A widely distributed breeder east of the Rocky Mountains, the Orchard Oriole shows a distinct preference for riparian zones, floodplains, marshes and the shorelines of large rivers and lakes. In addition, it often nests in shade trees and open shrublands, seeming to thrive in habitats with low-density human intrusion, such as farms and parklands. This species eats mostly arthropods, gleaned from foliage, but its diet also includes small, ripe fruit, as well as nectar in southern and overwintering localities. Only loosely territorial, it is often described as a “semicolonial” species in areas of prime habitat, but it is relatively solitary in marginal habitats. Areas of dense nesting often have multiple nests per tree that are easily seen during leafless seasons.
This Nearctic-Neotropical migrant leaves its wintering grounds in March and April, sometimes returning south as early as mid-July. It nests late and raises only a single brood, to accommodate this migratory schedule. The short time spent on the breeding grounds has made it difficult to define breeding-locality specimens in museum collections, because of the great number of transient birds on spring and autumn migration. Many presumed breeding specimens may actually be passage birds.
The Orchard Oriole is the smallest oriole in North America. Adult males (after-second-year) have distinctive black and chestnut plumage, while yearling males (hatch-year and second-year) are yellow-greenish with a black bib. Females of any age, and recent fledglings of both sexes, are similar to hatch-year and second-year males but lack the black bib. This sexual dichromatism allows easy separation of two age classes of males from females during all seasons. Such ease of identification of age and sex makes this an excellent species for modeling demographic dynamics.
Studies of the breeding biology of Orchard Orioles include Clawson 1980 (western Nebraska), Dennis 1948b (Mississippi Delta), Enstrom 1992a and Enstrom 1993 (Illinois), Sealy 1980a (Manitoba), Stevenson 1979 (Florida), and Thomas 1946b (Arkansas). The adaptive significance of delayed plumage maturation in the species has been studied by Enstrom 1992a and Enstrom 1993 , Rowher 1978 , and Rowher et al. 1980 . Spring and summer food habits and diet have been studied by Scharf and Kren 1997 , and Wunderle 1980 . Enstrom 1992b and Morton 1979a have studied feeding and dominance hierarchies of wintering Orchard Orioles in their neotropical habitats.