Icterus graduacauda

Geographic Range

Audubon’s orioles ( Icterus graduacauda ) are year-round residents of southern Texas, eastern and southern Mexico, and the northern tip of Central America. In Mexico, the range splits into two allopatric regions: the eastern range extends from northeastern Mexico (Tamaulipas and Nuevo Leon) south to central Veracruz; the western range extends from central Nayarit south to Oaxaca. There are also a few populations in Belize.

Audubon’s orioles are further divided into four subspecies: I. g. graduacauda , I. g. audubonii , I. g. nayaritensis and I. g. dickeyae . The Atlantic range is occupied by overlapping populations of I. g. graduacauda and I. g. audubonii , while I. g. nayaritensis and I. g. dickeyae inhabit the western range. Range overlap is not observed between I. g. nayaritensis and I. g. dickeyae . The range of I. g. audubonii is confined to southern Texas, Tamaulipas, and Nuevo Leon, while I. g. graduacauda is found in the Mexican states of Veracruz and San Luis Potosí. Sightings of I. g. dickeyae have been documented in southern Mexico from Guerrero east to Oaxaca. The subspecies I. g. nayaritensis inhabits the western portions of Nayarit and northwestern Jacisco.

• Biogeographic Regions
• nearctic nearctic
  • native native
• neotropical neotropical
  • native native

Habitat

Audubon’s orioles are a tropical species that prefers the edges of dense, established woodlands and riparian areas (Flood et al., 1990). They are found in a wide range of woodland habitats including scrubland, cloud forest, and both deciduous and evergreen forest (Flood et al., 1990; American Ornithologist’s Union, 1998). They are rarely found in open fields or in urban areas, though they may visit suburban feeding stations even in winter (Flood et al., 2002). Audubon’s orioles may be found at elevations of up to 2500 m, but avoid areas below 500 m in elevation (Flood et al., 1990).

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest
• scrub forest scrub forest

• Other Habitat Features
• suburban suburban
• riparian riparian

Physical Description

Audubon’s orioles are medium sized orioles and are nearly monomorphic, though males are slightly larger and brighter than females (Flood et al., 2002). Measurements range from 190 to 240 mm in length and average 42 grams in mass (Sibley, 2000), ranging in mass from 31 to 45 grams (Flood et al., 1990). They are the only yellow-bodied oriole species with a fully black hood in the New World (Jaramillo and Burke, 1999). In most cases the edges of the bib are poorly defined. The culmen is stout, black, and straight. The tail is black and is “long, narrow and obviously graduated” (Jaramillo and Burke, 1999).

In adult males, the wings and tail are black; wings have white-fringed greater coverts that form a wing bar (Jaramillo and Burke, 1999). The tail feathers are olive tipped, lending the tail a “graduated”, or stepped, appearance. The remiges and retrices (flight feathers) are white fringed. The body is generally yellow and the underparts are brighter than the back and nape, which are greenish-yellow (Flood et al., 2002). The median and lesser coverts form yellow epaulets. While it is difficult to determine the sex of an individual at a distance, adult females of this species are duller than males, with an olive green wash on the back and wings (Flood et al., 1990). Juvenile Audubon’s orioles have a pink lower mandible, a “greenish-gray” face, and yellow-tipped gray wings. Immature individuals resemble females, but have duller wings and greener upperparts (Jaramillo and Burke, 1999). Species bearing similar plumage include Scott's orioles ( Icterus parisorum ), Greater Antillean orioles ( Icterus dominicensis ), and black-cowled orioles ( Icterus prosthemelas ). Scott's orioles are distinguished from I. graduacauda by their more-prominent wing bars and yellow-fringed retrices; black-cowled orioles and Greater Antillean orioles are distinguished by the absence of wing bars (Flood et al., 2002).

Icterus graduacauda plumage varies geographically as described below:

Icterus g. audubonii generally fits the description given above.

Icterus g. graduacauda is smaller and lacks white fringes on the wings and tail.

Icterus g. nayaritensis exhibits a greater degree of sexual dimorphism, males being markedly brighter than females.

Icterus g. dickeyae is larger and brighter than the other three subspecies, with a cleaner border to the hood. The epaulets are more restricted, being orange-yellow only on the lesser coverts. The edges of the wings are yellow and the wing bars are absent. Additionally there may some black tipping or spotting on the back. Because of the dramatic differences in plumage, behavior, song, and habitat preference between I. g. dickeyae and the other three subspecies currently grouped under I. graduacauda , research efforts are considering whether this subspecies should be considered a separate species.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike
• male larger

Reproduction

Audubon’s orioles are monogamous.

