Unicolored Blackbird

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Quick Facts

  • Scientific Name: Agelasticus cyanopus
  • Spanish Name: Chango unicolor
  • Family: Icteridae
  • Known Nicknames: Unicolored Marshbird
  • Average Length: 18–22 cm / 7.1–8.7 in
  • Average Weight: 32–45 g / 1.1–1.6 oz
  • Wingspan: 25–30 cm / 9.8–11.8 in
  • Key Feature: Entirely glossy blue-black plumage in males; females are streaked brown
  • Primary Diet: Omnivore (insects, seeds, aquatic invertebrates)
  • Range: South America: from eastern Bolivia and southern Brazil to central Argentina and Uruguay
  • Habitat: Freshwater marshes, reedbeds, and wet grasslands
  • Social Structure: Gregarious; often in small flocks during non-breeding season
  • Nesting/Breeding: Deep cup nest woven among reeds or cattails over water
  • Conservation Status: Least Concern (LC)
  • Population Trend: Stable

The Unicolored Blackbird (*Agelasticus cyanopus*) is a striking South American songbird that defies its common name with a subtle yet dazzling secret. Males are entirely glossy black, appearing monochromatic at first glance, but in direct sunlight, their plumage reveals a brilliant, iridescent blue sheen, particularly on the wings and back—a feature hinted at by its scientific species name, *cyanopus* (meaning "blue-footed"). This bird is further distinguished by its pale, almost whitish-yellow eyes and a relatively slender, pointed bill, which it uses to expertly probe marsh vegetation for insects and seeds. A special feature of this species is its unusual social structure; it is one of the few blackbird species that breeds cooperatively, with multiple adults—often including offspring from previous broods—helping to defend the nest and feed the young in dense, freshwater marshes.

Fun Facts

Despite its name, the male Unicolored Blackbird isn't truly unicolored; in bright sunlight, its plumage flashes an iridescent, metallic blue or purple sheen, a trick of light that makes it look like a living jewel. This species has a bizarre breeding strategy: it is a brood parasite, meaning the female lays her eggs in the nests of other birds, particularly the Brown-and-yellow Marshbird, tricking them into raising her chicks. To pull this off, the female Unicolored Blackbird can lay an egg in under ten seconds, often while the host is away, and her chick hatches with a sharp egg-tooth to kill or outcompete the host’s own young.

Habitats & Distribution

The Unicolored Blackbird inhabits freshwater marshes, reed beds, and the edges of lakes and lagoons, often in areas with dense emergent vegetation like cattails and bulrushes. It is also found in flooded grasslands and wet savannas, preferring shallow water and muddy substrates for foraging. Its geographical range spans central and southern South America, occurring from southeastern Brazil and Paraguay through Uruguay and into central and eastern Argentina, with isolated populations in Bolivia. The distribution is largely continuous across the Pampas and Pantanal regions, extending south to northern Patagonia.

Behaviours & Reproduction

The Unicolored Blackbird is typically polygynous, with dominant males defending small territories that encompass the nesting sites of several females. Males perform conspicuous flight displays and sing from prominent perches to attract mates and deter rivals, while females alone build the deep, woven cup nest among cattails or reeds. Social structure is loosely colonial, with multiple nests in close proximity, but males provide no parental care. A unique reproductive strategy involves the female laying clutches of 2–4 eggs, which she incubates for about 13 days, and the young fledge after 11–14 days. Brood parasitism by the Shiny Cowbird is common, and females may abandon parasitized nests or attempt to eject the foreign egg.

Diet

The Unicolored Blackbird is primarily insectivorous, foraging for a wide variety of arthropods including beetles, grasshoppers, caterpillars, and spiders, which it typically gleans from vegetation or captures on the ground in marshes and wet grasslands. An interesting aspect of its diet is that it is also a facultative granivore, readily consuming grass seeds and small grains, especially during the non-breeding season when insect prey becomes less abundant. Furthermore, this species has been observed occasionally taking small aquatic invertebrates like snails and dragonfly larvae from shallow water, and it may opportunistically feed on fallen fruits from riparian trees, demonstrating a flexible foraging strategy that allows it to thrive in seasonally variable wetland habitats.

Colors

The male Unicolored Blackbird is entirely glossy black with a striking pale blue eye, while the female is a streaky brown with a pale supercilium, lacking the male's iridescence. This species has no unique camouflage adaptations beyond the female's cryptic brown streaking that blends with marsh vegetation.

Distribution Map

Places linked to this species. A sighting is never guaranteed.