system for flow visualization for analysis of the aerodynamic footprint. We then analyse the high-speed films by digitizing points on the body and wings of the animals allowing us to reconstruct the motion during the manoeuvers with high accuracy. The flow visualisation is analysed both qualitatively, i.e. by reconstructing and visualize the wake in three dimensions to investigate various structures of the wake and how and when these structures are formed, and quantitatively, by measuring the forces that are created and when that happens in the wing stroke and the manoeuver. Aerodynamics and kinematics are then linked together to provide a complete picture of how the manoeuver is performed.

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