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Molothrus bonariensis

Shiny Cowbird

  • Order: Passeriformes
  • Family: Icteridae
  • Polytypic 7 Subspecies

Authors: Lowther, Peter E



The Shiny Cowbird is a small, sexually dimorphic, brood-parasitic icterid. The male is entirely glossy violet blue-black or purplish black or (wings and tail) glossy greenish blue. The female dull grayish brown above. The color of the underparts in the female is geographically variable; many subspecies are dark brown below, but in some populations the female is much paler below (see Geographic Variation). Bill blackish gray, of medium length and conical. The size of the Shiny Cowbird also varies geographically. The smallest subspecies, minimus, of northern South American and the West Indies, is ca 18 cm in total length with a a mass of 31 - 40 g; the largest subspecies, cabanisii of northern Colombia, is ca 22 cm in length and with a mass of 55 - 65 g. The bill of the Shiny Cowbird is blackish gray, of medium length and conical.

Similar Species

The male Shiny Cowbird is similar to various other species of all-black New World passerines, but many potentially confusing species have no sexual dimorphism, e. g., Screaming Cowbirds (Molothrus rufoaxillaris). Even so, distinguishing Shiny and Screaming cowbirds may require some care. Useful field characters for identifying these two species (in addition to their different vocalizations) are the shape of the bill, the glossiness of plumage, and the color of the irides. The bill of the Shiny Cowbird is relatively longer, with a narrower base, than the stubbier, more finch-like bill of the Screaming Cowbird. The plumage of the Screaming Cowbird is less glossy than that of Shiny Cowbird, and Screaming Cowbird has chestnut or reddish brown irides, not dark brown as in Shiny (but this character can be difficult to see). Finally, in the hand, the Screaming Cowbird has diagnostic rufous underwing coverts.

In the United States and in the Caribbean, grackles (Quiscalus spp.), which also have glossy purple iridescence, are larger than Shiny Cowbird, and most taxa have yellow irides; the similar Brewer's Blackbird (Euphagus cyanocephalus) has a greenish iridescence and also has light-colored irides. The male Bronzed Cowbird (Molothrus aeneus) is larger than Shiny Cowbird and has bronzy iridescence, red irides and ruffled neck-feathers (Smith and Sprunt 1987); the female Bronzed Cowbird may be black or brownish gray. These two species overlap geographically only in southern Central America.

The Shiny Cowbird also is similar in size and proportions to Brown-headed Cowbird (Molothrus ater), but male Shiny Cowbird lacks the brown head. Female Shiny Cowbirds are grayish brown and are very similar to female Brown-headed Cowbird, but have a slightly longer, less conical bill and flatter head. Shiny also appears darker with possibly more prominent eye-stripe (Smith and Sprunt 1987), but comparison of these characters probably requires the presence of individuals of both species (Stevenson and Anderson 1994).


Development. No information.

Vocal array. The song, referred to as the "true courtship song" by Friedmann (1929), consists of three or 4 low "bubbling" purr notes followed by ascending series of short notes -- pe-tss-tseeee, and lasts about 2-3 s (Friedmann 1929, Selander 1964). First sounds of Song are combination of purring and bubbling, 3 "guttural notes are belched forth by the bird with considerable bodily quivering .... and may be written purrr purrr purrr but the rr's are softened so that the bubbling sound is produced. Closely following these notes come 3 high, rather glassy, thin notes, the first 2 short and somewhat run together, and the last one long and drawn out -- pe-tsss-tseeeee very much like those of [Brown-headed Cowbird] M. ater, but very slightly buzzier ... The last long drawn out note becomes thinner towards the end and ends, not abruptly, but merely by dying out ..." (Friedmann 1929: 71-72). Song described as prro-prro-prro-TSLEEyew by Sick (1993), or, in Portuguese, as prro-prro-prro-zlíjü (Sick 1985).

Twitter song. Consists of 3 short, distinct notes, followed by twitter of 6-8 or 9 syllables run together (Friedmann 1929).

Chatter (Rattle). This call is given most commonly given by the female. It consists of rapid chattering, similar to that of Brown-headed Cowbird, but slightly more metallic; given by female as starts to fly (Friedmann 1929). Female Chatter is similar to the call of other cowbird species (Fraga 1986).

