Monday, July 7, 2014

Arctic Tern! - 05 Jul 2014

I returned to Cell 3 at Pt. Mouillee late this afternoon to look for the Little Gull(s) and to see if I could get better digiscoped images. I was also anxious to see if my digiscoping rig modifications would make things easier for me in the field. They did.

I had spent the afternoon refining the Digidapter™setup to accommodate the Hoodman Loupe so that I could use the LCD viewfinder instead of the EVF.  I played w/ realigning the camera so that it sat square on the Digidapter™but found that I got a shadow when I rotated the Digidapter™ 90 degrees counterclockwise. When I rotated the camera 90 degrees clockwise, however, the image was clear, so I was successful there.  Another problem I ran into was the tripod screw position (1) on the Sony RX100 III - the Digidapter™camera screw crashed into the platform positioning screw on the right side of the camera (2) and prevented me from tightening the camera down. I resolved this by reversing the platform screw (large screw on top and small hex screw on bottom). To keep the camera position square I positioned the left platform screw (3) behind the camera so that tightening the camera down to the platform wouldn't rotate the camera. Now I can loosen the tripod screw, remove the camera to replace battery/card, and reposition it back on the Digidapter™ for perfect realignment. The modifications worked perfectly, as I would soon find out.

I arrived at the SW corner of Cell 3 shortly before 6 pm and found Adam Byrne, Phil Chu, and Bob and Laura Payne scoping the mudflats.  I innocently asked if they that seen anything interesting, and Phil replied, "Arctic Tern!". Adam was happy to see me since he only had his iPhone to try to photograph the tern through his scope at 70 yards.

The Arctic Tern was easy to find. Among the 'bright' white Forster's Terns and Bonaparte's Gulls sat a small dark gray tern with a bright red bill and short, short, short red feet. I was surprised at how dark the tern looked next to the other birds, and was informed that they can look even darker. I spent the next 30 minutes filling up a 16Gb card trying to get documentation images for Adam.

Phil was hoping to record a wing stretch so that he could see wing lining, so the Hoodman Loupe really came in handy while trying to maintain focus while using the focus-peaking capability of the Sony RX100 III. This involves focusing the lens w/ the left hand while depressing the shutter halfway with the right hand, which means camera shake! Luckily, I was able to get enough sharp images to 'confirm' identification.

The tern spent most of the time facing us while preening, but finally lifted off and flew forward a few feet at a time. It landed close enough to the Forster's Terns nearby to get a comparison of shape, size and posture.  When Phil mentioned that it had really short feet (tarsus) he was not kidding. According to Pyle (2008) Forster's Terns have tarsi that measure 21-27mm, while Common Terns have tarsi that measure 18-22 mm. Arctic Terns have tarsi that measure 13-17mm, which is significant enough to observe in the field. This difference is not always a reliable field characteristic, however, as posture and feather fluffing can lead to observable differences in leg length (Kaufman, 2011). But in this instance, the field mark was unmistakable!

I made sure to get some digiscoped images of the nearby Common Terns for comparison. Immediately evident was the longer-legged stance of most of the birds, but again, not reliable. Some of the adult Common Terns appeared quite gray, as well, with bills that were also almost entirely red. But close examination revealed a small amount of dark at the tips of the red bills that contrasted with the all-red (with hint of yellow tip) of the Arctic Tern.  Note, however, that depending upon age of birds and time of season (fall), the Common Tern bill can appear almost black.  Forster's Terns appear even lighter overall, with a taller stance and bright orange bills (dark tips) and feet.

Despite their gray coloration, however, the Common Terns showed significantly lighter chests and throats, while the Arctic Terns showed gray up to the chin area.  This was most evident when it stretched. There is no observable difference in shading between chest and back of the bird.

Kenn Kaufman (2011) has a wonderful writeup on the differences between the medium-sized terns (Common, Forster's, Arctic, and Roseate) that is worth reading. He describes in great detail all of the nuances and differences between these birds, including age, molt, wing projection, underwing, upper wing and tail patterns, and more. Its much easier reading than the more technical Pyle Guide, and helped me confirmed what I saw through the scope.

When the Arctic Tern finally flew off I managed to grab a series of flight shots as it headed east toward Lake Erie. These images helped me confirm identification and show differences between Arctic Terns and Common Terns (and Forster's, for that matter) that in most instances can only be recorded by a camera.

In flight, the first thing I noticed was the lack of 'dark' wedges in the wing tips of the Arctic Tern. This was significant, since Common Terns show dark feathers along the outer primaries (P5-P10) that help distinguish them from Forster's Terns, which typically show 0-1 dark primary feathers (P10) in flight.   In the image at left note how uniformly 'gray' the upper wing and primary feathers are compared to those of Common Terns in flight. The Arctic Tern shows a hint of black along the shaft of the P10 feather (lower right arrow). The tips of the primaries are dark gray and contrast with the white-tipped secondaries (upper left arrow). The composite image below shows this uniform gray upper wing during successive wing-flaps, while the underwings show only the gray-tipped primaries.

Compare with the 'dark wedges' seen with Common Terns in flight.

Now compare w/ Forster's Terns in flight. Only the wing tips show some darkening, while the chest and belly are almost completely white (in contrast to the dark gray chest/belly of the Arctic Tern).

With the tern now out of the picture we turned our attention to the rest of the field.  A Lesser Black-backed Gull (3rd yr bird) was observed near the American White Pelicans, whose numbers continue to swell (Adam had 25 birds today, and 62 birds would be seen by Andrew Sturgess on 07 July).  A Willet was seen along the north shore of Cell 3.  Short-billed Dowitchers have appeared since last night (22 birds), as have 5 Stilt Sandpipers!  After Adam and Phil left I spent a few minutes digiscoping the Stilt Sandpipers. I couldn't ignore the Black Terns or the Caspian Terns before heading back for the bike.

Just before reaching the car I spotted a Pied-billed Grebe that was sitting on the far dike. I couldn't leave without grabbing one last digiscoped image for the night.

Great day!


Pyle, P., Identification Guide to North American Birds, Part II, 2008,  Slate Creek Press, CA.

Kaufman, K., Field Guide to Advanced Birding, 2011, Houghton Mifflin Harcourt, NY.


Will Weber said...

Great images and discussion of tern IDs.

winerat said...

What a helpful and detailed explanation of these differences. Nothing like digiscoped pictures to illustrate and explore. Many thanks for the pictures and the discussion.

MedicineMan999 said...

I'm glad you are here and having already invented the wheel so-to-speak. Why? Because I have a Sony RX100iii, and a Swarovski 95mm ATX but more importantly I'm awaiting a Digidapter from Paul woohoo.

I've been using a Sony A6000 and like it BUT my choice of lens for it is the SEL35 F1.8 and using the Swarovski DCB-II I cannot get it to click in place.

Yes I sometimes use the TLS APO but often not.

I do want to give the RX100iii a go and you've already puzzled out the connectivity to the adapter=THANKS.

Of interest is your use of the Loupe versus the EVF. Can you tell me why you prefer this? I use a loupe on bodies like the 6D and 5Diii but never thought to use it on the Sony!

So thank you for being out there and contributing to a would-be wanna-be 'scoper.

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