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Thursday, April 28, 2011

Another learning experience

I have long thought that the main way one learns moth worthwhile skills is by doing it the wrong way for a while. Birding fits this rule to a tee. Tonight I pulled the trigger and made all the phone calls for what I thought was an adult Common Tern (a decent bird for Kent Co. and one I and several other Kent listers still need). I had first scanned the lake for 15 minutes without seeing any terns, so I packed up my gear and jumped in the car to leave. At this moment I noticed a tern flying just offshore from my car, got my binoculars on it in flight, and easily saw that it had a dark wedge in the mid primaries, grading from blackish around p6 to gray at p10, plus a fully black cap. This combination of features struck me as unique to adult Common Tern, so I began calling right away.

However, I never got a scope look at the bird in flight, as it was halfway across the lake by the time I got set up. I watched it land on a green buoy at the far NW corner of the lake, then drove up and was able to get a look from 300-400 feet, where I took these shots (below). The photos show a bird which structurally is a dead ringer for Forster's Tern (large bill with strong gonys, long, thick legs, heavyset appearance). Plumage (except for the primary pattern) also looks good for Forster's and wrong for Common: no gray on the underparts, and lots of white above the gape and below the front of the cap. Look for yourself:

So, how to reconcile the apparently contradictory traits? First off, these photos are enough to put me entirely in the Forster's Tern camp, regardless of primaries. This bird's structure and head/underparts plumage cannot be shown by any Common Tern of any sex and age that I am aware of. So what is going on with this primary pattern? The best explanation I have heard (from Adam Byrne) is that this is a third calendar year Forster's Tern with outer primaries which are older than what they would be on an after third calendar year Forster's Tern, and resultingly darker from the more advance state of wear. In a nutshell, adult Forster's Terns breed in May-July, then initiate primary molt at P1 in a descendent wave, which pauses around p5-p6 in September for fall migration. Once arriving on the winter grounds, primary molt resumes at p6 and also restarts at p1 (aka. serial descendant molt, or staffelmauser). The outer wave proceeds most slowly and is not completed until late in winter. Contrast this strategy with a second calendar year Forster's Tern, which begins molt in the early summer (since it is not breeding) while the adults are tending young and holding off on starting molt. This gives them a jump start of perhaps 2-3 months on the adults. I assume that this could easily translate into an early finish to the outer primary molt wave, perhaps by early winter? This would mean that as of April, a third calendar year bird might have outer primaries as old as 5-6 months, whereas the adult would only have 2-3 month old outer primaries. This could easily mean the difference between fully white outer primaries (as are standard on all fully adult Forster's) and gray outer primaries. This is the best explanation I can think of for how a bird could look like the one I found at Camp Lake. I would be very interested to hear additional alternative explanations, or from anyone who has seen a bird similar to mine.

Take home message: an all black cap and mid-primary black wedge in April does not necessarily translate to a Common Tern...

1 comment:

Happy_Peasant said...

There is a Forster's in this bird report I made today with some dark primaries. Maybe like yours?