A Hovering Happy Face

One of the delights of the Fall season is that Paddle-tailed Darners are out in numbers. They can be seen patrolling the shoreline of lakes, ponds, and ditches. They also hover frequently, as they keep a close watch on their territory. Here's an example of the hovering behavior:

These dragonflies are beautiful, elegant fliers.

When hovering, they flap their wings out of phase—but not 180˚ out of phase. Recall that 180˚ out of phase means that the wings are moving in opposite directions—one goes down when the other goes up. What is observed in this video, however, is a phase difference between the wings of 100˚. With this phase difference, the wings flap in such a way that when one is at it's maximum downward position, the other is at its horizontal location.

For a specific example, when the hindwings flap downward, the forewings follow a short time later, so that when the hindwings are at the bottom of their stroke, the forewings are at the horizontal position on their way down. When the forewings are at their maximum down position, the hindwings are horizontal on their way up. Flapping out of phase like this improves efficiency, allowing for minimum energy expenditure as they hover.

In contrast, when dragonflies want to accelerate and reach maximum speed they flap their wings together. This is less efficient, and hence requires more energy, but the tradeoff is worth it when the desire is to get moving quickly to chase a rival or a potential mate.

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:

http://www.youtube.com/watch?v=r8oLKmfMLBc

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.

Cap Sante Hilltopping

In my hometown of Anacortes, there's a nice park on a hill called Cap Sante.  Looking to the west from the summit of Cap Sante you get a great view of downtown Anacortes and the Cap Sante marina, as shown below.  The marina services both pleasure boats and commercial fishing boats, many of which fish for crabs and salmon in Alaska.

Downtown Anacortes as seen from Cap Sante.

Cap Sante is also a great location for watching sunsets:

Sunset from Cap Sante, in Anacortes, WA.

In addition, if you look to the east from Cap Sante, you get a magnificent view of Mount Baker, a 10,781 ft high volcano in the Cascade Mountain range.  Here's Mount Baker as seen from Cap Sante:

Mount Baker in the North Cascades, as seen from Cap Sante Park in Anacortes, WA.

What does Cap Sante and its wonderful views have to do with dragonflies?  Well, Cap Sante is a good place to observe hilltopping – a behavior where a group of dragonflies (or other insects) gather in considerable numbers above a prominent feature in the landscape.  In some cases, hilltopping is said to be a reproductive behavior – a way for insects to find mates.  In the case of the dragonflies at Cap Sante, it appears to be a gathering of immature individuals who are feeding together in a loose swarm.

The dragonflies at Cap Sante are mosaic darners, and they are clearly feeding as they fly back and forth over the top of the hill.  The darners really don't interact with one another – they seem focussed on feeding and maturing, before going back to a lake or pond to find a mate.

Here's a compilation of some slow-motion videos showing darners hilltopping over Cap Sante with Mount Baker in the background.  The dragonflies fly and soar almost like hawks.  Notice the interesting flight maneuver at about 18 s, where a darner snags a meal.

Extreme Maneuver

Dragonflies are well know for their acrobatic abilities in flight.  Here's an extreme example:

This dragonfly, as you might have guessed, is a male Happy-face (Paddle-tailed Darner).  It was patrolling back and forth along its territory, a walking path in front of some bushes near a water-filled ditch.  Most of my shots show him in normal flight, but in this shot he is apparently making an extreme turn to the right.  I think fighter jets would have a hard time keeping up with this guy.

Here's what he looks like most of the time.  Notice the heavy eyebrows of the "happy face," and the fact that the front two legs are tucked up behind his head.