It is the only Old World warbler to have colonised Hawaiʻi, and there is no fossil evidence that the species ever had a distribution beyond these two islands.

Millerbirds form long-term pair bonds and defend territories over a number of years.

Breeding occurs variably from January to September depending on food availability.

A small warbler that occurs only on Nihoa and Laysan in the Northwestern Hawaiian Islands.

It does not occur on the main Hawaiian Islands and prefers dense, low vegetation.