• Mating System
• monogamous monogamous

Nests of Audubon’s orioles are semi-pensile (dangling) and cup shaped; the nest is built so that it hangs at the rim from a branch. Unlike other closely related species, this species prefers to build its nest close to the trunk rather than on smaller branches. Nests are usually built from between 12.9 to 33 m above the ground in several different species of large trees. In Texas, Audubon’s orioles prefer mesquite trees and make their nests 1.8 to 4.3 m above the ground. Nests are usually constructed of fine plant fibers, primarily from strands of the leaves of palmetto plants (Flood et al., 1990). According to Flood et al. (2002): “Audubon’s oriole nests are small relative to the body size of the birds that built them.”

Breeding occurs during wet seasons and brood sizes range from three to five eggs. Due to the long breeding season, mating pairs will occasionally attempt to raise a second brood. Eggs are light blue with evenly distributed dark markings. Occasionally, eggs have large dark blotches clustered around the broad end of the egg (Jaramillo and Burke, 1999; Flood et al., 1990).

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Only females incubate the eggs; both parents however, play a role in the care of the nestlings, feeding and protecting the young as well as cleaning the nest. A mated pair will defend the nest against predators and nest parasites, charging and sometimes striking the target (Flood et al., 2002). Adults remove fecal sacs from the nests, often eating them (Flood et al., 1990). It takes eleven days for newly hatched Audubon’s orioles to fledge (Jaramillo and Burke, 1999)

• Parental Investment
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifespan/Longevity

Due to a lack of data on this species, longevity is unknown.

Behavior

Audubon’s orioles can usually be seen alone or in pairs. Mated pairs often forage together. In the winter, Audubon’s orioles may join mixed-species flocks containing other, similarly sized birds including green jays ( Cyanocorax yncas ), tanagers ( Thraupidae ), and other orioles ( Icterus ) (Flood et al., 2002). Sometimes this species is also seen in small conspecific flocks of eight to ten individuals (Bendire, 1895; Flood et al., 2002). Audubon’s orioles seem to be territorial during the breeding season, but do not seem to exhibit this behavior during the winter months (Flood et al., 2002)

• Key Behaviors
• arboreal arboreal
• flies
• diurnal diurnal
• motile motile
• sedentary sedentary
• territorial territorial
• social social

Home Range

Due to a lack of data, the home range size of this species is unknown.

Communication and Perception

Both male and female Audubon’s orioles sing and their song structures seem to be identical. Mated pairs answer each others' calls, possibly to reinforce pair bonding, though not all mated pairs use song extensively (Jaramillo and Burke, 1999). The species overall is less vocal than North American Icterus species (Flood et al., 1990). During the breeding season, mated pairs respond to calls of nearby conspecifics. It is hypothesized that in this species song is used in territory defense (Flood et al., 2002). It is believed that this species sings less frequently during cold weather (Flood et al., 1990). As of 2002, there is no information regarding possible geographic variation in song (Flood et al., 2002).

The Audubon’s oriole voice have been described by as “low, mellow, humanlike whistle”. It consists of a series of non-repeating notes that are low in frequency and with little frequency modulation. To produce variation in the song, Audubon’s orioles omit notes or clips the end of the series (Flood et al., 1990). These characteristics are unlike most other New World species, especially those formerly incorporated into the northern oriole species: Baltimore orioles ( Icterus galbula ) and Bullock's orioles ( Icterus bullockii ). These vocal characteristics are similar to those of yellow-backed orioles ( Icterus chrysater ) and bar-winged orioles ( Icterus maculialatus ) (Davis, 1972; Flood et al., 2002). Audubon’s oriole alarm calls are described by Jaramillo and Burke (1999) as a “nasal nyyyee”, the amount of time between repetitions decreasing as the bird becomes more excited.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Audubon’s orioles are primarily insectivorous, consuming beetles ( Coleoptera ), butterflies ( Lepidoptera ), and spiders ( Araneae ). They also eat orchard fruit including plums, peaches, and apples; hackberries; sunflower seeds; and mesquite beans ( Prosopis species). They occasionally visit suburban hummingbird and seed feeders (Flood et al., 2002).

