Sunday, January 29, 2012

Dragonflying at Mount Shuksun

On October 17, 2011, Betsy and I went to Picture Lake near Mount Baker.  Picture Lake is just before you get to the Mount Baker ski area, and it provides an incredible reflected view of Mount Shuksun.  A little later in the season the fall colors would be more prominent, yielding an even more impressive view.

Betsy, with Picture Lake and Mount Shuksun in the background.  This picture was taken shortly after we arrived, when there was still a bit of fog near the summit of Shuksun.  The temperature at this point was a brisk 46 ˚F, but the sun and lack of a breeze made it feel quite comfortable.  It warmed up during the time we were there, and by lunchtime it was 64 ˚F.

This is me at Picture Lake a little while later after the fog had burned off.  When we first arrived we didn't see any dragonflies to whisper to, but shortly thereafter the activity was pretty good.

The greatest activity that day was provided by the Ringed Emeralds.  The first one we saw was this dead male found along the shoreline.  A darner helped us find it by stopping on his patrol of the shoreline to check it out more carefully – in case it might happen to be a female darner.  The Ringed Emerald has an iridescent green thorax, green eyes, and thin white rings at the base of each abdominal segment.

We saw a few darners at the lake.  All that we could see well were Paddle-tailed Darners – Mr. Happy Face.  

The Ringed Emeralds were plentiful as the air warmed up a few degrees.  You could see them everywhere along the shoreline: pairs flying in tandem; males chasing females and other males; females laying eggs with males hovering nearby for protection.  Here a female dips her abdomen into the water to deposit eggs (water temperature was 46 ˚F).  

The water was so clear that I spent some time looking to see if I could spot the eggs the female was laying.  With a little practice I could.  The eggs came out in a small mucus string of maybe 6 or 8 greenish eggs, a lot like miniature frog eggs.  Strings start out in a tight ball, but then twist and straighten out as they drift slowly to the bottom.

Saturday, January 28, 2012

Splash-Dunk Behavior: The Dragon Splash

Final Approach

A Paddle-tailed Darner on its "final approach", preparing for a splash/dunk.  The darner is just to the left of center, and its reflection can be seen in the water.


The darner plows into the water sending up a considerable quantity of water to an impressive height.  If we were to scale the darner up to human size, say with a length of about 6 feet, the height of the splash would be over 30 feet.  Quite an impressive splash.

The Dragon Splash

A head-on view of a darner splash.  The left and right lobes of the splash are produced by the wings; the central lobe is due to the body.  To me the splash looks a bit like a Loch Ness monster with wings.  I refer to this as the "dragon splash."

Sticking Frequency versus Temperature

When a darner does a splash-dunk, it is sometimes unable to get back out of the water.  It becomes stuck, and will die (unless it's close enough for me to save it), either by drowning or being eaten by a fish. 

The plot below shows data for the frequency for becoming stuck – the sticking frequency – as a function of the date of observation.  The first observation of a darner getting stuck (a sticking event) was about the middle of the season (9/19/11) and we continued to see splash-dunks and sticking events until the end of October.  The plot also shows the temperature versus date of observation in ˚F, using the same vertical axis as for the sticking frequency.

No sticking events were seen when the temperature was above 65 ˚F.  In fact, we saw 91 dunks before observing the first sticking event.  As the season continued, and the temperature dropped, and the sticking frequency increased.

Notice the nice correlation between temperature and sticking frequency.  By the end of the flight season the temperature was in the 40s and the sticking frequency was about 25%.  At temperatures this low, the flight mechanism is inefficient, and the dragonfly is often just not strong enough to break free of the water.  Thus, a dragonfly splash-dunking at the end of the season has a one in four chance of not making it back out again.  Quite a high mortality rate.  Clearly the benefit of splash-dunking (possibly to clean their bodies and wings) is fairly significant for the population to continue the behavior in the face of a 25% mortality.

The next plot shows the sticking frequency as a function of temperature in ˚F.  The data indicate a smooth, systematic drop off in sticking frequency with increasing temperature.

The drop-off in sticking frequency is approximately exponential, as indicated by the smooth curve that gives a good fit to the data.  The exponential curve predicts a small probability for sticking events at 65 ˚F and higher, but none were observed.  The probability of these events is so low that it will take more observations to get good results in this temperature range.

Splash-Dunk Analysis

When taking data in the field you have no idea how things will turn out in the end.  The plot shown here presents splash/dunk data for 2011.  When recording the data I had no idea the end result would be so pleasing to the eye.

Here's the idea behind the plot.  When dragonflies are observed to splash/dunk, they often hit the water multiple times – doing one dunk after another after another.  We've observed as many as 6 dunks in a row before the dragonfly gains altitude and does a spin/dry at 1,000 rpm.  The data for 2011 consisted of observations of 148 events combining for a total of 288 dunks.  Most events consisted of only a single dunk (62 such events), and the average number of dunks per event was 1.9.

The exponential drop off in the number of dunks suggests that dunks are independent events – each dunk is independent of the dunks that precede it.  Roughly speaking, once a dragonfly dunks there is about a 50/50 chance that it will dunk again.  This is true whether the dragonfly has only dunked once, or has already dunked 5 times.  The previous history doesn't matter.  In a sense, you might say that dragonflies "live in the moment," deciding after each dunk whether to dunk again with no regard for what has happened in the past.

Monday, January 23, 2012

Interacting With Dragonflies

Autumn Meadowhawks in the wheel position.

Interacting with dragonflies is part of the fun of dragonflying.  In the photo above a pair of Autumn Meadowhawks in the wheel position rests on my finger.  These are the latest flying dragonflies in our area, and they are very comfortable with landing on people or sitting on your finger.  Betsy and I have given them the title of "Friendliest Dragonfly."

The pleasures of interacting with dragonflies.

Nature is a delight.

One day while taking pictures of dragonflies I had a Blue-eyed Darner on my finger.  A father and his daughter stopped and asked what I was doing.  When I told them, the girl said, "Can I hold it?"  I said sure, and transferred the cooperative dragonfly from my finger to her hand.  As you can see, she was delighted with the experience.  It was a beautiful dragonfly, with incredible blue colors.  The dragonfly sat on her hand for about five minutes.  We eventually walked closer to the shore, she held her hand out, and the dragonfly took off.  She said, "Will it come back?"  "No," I said, "it has a busy life and lots of things to do."

The dragonfly had not been caught with a net or manipulated in anyway – it was a "free range" dragonfly.  I simply lifted it up from the bushes on my fingertip.  It was free to fly away any time it wanted.