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As a result of frequent hybridization between the Bullock's (I. bullockii) and Baltimore Orioles where their ranges meet in the Great Plains, these two orioles were at one time treated as conspecific (i.e., members of the same species), representing two forms of a species that was known as the Northern Oriole. However, genetic studies have indicated that these two species are not even each other’s closest relatives (for a full discussion of this issue, see Jacobsen and Omland 2011).
Baltimore Orioles breed in deciduous and mixed woodlands, usually in open woods or along edges rather than in the interior of dense forests. They may be common in towns and other relatively developed areas with appropriate trees (especially elms). The familiar nest of the Baltimore Oriole is a hanging pouch woven of plant fibers and may be seen in shade trees in towns and suburbs. The nest is typically attached firmly by its rim near the end of a slender drooping branch 6 to 9 (sometimes 2 to 18 or more) meters above the ground. There are 4 to 5 eggs (range 3 to 6) eggs. Eggs are incubated by the female for around 12 to 14 days. Nestlings are fed by both parents and leave the nest around 12 to 14 days after hatching.The diet include mainly insects in summer, especially caterpillars, including hairy types avoided by many other birds. They also consume many berries and sometimes cultivated fruit. They may feed on nectar from some flowers and will take sugar water at feeders. The liquid, musical tones of the Baltimore Oriole's song floating down from the treetops are a familiar harbinger of spring in the eastern United States.
(Kaufman 1996; AOU 1998)