Yes, N sounds reasonable to me. I guess in theory, if this is only ever used for incubating or brooding birds and you are confident in the ID, in theory it could be ON but I hesitate to make that a general rule (can other species be confirmed by call alone). I wasn't even aware this call existed, do you have a link to audio of it?
I learned it in the field. It's like "Chip. Chip-chip, chip-chip", made as the bird is flying low to or from the nest. I did Swamp Sparrow nest monitoring for a PhD student from Purdue in the summer of 2008, right after I did the pilot year of marsh bird surveys for Ryan and Andy, so I spent all day standing in the Marshes at Westford and Slinger outside Beaver Dam listening for that call. There's literature but I can't find audio, unless it'sburied somewhere in Xeno Canto.
"The really special vocalization of the Swamp Sparrow is not the song of the male. Females utter a loud series of chips as they leave a nest they have been incubating or brooding. This 'nest departure call" is the Rosetta stone of Swamp Sparrow biology. It allows ornithogists to find their well-concealed nests, which are grassy cups, hidden within tufts of grasses or sedges. Presumably they do not give this call to make the life of researchers easier. But why they give a call that gives away the location of their nest is a great mystery.
One clue is that the few other species that also have such a vocalization breed in dense marsh vegetation as well. One theory holds that the female is letting the male know she is leaving the nest and should not be attacked by her aggressive mate. The males might be prone to erroneous attacks because of the high density of sparrows and the thickness of the ground cover."
I guess I'll stick with N. I don't think they give a fake departure call to fool other birds, but how would I know?
Wow, this is GREAT information. I've been trying to track Swamp Sparrows for weeks to observe any nesting behavior, but have so far been unsuccessful. I know that I've been very close to nests, but saw nothing, while the males continued their melodic songs nearby. That said, having essentially given up, I tried one more time a week ago, and voila, saw a recently fledged young Swamp Sparrow in a known nest site. Whew!