Monday, January 6, 2014

A Hovering Darner, Part I

Last summer, Betsy and I spent a delightful afternoon at Beaver Pond, near Winthrop, WA.  It's a medium-sized pond with a tall snag topped by an Osprey nest, and surrounded by a wide flat trail lined with towering Ponderosa Pines and ghostly Quaking Aspen.  The day was sunny and warm, and buzzing with activity of all types.

At one point we sat on a bench to relax, and a friendly male Happy-face Darner (shown below) began to hover at eye level, just inches away from us.  After a few seconds of hovering, he would dart off after a rival, and then return to continue guarding his territory.  He did this time after time, repeatedly giving us a great view.

A male Happy-face Darner (aka, the Paddle-tailed Darner) hovers in front of us, keeping us company.

At first, I wasn't going to take a video of his hovering – after all, I have plenty of good hovering videos from previous years.  But then I noticed something that I had noticed before, but hadn't given much thought.  As the darner hovered, it was "bobbing" up and down.  This doesn't seem too surprising, given that the hovering is produced by wings that are flapping up and down.  What was surprising, though, was that the bobbing was at a much lower frequency than the wingbeats.  Why such a difference, and just what is the bobbing frequency?

With these questions in mind, I decided to go ahead and take a video of the hovering.  The total 5 second video can be found at the following link:

The video was shot at 30 frames per second, and so I made screen captures of each of the 150 frames in the total clip.  I digitized each frame by measuring the distance from a prominent point in the background (a crossing of two branches) to a fixed point on the body of the darner.  This removed effects of the camera moving slightly during the clip.  I measured to the head of the darner, to his thorax, and to the tip of his tail – all three cases gave essentially the same results.  The results given here were recorded for distances measured to the darner's head.

The graph below shows a plot of all 150 data points for the 5 seconds of hovering.  The "cluster" of points is the hover itself, and the points going off to the right show the darner's takeoff to challenge a rival.  Notice that this is a plot of vertical position versus horizontal position.

To see what's happening during the hover, we expand the cluster of points, and plot them as vertical position versus time.  The result is shown below:

The first thing we notice in this plot is that the darner is indeed bobbing up and down as it hovers, and in a fairly regular way.  In fact, it's clear there are about 4 - 5 "bobs" per second, as compared with the wingbeats, which are at the rate of 35 - 40 per second.  The second thing we notice is that the average height of the hover is decreasing with time at a steady rate of about 0.2 cm per second.  This is something I hadn't noticed in real time as I observed the hover.

The analysis of the hover will continue in a subsequent post.

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