Here you have a single Marsh Wren singing on territory in a salt marsh at the edge of Rockville, S.C. Migratory Marsh Wrens nest and sing in the freshwater marshes of eastern North America, but only down to 40 degrees N. latitude. On the other hand, the narrow strip of coastal salt marshes is home to resident populations, all the way to Texas. The ones in South Carolina are subspecies griseus, aka Worthington's Marsh Wren. They are grayer than the more richly-colored birds of northern and western locales. We have talked here about song-type variety, how in some species a bird has only one song-type, in others he repeats one many times before switching, and in others he switches with each song. That is called "immediate variety." This bird is clearly switching with every song, and in a cursory check it seems to me that every song in the cut is unique. That would be 42 song-types. Feel free to check my work on this. Let's now see how these observations fit into the larger Marsh Wren picture.
Actually, the Marsh Wren is a coast-to-coast species. In The Singing Life of Birds, Don Kroodsma tells the engaging story of how he and Jerry Verner went out, in Illinois, to replicate Verner's finding, in Washington State, that neighboring Marsh Wrens have repertoires of over 100 song-types, that they never repeat, and that they match each other's song-types with every song. They found none of that. [Note to budding ornithologists: replicate your results in another area before generalizing.] Eventually Kroodsma quantified a typical eastern Marsh Wren repertoire size as in the 30s, and found subtle but reliable differences in the introductory notes and the acoustics of the trills between east and west. Then he found the contact zone in Nebraska, and learned there, and in Saskatchewan, that eastern and western singers coexisted without intermediacy. Then he found DNA differences between the two populations. It seemed that an east-west split of the species was eminent. But that has not happened. Perhaps because, according to Pieplow (2017), some interbreding has now been documented, and some birds do sing intermediate songs. In my opinion, a small amount of interbreeding at range interfaces does not invalidate separate species status for the two populations.
And now, back to this one Worthington's Marsh Wren, singing with immediate variety and a repertoire that seems considerably above 40 (because every song was unique, and hence the limit has not been approached). This form, this one subspecies, is treated separately in eBird, even from the form that is resident in North Carolina, so its distinctiveness is appreciated. Its range extends roughly from Charleston to Jacksonville. It was not included in Kroodsma's studies, and Pieplow's characterization of the eastern Marsh Wren does not fit well with what we see and hear in this cut, which is in my experience completely typical of the Marsh Wrens I encounter frequently in coastal South Carolina. So, it seems that I have a new question to ask as I range up the coast: How many song types does a typical Worthington's Marsh Wren have? But, don't think I am suggesting this bird is like the westerns. His song is eastern. Also, knowing of Verner's findings, I recorded this bird and two others on June 28, 1992 (see below). It's not a very good tape, but there was no evidence of song-matching, and this bird once sang the same song four times in a row. So, maybe Worthington's is just a law unto itself. Geographic variation, it's wonderful. It's the grist of evolution, and a pitfall for over-generalization.