Although highly social throughout the year, the tricoloured blackbird’s gregarious behaviour becomes most apparent during the breeding season (April to July), when huge colonies may form, consisting of tens of thousands of birds (2) (3). Within the vast colony, many activities are remarkably synchronous, such as nesting, foraging and the males’ singing (5). Breeding pairs, which only stay together for a single nesting effort, maintain a small territory of a few square metres around their nest (4). The female tricoloured blackbird builds the nest alone, collecting dry leaves which are dipped in water and woven around strong, upright plant stems, usually around a metre above the ground. A layer of mud and softer materials is then added to help cushion the clutch of three to five eggs, which are incubated by the female for around 12 days. After hatching, the chicks are fed by both parent birds for 10 to 14 days before fledging. Interestingly, adults encourage the fledglings to disperse from the colony by tempting them with food, and then fly away from the colony with the young bird in pursuit (4).
Despite feeding mainly upon grains, the tricoloured blackbird is opportunistic and will take a variety of other foods when available, such as insects (particularly grasshoppers) and snails (2) (4). This species usually only forages within five to six kilometres from the colony, hence the proximity of good foraging sites is one of the requirements for colony formation (6). During the winter, the tricoloured blackbird roosts and forages communally, and many colonies withdraw from their breeding grounds and concentrate around the central coast of California and the Sacramento-San Joaquin Delta (4).