Icterus pustulatus
streak-backed oriole

Icterus pustulatus, also known as streak-backed orioles, have a range that extends along most of the Pacific coast of Central America. They occur from northwestern Mexico in northeastern Sonora and western Chihuahua south into into Sinaloa, along the coast. Streak-backed orioles are common throughout the rest of the western slope of Mexico. They are seldom observed in Guatemala, but documentation of their range resumes in El Salvador and into central Honduras and western Nicaragua. They have very rarely been seen in the southwestern United States. (Jaramillo and Burke, 1999)

In their book, "New World Blackbirds: The Icterids," Alvaro Jaramillo and Peter Burke assert that there are six subspecies of I. pustulatus, divided among three groups. (Jaramillo and Burke, 1999)

The northernmost group is the scarlet-headed oriole group. This includes I. p. pustulatus, which occurs south from Colima to Northern Oaxaca and east from Guanajuarto to Merelos, Puebla, and West Vercuz, and I. p. microstictus, which can be seen from Sonora and Chihuahua, south to Jalisco. The northern subspecies are partially migratory. While little is known of their seasonal movements (Jaramillo and Burke 1999), some I. p. microstictus have been observed wintering to the south, in the state of Guerrero (Howell and Webb 1995). (Howell and Webb, 1995; Jaramillo and Burke, 1999)

The Tres Marías oriole group consists only of I.p. graysonii. This subspecies is restricted to the Tres Marías islands which lie along the coast of Nayarit, Mexico. (Jaramillo and Burke, 1999)

Finally, the largest group, and found farthest to the south, is the streak-backed oriole group. Members of this group include I. p. alticola, found mostly in dry areas of southern Guatemala and central Honduras. Icterus pustulatus sclateri is the most southerly population and may occasionally breed with I. p. alticola, as morphologically intermediate specimens have been found. Icterus pustulatus formosus is found in the extreme south of Mexico and northwestern Guatemala. This subspecies is endemic to the vicinity of Volcán San Miguel, in southern El Salvador. Lastly, I. p. maximus is endemic to the arid Rio Negro Valley in Guatemala (Jaramillo and Burke 2000). In spite of their morphological similarity, all of these subspecies are very closely related and have diverged very recently based on mitiochondrial DNA data (Nandadevi Cortes-Rodriguez, Adolfo Navarro, UNAM Mexico; Kevin Omland, UMBC - unpublished data). (Jaramillo and Burke, 1999)

Streak-backed orioles live in arid to semiarid areas of brushy woodland and scrub. They seem to have an affinity for thorny bushes like Mimosa in open woodlands, as well as areas of secondary growth and agriculture. To the northern end of their range, they can be found in riparian areas in deserts. (Jaramillo and Burke, 1999)

According to Howell and Webb, streak-backed orioles are seen both in solitude and in small groups. They also seem to associate with other orioles, often nesting near Altamira orioles and spot-breasted orioles. (Howell and Webb, 1995)

In the northernmost group, the scarlet-headed orioles, red tones in the feathers of the heads are apparent, and there is distinct sexual dimorphism; males are much brighter in color than females. These include I. p. pustulatus and I. p. microstictus. Icterus pustulatus pustulatus are slightly larger than I. p. microstictus, and the females bear a greater resemblance to males. (Jaramillo and Burke, 1999)

Tres Marías orioles are more pale than I. p. pustulatus, and there is almost no black streaking on their backs. Females resemble males, excluding their olive colored tails (Jaramillo and Burke, 1999)

Orioles of the streak-backed group are larger, have entirely orange heads, and are almost completely monchromatic. Females can occasionally have some olive coloration. The back of I. p. alticola is the least streaked of all the subspecies and is sometimes solid black. Icterus pustulatus sclateris is often larger than other subspecies, and has a bright yellow head and broadly streaked back. Icterus pustulatus formosus has a brightly colored yellow-orange head. Icterus pustulatus pustuloides closely resembles I. p. aticola except for a reddish-orange head that is much like that of the northern populations. Icterus pustulatus maximus also resembles I. p. aticola, but is yellow, instead of orange. (Jaramillo and Burke, 1999)

There is no published information on lifespan in I. pustulatus. Baltimore orioles (I. galbula) have been recorded living up to 12 years in the wild.

Streak-backed orioles forage in pairs or as family groups, and occasionally they form larger groups. Eguiarte et al. (1987) documented that, while foraging in Pseudobombax ellipticum trees in Morelos, Mexico, streak-backed orioles behaved submissively to black-vented orioles (Icterus wagleri) during confrontations. On the other hand, streak-backed orioles consistently won confrontations with wintering Bullock's orioles (Icterus bullockii) on the same tree. Streak-backed orioles are most closely related to migratory Bullock's orioles of the western U United States (Omland et al., 1999). (Howell and Webb, 1995; Jaramillo and Burke, 1999; Omland, Lanyon, and Fritz, 1999)

The northern subspecies of I. pustulatus are partially migratory. While little is known of their seasonal movements (Jaramillo and Burke 1999), some I. p. microstictus have been observed wintering to the south, in the state of Guerrero (Howell and Webb 1995). (Howell and Webb, 1995; Jaramillo and Burke, 1999)

Home Range

There is no published information on home range or territory sizes in I. pustulatus.

Stokes and Stokes (2000) describe streak-backed orioles as having a gentle "wheet" call. Howell and Webb (1995) also describe a "jerky, discordant, jangling warble," which usually ends in a raspy rattle. (Howell and Webb, 1995; Stokes and Stokes, 2000)

Icterus pustulatus is primarily insectivorous. However, individuals do feed on fruit, seeds, and nectar as well. (Jaramillo and Burke, 1999)

There is little known about response to predators in streak-backed orioles. It is likely that avian predators occasionally take adults and that eggs and nestlings may be taken by arboreal predators, such as snakes and Bassariscus species. Other Icterus species respond to predators with warning calls and mobbing behavior. By building pendulant nests in trees, these orioles are avoiding predation on eggs and nestlings by many potential predators. (Jaramillo and Burke, 1999)

Streak-backed orioles are frugivorous and participate in the dispersal of seeds. They are also instrumental in modulating the population of various insect species, their primary prey.

While never observed to successfully fledge, a nest parasite, bronzed cowbirds (Molothrus aeneus), are known to parasitize the nests of streak-backed orioles. (Jaramillo and Burke, 1999)

As these birds sometimes eat fruit, they may cause some degree of damage to fruit crops, such as mangos.

Streak-backed orioles are attractive birds and attract the attention of ornithologists and bird enthusiasts. Research on their behavior, plumage, and genome will provide insights into the evolution of migration and sexual dimorphism.

Streak-backed orioles have no special conservation status. They are widespread in agriculturally disturbed habitats. Therefore, it is unlikely that the species is threatened (Omland, personal observations).

Streak-backed orioles demostrate a consistent trend within the genus Icterus. Males and females of sedentary, non-migratory populations are monomorphic, meaning their plumage characteristics are very similar. On the other hand, migratory populations are generally dimorphic, meaning the two sexes have different plumage characteristics, with males generally have more colorful plumage. Streak-backed orioles provide a novel opportunity for study, as there are populations that are both migratory-dimorphic, and sedentary-monomporphic. Dr. Kevin Omland and his post-doctoral student, Dr. Troy Murphy, are investigating this correlation between the loss of elaborate female plumage and the evolution of migration in streak-backed orioles. For more details on this research, please visit Dr. Troy Murphy's website at: http://www.umbc.edu/biosci/Faculty/OmlandLabWebpage/NewPages/Murphy_Troy/SBORresearch.html