The Chestnut Mannikin is also known as the Black-headed Munia. The ones found in Hawaii escaped from cages and are fairly widespread in Kauai. I wasn't very lucky on seeing them and only saw these ones briefly before they took off. They were fairly fair away and the photos are marginal.
Chestnut Mannikins are seed eaters. They will land on the tall seed-bearing stalks of a weed and walk out to the end to eat the seeds.