Flight Whistle. Given only by the male. A "plaintive whistle ... high, clear, thin, and glassy" (Friedmann 1929: 73). Possibly equivalent to Brown-headed Cowbird's Flight Whistle (Selander 1964); usually preceded by chuck, sometimes repeated, heard a few times to be preceded by Chatter (Friedmann 1929).

Call note. Chuck, commonly used by both sexes.

Battle Call. Heard only from birds fighting; perhaps merely same as chuck, but harsher and more grating, usually rapidly repeated and each note is more drawn-out than "normal" chuck (Friedmann 1929).

No geographic variation of song or calls described.

Phenology. Song reported from early September-early December (breeding season) in Argentina (Friedmann 1929).

Daily pattern of vocalizing. No information.

Places of vocalizing. From trees and ground, and in flight.

Repertoire and delivery of songs. No information.

Social context and presumed functions. Song serves in territory defense and/or self advertisement. For context of calls, see above.

Nonvocal Sounds

None known.

Detailed Description (appearance)

Following based on Friedmann 1929, unless otherwise indicated.

Adult male (Definitive Basic plumage): Upper- and underparts lustrous black with iridescent purplish and bluish gloss; wing and tail with greenish gloss.

Adult female (Definitive Basic plumage): Head, neck, back, scapulars and interscapulars grayish olive-brown; rump and uppertail coverts grayer and less olive than back; wings and tail dusky brown with grayish brown edgings; underparts hair-brown.

There is no Prealternate molt, but feather wear results in plumage becoming more bluish in males. 

Immature (Formative plumage):  Similar to Definitive Basic, but retains some feathers from the Juvenile (First Basic) plumage, especially on the underwing coverts, tertials, or on the body. These brown, retained juvenile feathers are relatively obvious on the otherwise blackish male, but the female in First Basic plumage can be difficult to distinguish from adult female (Definitive Basic); the primaries may be more worn (Jaramillo and Burke 1999).

Juvenile (First Basic plumage): In most subspecies (other than northern minimus), shows sexual dimorphism in First Basic plumage; this is the only species of cowbird that is dimorphic at this stage (Jaramillo and Burke 1999).

Male. Varying from dark chaetura drab to fuscous black on upperparts; wing coverts margined with dusky grayish brown, creating 2 wing-bars; underparts dull buffy-gray, heavily streaked with sooty grayish-brown to fuscous black. Abdomen washed with yellowish buff. As in Brown-headed Cowbird (Molothrus ater), underwing coverts of Juvenile male may average darker than those of Juvenile female (Pyle 1997).

Female. Varying from light tawny olive to buffy brown; wing coverts margined with grayish tawny, creating 2 wing-bars; underparts varying from dull light buffy to light grayish, slightly washed with buffy and distinctly, but not always conspicuously, streaked with light grayish brown.

Hatchling: Natal down mouse-gray.

Bare Parts

Unless otherwise noted, colors are from W. Post, using color names and numbers from Smithe 1975.

Iris: plumbeous, or (Jaramillo and Burke 1999) dark brown.

Bill and gape: blackish neutral gray (82); in juveniles, base of bill plumbeous (78); tongue mauve (172 C) to deep vinaceous (3); mouth deep vinaceous.

Tarsi and toes: blackish neutral gray; foot pads dark neutral gray (83).



All measurements for after hatch-year M. b. minimus; wing length from banded birds; all other measurements from fresh specimens (W. Post).

Bill length. Males, mean 12.8 mm ± 0.5 SD, range 12.0-14.0, n=16; females, mean 11.7 mm ± 0.6 SD, range 10.8-12.7, n = 9.

Bill width. Males, mean 5.6 mm ± 0.6 SD, range 5.0-6.8, n=15; females, mean 5.4 mm ± 0.5 SD, range 4.9-6.1, n=8.

Bill depth. Males, mean 7.3 mm ± 0.4 SD, range 6.9-8.3, n=16; females, mean 6.9 mm ± 0.3 SD, 6.5-7.2, n=8.

