The Cook Reed Warbler is an island reed warbler of the Cook Islands. Bell’s comparative study covers populations on Mangaia and Mitiaro and records the species beside lakes, ponds, and rivers.
Bell’s comparative study lists the Cook Reed Warbler among island reed warblers with yellow underparts. It also describes its pale feather edging as less marked than in other East Polynesian species.
Bell’s paper explicitly names the Cook Reed Warbler as one of the species that occurs beside lakes, ponds, or rivers.
An extinction-risk assessment has not been added yet.