• Primary Diet
• carnivore carnivore
  • insectivore insectivore

• Animal Foods
• insects
• terrestrial non-insect arthropods

• Plant Foods
• seeds, grains, and nuts
• fruit
• nectar

Predation

The nests of Icterus graduacauda are frequently parasitized by bronzed cowbirds ( Molothrus aeneus ) and squirrel cuckoos ( Piaya cayana ) (Jaramillo and Burke, 1999).

Most of the major predators of Audubon’s orioles feed on their eggs or young. These predators include snakes; ants; birds, including brown jays ( Cyanocorax morio ); and mammals, especially white-nosed coatis ( Nausa naurica ) (Flood, 2002).

Ecosystem Roles

There is little information published regarding ecosystem roles. However, it is speculated that these orioles help to propagate several species of flowering plants by scattering their seeds through droppings. In addition, eggs and young are prey to several species (see predation section). Audobon's orioles may aid in regulating populations of the insect species that form a large part of their diet.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

There is little information documenting Audubon’s oriole impact on humans, either positively or negatively. Their potential impact on plant regeneration through seed dispersal may help humans and they may help to regulate some insect populations.

• Positive Impacts
• controls pest population

Economic Importance for Humans: Negative

There are no negative impacts of Audubon’s orioles on humans.

Conservation Status

Audubon’s orioles have experienced a marked decline in population in southern Texas due to nest parasitism by bronzed cowbirds . In addition, the species has been affected by human activities, including the clearing of habitat for human development. It is also suggested that Audubon’s orioles are sensitive to human interference during the breeding season (Flood et al., 2002).

Other Comments

Audubon’s orioles are similar in coloration to their closest relative, Icterus chrysater , commonly known as yellow-backed orioles (Hofmann et al., 2006). In I. chrysater and I. graduacauda , both sexes have songs of similar complexity (Price et al., 2007). This is typical of tropical oriole species and has been shown to be the ancestral state of the genus Icterus (Hofmann et al., 2008). Analysis of cytrochrome-b and ND-2 genes in genus Icterus strongly suggests that the two species are each other’s closest relatives, having 1.7% genetic divergence in the target sequences (Omland et al., 1999). This suggests that the two clades may have diverged from one another less than one million years ago, assuming a molecular clock of approximately 2% divergence per million years (Omland et al., 1999).

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Matthew Murphy (author), University of Maryland, Baltimore County, Kevin Omland (editor, instructor), University of Maryland, Baltimore County.

References

American Ornithologists' Union, 1998. Check-List of North American Birds . Washington, DC: American Ornithologists' Union.

Bendire, C. 1895. Life histories of North American birds, US National Museum Special Bulletin 3. . Washington, DC: Government Printing Office.

Davis, L. 1972. A field guide to the birds of Mexico and Central America . Austin, TX: University of Texas Press.

Flood, N., J. Rising, T. Brush. 1990. Aspects of the Breeding Biology of Audubon’s Oriole. Journal of Field Ornithology , 61(3): 290-302.

Flood, N., J. Rising, T. Brush. 2002. Audubon's Oriole (Icterus graduacauda). Pp. 1-16 in The Birds of North America , Vol. 691. Philadelphia, PA: The Birds of North America, Inc..

Hofmann, C., T. Cronin, K. Omland. 2008. Carotenoids and melanins contribute to sexual dichromatism in New World orioles. The Auk , in press: in press.

Hofmann, C., T. Cronin, K. Omland. 2006. Using spectral data to reconstruct evolutionary changes in coloration: carotenoid color evolution in New World orioles. Evolution , 60: 1680-1691.

Howell, S., S. Webb. 1995. A Guide to the Birds of Mexico and Northern Central America . New York: Oxford University Press, Inc..

Jaramillo, A., P. Burke. 1999. New World Blackbirds: The Icterids . Princeton, NJ: Princeton University Press.

Omland, K., S. Lanyon, S. Fritz. 1999. A molecular phylogeny of the New World orioles (Icterus): the importance of dense taxon sampling. Molecular Phylogenetics and Evolution , 12: 224-239.

Price, J., N. Friedman, K. Omland. 2007. Song and plumage evolution in the New World orioles (Icterus) show similar lability and convergence in patterns. Evolution , 61: 850-863.

Sibley, D. 2000. The Sibley Guide to Birds . New York: Alfred A. Knopf, Inc..