Tuesday, August 15, 2017

Search for Cave Swallow at Harriman Lake Park

August 7, 2017

Richard Stevens: email sent to cobirders listserve:

Hello cobirders,
Bird report below, trust me.

Anyone seeing a nut standing in the rain for three+ hours at Harriman Lake Park this afternoon, it was me!  I rigged up a tripod with an umbrella and another with my camera and took shots (can they still be called photos?) of the swallows flying around the western end of the lake.

While studying the swallows I noticed that the species tend to fly different patterns and speeds.  After a while, I would look without binoculars and guess species; then use binoculars to confirm.  Accurate rate was close to perfect.

That got me thinking about the Strouhal number and whether I could calculate what I was seeing.  For those not knowing, Strouhal number may be used to calculate flight speed of birds (in some cases when flight is in straight line).  The number is equal to frequency (of wing beats) times amplitude (vertical distance traveled by wing tip during a flapping stroke) divided by forward speed (distance traveled per second).

Took over 1400 photos and stopwatch measurements, it will take a few days to summarize my data, depends on how well the photos come out, I was able to get 1/2000 shutter speed out of my camera during some of the brighter moments in the cloudy skies.

All this was brought to mind as one particular swallow was much faster and flying straighter than most of the rest.

Swallow count was hundreds of Barn Swallows, dozens of Tree Swallows, four Violet-green Swallows, two Cliff Swallows and the speedy swallow.

I watched a potential Cave Swallow (speedy) at one stretch for 12 minutes as it flew towards me and parallel many times.  It eventually flew to the eastern end of the lake.  Many of the swallows would rest on the rocks at the southwest corner of the lake.  Regrettably the possible Cave Swallow did not.

The speedy swallow had a buffy throat that extended behind the nape, rusty forehead, and no white "headlights" on forehead.  Southwestern race of Cliff Swallow does show rusty forehead but also dark chestnut throat which "speedy" lacked.  I hope I captured a photo of the mystery bird!

Rain increased, I departed.

Another bird of note was a Red-eyed Vireo.  It was along the southern side of the lake up the "canal" that runs south.

Continued Good Birding!

Directions to birding spots and maps on CoBus website:


NOTE: after looking through hundreds of photos (most were fuzzy, out of focus) I could not confirm that a Cave Swallow was present.  I probably will address "swallow flying speed" in September, 2017 "Colorado Field Notes" 

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