A typical reed finch, the chestnut-breasted mannikin flocks in groups of up to several hundred birds. They bunch into tight-flying packs, calling and executing all manoeuvres in rapid unison. They also feed, drink, rest, sleep and sing together; fighting is rare but ritualized. They do not preen one another and never build roost nests, clustering instead to sleep in thick grass areas or reeds. Bonds and coherence in flocks are kept by ‘jingling’ and peering.
Chestnut-breasted mannikins inhabit banks of rank grasses and reeds on the margins of swamps, mangroves and rivers around northern and eastern coastal Australia. There they are locally nomadic, more so in the northwest than the east.
They mostly eat half-ripe grass seeds, gathering while clambering nimbly among seeding stems, one foot after the other. They land among the upright stems, reach out to a laden spike, pull it down and clamp it with one foot while they pick out the seeds. At the start of breeding season they also catch flying termites on the wing. Drinking is by scooping, then the birds tilt their heads back to swallow.
The chestnut-breasted mannikin may breed through all months, but have a preference for summer and autumn. They build a spherical nest with dry blades of grass. The nest is compressed at the side, somewhat resembling a bottle lying on its side. The birds line the nest with fine grass stems. Nests are built within 2m of the ground, usually in dense clumps of grass and reeds or bushes. 4-6 eggs are laid and incubated by both sexes for 12-13 days.