Adult birds are 17 to 20 cm long, somewhat smaller than a robin. There is sexual dimorphism in plumage and in size; males are 1-5% larger than females in a variety of measurements. Adult males have a black head, bill, and back, and a bright orange breast, rump, and underparts. Their wings are black with orange and white wing bars, and the tail is orange with black streaks. Adult females are paler than males, olive-brown to orange. Their wings are brown with white wing bars, and the bill is gray. She may have traces of black on her head. Immature animals are variable, but typically resemble the female. Males take over a year to reach adult plumage. (National Geographic Society, 1999; Peterson, 1980; Tekiela, 1999)
Baltimore orioles are neotropical migrants. They spend summers in the Nearctic, primarily the eastern United States. They breed from Wisconsin to Maine and south to central Mississippi and Alabama, northern Georgia, and western South Carolina and North Carolina. They winter in the neotropics as far north as Mexico and sometimes the southern coast of the United States. (Harrison, 1975; National Geographic Society, 1999; Peterson, 1980; Peterson, 1990)
Baltimore orioles prefer open woods, with a strong preference for deciduous over coniferous trees. They are very adaptable, however, and can be found breeding in a variety of habitats. They are rare on farmlands but have adapted well to urban parks and suburban landscapes. In Mexico, they winter in flowering canopy trees over shade coffee plantations. (Greenberg, et al., 1997; Jobin, et al., 1998; Peterson, 1980; Rising and Flood, 1998)
Baltimore orioles usually find one mate for a breeding season, but may mate with more than one other bird as well.
In the spring, males try to attract mates to their territory by singing or chattering while hopping from perch to perch in front of her. Males give a bow display, bowing with wings lowered and tail fanned. Interested females sing and give calls or a wing-quiver display in response. The wing-quiver display involves leaning forward, often with tail partly fanned, and fluttering or quivering slightly lowered wings.
Males arrive on breeding grounds in the spring a few days before females. Courtship displays by the male consist of bowing, to show off the bright orange front and black back, and singing. The female builds a woven pouch nest hanging from the end branches of trees, well concealed by leaves. She builds a new nest each year with little or no help from the male. Baltimore orioles prefer to build nests in elms, maples, willow, or apples, twenty-five to thirty feet above the ground. Any available plant and animal fiber may be used in nest-building.
The female lays four to six eggs, typically four. The eggs are pale grayish or bluish white, irregularly blotched and streaked with browns and black. The female incubates them for twelve to fourteen days. Both parents feed the nestlings. Fledglings will stay with their parents for two weeks, and are fed by both parents during that period. Baltimore orioles lay only one brood per season.
The female alone broods nestlings; the male occasionally feeds the brooding female, but she usually forages for herself. Parents feed nestlings by bringing up already-eaten food from their crops during the first few days of the nesting period.
The oldest recorded Baltimore oriole in the wild lived to 11 years and 7 months old. They have been recorded living 14 years in captivity. (Rising and Flood, 1998)
Baltimore orioles are not gregarious. Like most blackbirds, their flight is strong and direct. These birds are active during the day and migrate between summer and winter ranges.
Territory size varies with habitat quality, food availability, population density, and time of breeding season. It is largest when the male is attempting to attract a female and smallest after eggs are laid. (Rising and Flood, 1998)
The male sings all summer. His song is rich and flute-like, with each individual having a distinct song. The female song is generally shorter and simpler. The call, from both sexes, is a whistled "hew-li." Nestlings beg loudly.
Baltimore orioles also use postures and movements to communicate, such as male courtship displays, female wing-flutter displays, and nestling wing-flutters when begging for food.
Baltimore orioles eat primarily caterpillars, including many pest species. They also eat other insects, some small fruits, and nectar. They are an important predator of the nuisance forest tent caterpillar, which it eats in both its larval and pupal forms. Large larvae are seized and smashed against a twig to break them open and avoid the setae (stiff, hair-like structures). Pupae are pulled out of their cocoon.
In suburban and rural areas, Baltimore orioles can be attracted to feeders by providing orange halves, grape jelly, or artificial nectar. Adults who come to feeders will take their young to the feeder once they are fledged.
Baltimore orioles have many different kinds of predators, including larger birds and mammals. Most predators take eggs, nestlings, or fledglings. In western Massachusetts, avain predators caused 16% of egg losses and 9% of nestling and fledgling losses. In response to predators, both males and females give alarm calls, and chase and mob (harass) predators. (Rising and Flood, 1998)
Baltimore orioles are important predators on insects in the communities in which they live. Because they live in forested areas and prey on caterpillars, the lifestage at which many insects do most damage to plants, they are especially important in protecting forest trees from damage.
Baltimore orioles may occasionally damage crops of peas or small fruits. (Bent, 1965)
Baltimore orioles are attractive songbirds that will come to feeders. They are generally liked by both serious birdwatchers and casual backyard enthusiasts for both their appearance and song. They are also important predators on some insect pests such as forest tent caterpillars. (Bent, 1965; Parry, et al., 197)
Baltimore orioles are protected by the Migratory Bird Treaty Act. They may be at risk due to habitat loss, as they prefer wooded areas, but this is not well documented. Although direct human impacts on oriole populations are unknown, the increase in number of orioles wintering in temperate North America may be due to an increase in bird feeders in backyards and elsewhere. (Rising and Flood, 1998)
Baltimore orioles were previously considered a supspecies of northern orioles (Icterus galbula galbula) along with Bullock's orioles (Icterus glabula bullockii), a western North American oriole. They are currently considered separate species: Baltimore orioles, Icterus galbula, and Bullock's orioles, Icterus bullockii. Some hybridization occurs in the Great Plains where these species overlap. (Gill, 1995; Peterson, 1990)
Kathleen Bachynski (author, editor), University of Michigan-Ann Arbor, Tanya Dewey (editor), Animal Diversity Web.
Sara Kennedy (author), University of Michigan-Ann Arbor, Terry Root (editor), University of Michigan-Ann Arbor.
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Gill, F. 1995. Ornithology. New York: W.H. Freemand and Company.
Greenberg, R., P. Bichier, J. Sterling. 1997. Bird populations in rustic and planted shade coffee plantations of Eastern Chiapas, Mexico. Biotropica, 29 (4): 501-514.
Harrison, H. 1975. A field guide to birds' nests. Boston: Houghton Mifflin Company.
Jobin, B., J. Des Granges, C. Boutin. 1998. Farmland habitat use by breeding birds in southern Quebec. Canadian Field-Naturalist, : 611-618.
National Geographic Society, 1999. Field guide to the birds of North America. Washington, D.C.: National Geographic Society.
Parry, D., J. Spence, W. Volney. 197. Responses of natural enemies to experimentally increased populations of the forest tent caterpillar *Malacosoma disstria*. Ecological Entomology, : 97-108.
Peterson, R. 1980. A field guide to the birds. Boston: Houghton Mifflin Company.
Peterson, R. 1990. A field guide to western birds. Boston: Houghton Mifflin Company.
Rising, J., N. Flood. 1998. Baltimore Oriole. The Birds of North America, No. 384: 1-32.
Tekiela, S. 1999. Birds of Michigan field guide. Cambridge, Minnesota: Adventure Publications, Inc..