Features separating this species from Great Reed Warbler include:-
The supercilium extends well behind the eye.
Longer bill
Thinner bill compared to the Thrush like bill of Great Reed Warbler
Grey rather than brownish leg colour.
Short wing length - primary extension past tertials about 2/3rds the length of tertials (equal to tertial length in Great Reed Warbler).
Primary tips showing are normally 6-7 in brunnescens Clamorous Reed Warbler although can be as little as 4-5, and 7-8 for Great Reed Warbler.
Less warm toned upperparts than nominate Great Reed Warbler.
