When I found this bird on Saturday at 6pm, my initial interest was drawn to the constant mimicry from a bird low down in the willow tree (with the horizontal bough). By the time it got on to the 5th consecutive species, I was locked on to get a view of what hopefully was a Marsh Warbler. It didn't stop singing for the first 45 minutes, and went through at least 11 species in that time (Common Blackbird, Blue Tit (one of its favourites), House Sparrow, Common Whitethroat, Goldfinch, Robin, Linnet, Barn Swallow, Wren, Great Tit). Each was a perfect rendition without hiccup and jumped from one to the next without a break. Not once did it stop to give any acro style notes or churrrs, nor were there any repeated phrases. Many of the views I got were down its gape! - as Jan Hein described well, but it was keeping low and active enough for me not to get any decent views of the primary pattern.
What a contrast to Monday night then, when in an hour it uttered one phrase which was more akin to the descriptions on this site, a bit quiet and an indistinct attempt at mimicking a Blue Tit with many churrrs, and almost an element of phraseology.
I'm left wondering if the bird had been in for a few days before Saturday,and having got no responses from any other Marsh Warblers, is reducing his repertoire. I was involved with wardening the Marsh Warblers in Kent in the late 90s and they usually sang continously on arrival and then over time went quiet, to the extent that some thought they had left the site(unfortunately that didn't apply to the abuse from the eggers).
Fortunately, this bird seems very site-loyal to that part of the field, which contains some of its favourite food plants (with thanks to Phil for that snippet!), and it'll be interesting to see how it continues to behave during its stay (Jonathan Braggs)