The omnivorous diet of the chalk-browed mockingbird consists of various insects, spiders, worms, fruit, seeds and berries, and occasionally the eggs and nestlings of other birds (2). Although this mockingbird forages mainly on the ground, it will regularly perch in low bushes or trees, especially during the breeding season when the males spend long periods singing (4). Breeding occurs from September to January with each monogamous pair often accompanied by several helpers that assist with territorial defence, nest-guarding and feeding of young. A clutch of three to four eggs is laid in a small, loosely constructed nest and incubated for 12 to 15 days. After hatching the young are confined to the nest for another 12 to 15 days, and are fed by the parents for around a week after fledging. The juveniles then remain in their parental territory through the non-breeding season and potentially as helpers over the subsequent breeding season (2). The nests of the chalk-browed mockingbird are commonly parasitized by shiny cowbirds which lay their own eggs in the nests of the mockingbirds (2) (5). This typically results in the mockingbird unwittingly incubating the eggs of the shiny cowbird and rearing its young, often to the detriment of its own (5) (6).
During Charles Darwin's voyage aboard the Beagle, the eminent naturalist was clearly taken by a bird he described in Uruguay as “possessing a song far superior to that of any other bird in the country” (3). This bird was the chalk-browed mockingbird, a small, ubiquitous passerine, with a conspicuous white supercillium, or 'eyebrow', sandwiched between a blackish eye stripe and a dark grey crown (2) (4). The upperparts of its body are predominately greyish brown, while the flight feathers and the long semi-erect tail are blackish, with white tips. Below, the throat and belly are greyish white, but the flanks are often streaked with darker markings. Across the chalk-browed mockingbird's broad range, four subspecies, which exhibit minor differences in body size, bill length and plumage, are currently recognised: Mimus saturninus saturninus, M. s. frater, M. s. arenaceus, and M. s. modulator (2).
The chalk-browed mockingbird occurs in southern Suriname and in large parts of central, southern and northeastern Brazil, west into Bolivia and Paraguay, and south into Uruguay and northern Argentina (2) (4).