Perhaps the least known of the orioles that regularly breed in North America, this species has a well-deserved reputation for being secretive. Although often drawn to edges, like other orioles, it more commonly forages and breeds in dense vegetation, making it difficult to locate and study. Its well-concealed nest is cup-shaped, resembling those of Orchard (I. spurius) or Scott's (I. parisorum) orioles more than the familiar—and much easier to find—pendant nests of the Baltimore (I. galbula), Bullock's (I. bullockii), and Altamira (I. gularis) orioles.
Partly as a result of these features, few data have been collected on the species, and its precise distribution and abundance remain poorly known. It is found in diverse forested and brushland habitats over a wide elevational range, showing perhaps broader habitat preferences than most of its congeners. Many of these habitats, however, have been cleared for agriculture, forestry, or urban development. This, in addition to heavy parasitism by the Bronzed Cowbird (Molothrus aeneus), has caused the species' numbers to decline in recent decades, at least in Texas, the only U.S. state in which it is resident. It is currently on the National Audubon Society's WatchList, partly because of our lack of knowledge about its population status.
In keeping with its apparent penchant for secrecy, Audubon's Orioles commonly sing from inconspicuous perches. Described as a slow, mellow, humanlike whistle, the song consists of a series of almost pure tones; compared to the familiar song of the Baltimore Oriole, typical Audubon's songs are longer, but less complex (in terms of number and arrangement of specific notes). They also show less frequency modulation and cover a narrower range of frequencies. Both males and females sing, and their songs appear to be equal in frequency and complexity; members of a pair sing or call regularly to each other as they forage or come and go from the nest.
Monogamous, and resident throughout its range, Audubon's Oriole typically begins breeding with the onset of the rainy season in April. Anecdotal evidence suggests that some pairs may attempt a second brood, as breeding continues through July and even into September, at least in some localities. Many authors have commented on the fact that pairs are seen together more constantly than is typical for many avian species and that these pairs remain in seemingly constant vocal contact with each other. This is particularly true in the breeding season; although pairs are often seen together during the winter, they also forage in mixed-species flocks at this time.
Male and female Audubon's Orioles also share other similarities. Although females can be somewhat duller than males, the species is essentially monochromatic, and the sexes cannot reliably be separated on the basis of plumage color. This species is the only black-hooded oriole (entirely black head and breast but not back). Indeed, it was formerly known as the Black-headed Oriole, but this name was changed in 1983 to Audubon's Oriole, a name that had some historical precedent, to avoid confusion with an Old World group of species in the genus Oriolus ( American Ornithologists' Union 1998a ). Their heads are thus much more distinctive than the tails that are recognized in their scientific name: the specific epithet, graduacauda, literally means “graduated tail” (from the Latin gradus, “a step,” and cauda, “tail”). The significance of this is somewhat unclear ( Choate 1985 ), although the olive tipping on the tail feathers of females might appear graduated on some individuals (see Jaramillo and Burke 1999 : Fig. 49.1).
Although general descriptions of, and notes on, the species are common (e.g., in books or papers on the avifauna of specific regions or localities), there has been little systematic study of this oriole. Since Bent's ( Bent 1958 ) review of the species, the main published sources of information have been Oberholser's ( Oberholser 1974c ) description of the species in Texas and Flood's ( Flood 1990 ) observations of the behavior of several breeding pairs in Mexico. Although brief and somewhat limited, this latter study revealed several differences between what had been understood of the species in Texas and what seemed to be true for Mexican individuals. In 1999, Jaramillo and Burke included a summary of this species in their book on New World blackbirds. As this book showed, more study of Audubon's Oriole is clearly needed, particularly in light of its apparent decrease in abundance in parts of its range.