Wednesday, August 6, 2014

the Medium terns in Late Summer

I walked Tiscornia to Jean Klock yesterday without shorebirds of interest, but there were some quite tame terns in with the gulls.  In the spring the terns are fairly flighty, these ones have clearly become accustomed to people.  Which was good since I was having trouble telling them apart from a distance.

The problem is that you don't really think of them as having different plumage in the fall as the spring, but they change enough to be troublesome.  The Common is on the left above and the Forster's on the right.  When I'm up close, the shorter legs and trimmer build of Common stands out.  At the right angle the darker mantle of Common is helpful, but at a lot of sun angles there's not a ton of difference even with direct comparison.  With direct comparison the redder bill and legs of Common is frequently a shade darker in the field than the orange of Forster's

Here's a Forster's showing another problem with fall ID:
 It's either moulting or has broken off its outer tail feathers meaning that the tail falls short of the end of the wings as Common generally does.  In spring Forster's tail will extend an inch past the primaries.

This bird, a little farther along in moulting (as evidenced by the cap) has the tail at the proper length.
The primaries are also noticeably darker in the fall.  Tern primaries have a fine bloom on the feathers which gives fresh feathers a much whiter look.  With that worn off Forster's primaries become medium cold gray at some angles rather than the brighter white of spring.

It varies though, upon both individuals and the angle.

Moving to Common, they also have darker wings than they exhibit in spring, quite blackish when folded.
 The above group are 3 Commons in front (with the front most bird still having grayish underparts and a full cap with the Common on the right being 1 year old) and a longer-legged rangier Forster's in the back.

The primaries aren't as dark in the spread wing than as when folded, which doesn't make it easier to tell them apart.

Finally the one-year old Common's that are present.
The dark carpal bar (which juvies will also show when they arrive) is a dead giveaway that it's a Common.
The (quite faded) one-year-olds are quite pale in flight.
The other ID difference you may have noticed is that a portion of the Common Terns are banded, I didn't see any banded Forster's.
Speaking of which, I added a comment from one of the plover researchers, one of the Piping Plover juvies is from Ludington and one from Sleeping Bear.


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