One of the few times I encountered Tundra Swans this winter was when I spotted these three birds. It wasn't for lack of looking. Sometimes it is a matter of access. I knew where swans were a couple of times, but there was no public access to that particular piece of shoreline.
Anyway, there was a seagull hanging with these three who was pretty obnoxious. He would get so close, he would actually be bumping up against them. I thought it was a bit strange, but they didn't seem to mind.
The Tundra Swans eat what is known as SAV, subaquatic vegetation, so they are usually found in water no deeper than the length of their neck. That can be a helpful tip for finding them. They are seldom found in deep water and can usually be found along the shoreline or in shallow ponds and marshes.
While watching the interaction of the swans and seagull, the answer revealed itself, as to why this seagull was staying so close. One of the swans pulled up some grass and brought up a razor clam with it. That was what the seagull was waiting for. He grabbed it and flew off triumphant!
I have seen two other instances of one bird species using another to obtain food. One was seagulls and diving ducks and the other was Snowy Egrets and dabbling ducks. It doesn't fit the definition of a parasitic relationship, but neither is it a symbiotic relationship. I'm not sure what you would call it.
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