Friday, September 27, 2013

Darner Hovering: Bobbing With Bursts

Hovering is an energy-intensive activity.  It's not surprising, then, that dragonflies might employ strategies to reduce the effort – or at least rest the muscles a bit.  Here's an example.

I was observing a Happy-face Darner (Paddle-tailed Darner) hovering at Beaver Pond in Winthrop, WA.  As it hovered, I noticed it was bobbing up-and-down in a regular manner.  This might not seem surprising, considering that its wingbeats don't provide constant lift.  The frequency of the "bobbing" motion was much smaller than the frequency of the wingbeats, however.  The wings beat at about 35-40 beats per second, whereas the bobbing motion was clearly only a fraction of that frequency.  I decided to record a video to study the hovering in more detail.

Here's one of the videos I recorded.  It's recorded in "real time"; that is, normal speed.

A better view can be had at the following link to the Dragonfly Whisperer Channel on YouTube:

As you can see, the wings are beating so fast they're just a blur.  In contrast, the body bobs up-and-down at a much slower rate.

I made frame-by-frame measurements of the flight level of the darner from the video and have plotted the results below.

Data from a video showing the up-and-down bobbing motion of a hovering darner.

The data show a clear periodicity to the flight level.  The darner is indeed bobbing up-and-down with quite a well-defined frequency.

The bobbing motion has a period of about 0.2 s, which corresponds to 5 bobs per second – or roughly one bobbing cycle for every 8 wingbeats.  The angular frequency of this motion, w, is about 31 radians per second, and the amplitude, A, is about 0.4 cm.  It follows that the maximum upward and downward acceleration is roughly Aw^2 = 3.8 m/s^2, which is just under 40% of the acceleration due to gravity.  Thus, the darner is never in free fall; it's always getting lift from the wings, but the lift varies with a period of 0.2 s.

It seems, then, that the darner alternates weaker and stronger wingbeats as it hovers, a little like the flight of finches and woodpeckers.  This is indicated in the plot below.  With this strategy, the darner can give a burst of 4 strong wingbeats, rest for 4 wingbeats, give another burst of 4 strong wingbeats, rest for 4 wingbeats, and so on.  Perhaps the intermittent rest helps it hover for long periods of time without getting fatigued.

Same plot as above, but with times of weak and strong wingbeats indicated.

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