The American blackbirds are common and conspicuous birds over much of North and South America, and their habit of forming large flocks outside the breeding season attracts the attention of even casual observers of birds. The family also includes cowbirds, most of which are brood parasites, laying their eggs in the nests of other species. Sometimes very closely related blackbirds have strikingly different social systems. The majority of species are tropical. There are centers of species richness in southern Mexico (24 species) and Colombia (27 species).
Both regions with diverse habitats. Many species are found in temperate areas with an abundance of marshes. Such as northern Argentina and adjacent Uruguay (19 species), and the Midwest of the USA (10 species). Blackbirds breed in all habitat types but especially in open environments such as grasslands, savannas, and marshes. Forest species favor edges and disturbed sites rather than mature forests, but a few tropical species breed in primary forests. Blackbirds are generalized foragers, eating a wide variety of invertebrates and plant materials. Many species are insectivorous during the breed-in season but seed-eaters during the remainder of the year.
Blackbirds are medium-sized birds, rang-in in size from that of a large sparrow (females of some orioles and tropical marsh-nesting species) to that of a crow (tropical oropendolas). Bills and eyes are brightly colored in many species but legs are dull. Inman tropical species, males and females are alike in plumage: striking sexual differences are found among high latitude migratory species and among species in which males hold harems (polygynous) atoll latitudes. Females of all species achieve adult plumage within one year, but males of many polygynous species retain subadult plumage until they are two years old.
Juvenile plumages are always female-like and sub-adult plumages of males are intermediate between those of females and adult males. During the non-breeding season, blackbird flocks may be extremely large. Social groups during the breeding season are much smaller, but the Tricolored blackbird of California breeds in dense colonies that may contain over I00, WO birds. Territorial blackbirds breed in all habitat types occupied by the family. Colonial breeders are found principally among marsh-nesters. Species that breed in isolated trees in open savannas where nesting sites are limited but feeding areas are widely dispersed, and among the partly fruit-eating tropical oropendolas and caciques that form conspicuous colonies insolated trees in forest clearings.
Among the blackbirds can be found most of the social systems of the avian world. Most species breed as mated pairs on large territories, but there are also colonial species, species with highly clumped territories, and monogamous, polygynous, and promiscuous species. A few species live in year-round flocks and up to eight individuals may attend a single nest. Among the brood-parasitic cowbirds are two species (Brown-headed cowbird of North America and Shiny cowbird of South America) that lay their eggs in the nests of hundreds of other species of birds.
Two others (Red-eyed cowbird and Giant cowbird) parasitize primarily other members of the blackbird family, especially orioles, caciques, and oropendolas. The Bay-winged cowbird takes over active or inactive nests of other species of birds but incubates its eggs and feeds its nestlings. Finally, the Screaming cowbird is known to parasitize but a single host, the Bay-winged cowbird.
Adaptations of cowbirds for brood parasitism include short incubation times, the habit of throwing out an egg of the host for every egg they deposit, and a tendency towards mimicry of the eggs and nestlings of their hosts. Regardless of the form of breeding social organization, male and female blackbirds assume very different roles. Nests are built exclusively by females in every species that has been studied except two. Incubation of eggs by males has never been reported, even among species with identical males and females. Males do not bring food to their incubating spouses but many stand guard near the nest.
Males of most monogamous species feed nestlings and fledglings, but males of only about one-third of the polygamous species do so. Polygynous males feed preferentially at the nest containing the oldest nestlings. Among monogamous species, males and females have about the same number of vocalizations, but males of poly-genius species utter a greater variety of sounds than females. Among West Indian species, several of them are restricted to single islands where their populations were never very large.
Those considered threatened (but not yet listed by the ICBP) include the St. Lucia oriole, the Martinique oriole, the Montserrat oriole, and the Jamaican blackbird. In South America the tropical, the chop. And the Shiny cowbird is often kept as caged bird because of its beautiful songs.