This is what BWP has to say about Marsh Warbler. (bits that don't fit are highlighted in italics.) Vol Vl p172-173
By summer, wear removes pale bright tinge above and below so that whole bird becomes duller and greyer, though often still more olivaceous and much more earth-brown on rump than western A. scipaceus scirpaceus. Following autumn moult plumage again becomes washed with green and olive but less vividly so than in spring.
Within unstreaked Acrocephalus, A. palustris is pumpest, heaviest and longest-winged, with (1) typically rather short, wide-based bill...(2) rather domed crown, typically lacking as low a forehead as A. scirpaceus and A. dumetorum and thus tending to enhance short-billed appearance, (3) seemingly fuller chin and throat enhancing round-headed appearance, (4) rather long wings with folded wing point at least three quarter length of exposed tertials, displaying fully (due to bright feather fringes) 8-9 primary tips (with noticably even spacing indicative of least round shape within unstreaked Acrocephalus) and extending to end of, or even beyond, upper tail-coverts, (5) pear-shaped or pot bellied body which gives impression of bird carrying most weight lower than other Acrocephalus and (6) long full under tail-coverts, cloaking three quarters of tail more obviously than A. dumetorum. In addition, rather less agile or energetic than A. scirpaceus with rather upright carriage both at rest and on the move, with tail held down (and rarely above the wing points),
Darrell