Their throat and underparts are pale yellowish with a brownish to olive wash on the breast.

[5][6] The Cocos tyrannulet feeds mostly on arthropods but also occasionally includes fruit in its diet.

Both sexes construct the nest, an open cup of plant fibers and fungal rhizomorphs lined with feathers.

It is typically firmly attached to twigs and vines about 10 to 25 m (35 to 80 ft) above the ground in a tree top.

[5] As of late 2024 xeno-canto had no recordings of Cocos tyrannulet vocalizations and the Cornell Lab of Ornithology's Macaulay Library had eight.

It has a very small range; its estimated population of between 6000 and 15,000 mature individuals is believed to be stable.

"Rats and cats are potential predators, and feral deer, pigs and goats graze suitable habitat.

[1] The island is a national park with no permanent settlements and its forest cover remains mostly intact.