A Tern Mystery

While I know there is one mystery still outstanding (the skull, see a few posts below) here is another which I have no answer to.

Black-necked Swans and Hudsonian Godwits (over 1000)

I was out at la Bahia de Caulin, an awesome shorebird (and oyster) spot, poking around for birds when I noticed an immature tern roosting with the gulls. Immature terns can be quite interesting cases for identification and though I suspected this was a South American Tern, I got closer to make sure. I was right, but as I neared, I realized that something else was very interesting about this bird.

Immature South American Tern

That's right, it had only one wing. The mud in the picture actually was not on the bird until I tried to pick it up, at which point it turned on its back and tried to swat me with its legs and its one wing and bite me. Had I been looking at the other side of the tern, I would have walked right by as if nothing was wrong. There was no blood at the wound (though the bone was sticking out), nor was the tern acting odd. It was in the middle of a muddy seashore with nothing for hundreds of meters in any direction. How did it get there (I guess it must have walked)? Where was its wing? What took just its wing? Why was there no blood?

Note the bone just up and to the left of the first joint of my thumb

My guess is that it got tangled in a fishing/seaweed net and somehow lost its wing getting out, but that is just a guess based on how much fishing/seaweed collecting (passively using nets) goes on in this bay. Maybe, as it is an immature, the parents were still feeding it and that is why it was in such good shape. But that is all hypothetical, the truth is I have no idea why this one-winged tern was in apparent good condition on the beach today, and don't expect ever to know.

A two-winged adult, Ushuaia harbor, Dec 2010. 

Ouch.