Wing length. In breeding season, chord from "wrist" to tip of longest primary: males, mean 96.5 mm ± 2.6 SD, range 90.0-102.0, n=42; females, mean 85.8 mm ± 1.8 SD, range 81.5-89.5, n=60.

Tail Length. Males, mean 70.9 mm ± 3.8 SD, range 65.1-78.0, n=16; females, mean 61.5 mm ± 2.4, range 56.0-65.0, n=9.

Tarsus length. Males, mean 23.3 mm ± 0.8 SD, range 22.0-24.5, n=12; females, mean 21.9 mm ± 1.0 SD, range 20.5-23.6, n=8.

Brain size. Mean hippocampus volume (estimated from figure): 29.5 mm3 for females, 27.5 mm3 for males; mean telencephalon volume, 800 mm3 for females, 925 mm3 for males (n=8 females, 9 males; Reboreda et al. 1996).


The following data for M. b. minimus captured in mist-nets during breeding season (1 June-15 July) in St. Lucia (W. Post). Adult (after-second-year) males -- mean 40.0 g ± 2.94 SD, range 32.1-45.5, n=42); second-year males -- mean 37.9 g (range 34.3-41.2, n=6); juvenile (hatch-year) males -- mean 37.7 g ± 2.4 SD, range 29.1-40.8, n=25). After-hatch-year females -- mean 34.1 g ± 3.0 SD, range 29.5-39.2, n=48; after-hatch-year females with palpable eggs -- mean 36.4 g ± 2.5 SD, range 31.3-39.6, n=14. Mass varies little throughout year. In Puerto Rico, mass of decoy-trapped females peaked in breeding season (June; 33.3 g), was lowest in winter (January and February; 30.3 g; Wiley 1988).
Within same subspecies, apparent variation between geographic areas: Birds in Greater Antilles weigh more in spring than do comparable, but less vagile, individuals in Lesser Antilles (Post et al. 1993).  Mean mass of birds from Trinidad: females 32.32 g ± 1.75 SD (n=13; Manolis 1982).  Mean mass of birds from Colombia: males 63.7 g ± 1.3 SD (n=5), females 55.6 g ± 3.8 SD (n=24); females at first molt 45.2 g ± 4.4 SD (n=9); 2 males at first molt, 49 g and 51 g (Kattan 1993).  Mean mass of birds from Argentina: males, 55.5 g ± 7.6 SD (n=69); females, 44.9 g ± 4.3 SD (n=78; Mason 1987).


Definitive Basic plumage is acquired by a complete Prebasic molt complete (Pyle 1997, Jaramillo and Burke 1999). Occurs July-October in northern populations (Pyle 1997); February-April in nominate bonariensis (Jaramillo and Burke 1999).

Formative plumage is acquired by an incomplete Preformative molt. Most feathers are replaced, except some underwing-coverts probably retained and usually, in about 80% of birds reported in tropical America, some Juvenile (First Basic) flight-feathers (probably among S2 - S6, rectrices, and perhaps P10) also are retained. More study needed to determine whether molting strategies of North American populations diverged from those of tropical populations. In northern populations, molt probably occurs primarily July-October (Pyle 1997); the molt in nominate bonariensis is January-March (Jaramillo and Burke 1999).

Geographic Variation

Geographic variation represented by seven described subspecies (Blake 1968). Among these M. b. occidentalis, of western South America, is palest; M. b. cabanisii, of northwestern South America, is the largest, M. b. minimus, of northeastern South America north through the Antilles to south Florida, is the smallest.
Characters used to differentiate subspecies include the coloration of both male and female, overall size based on wing and tail length, and bill size and shape. Males of nominate M. b. bonariensis of eastern and southern Brazil and M. b. minimus which has invaded North America have similar plumage but differ in size; females of these two subspecies generally similar, excepting size, but M. b. bonariensis with top of head lighter and scapulars without distinct streaking.

Molothrus bonariensis cabanisii Cassin, 1866: Eastern Panama, tropical and lower subtropical zones of Colombia west of the eastern Andes, and eastern slope of eastern Andes; intergrades with bonariensis in southeastern Colombia. The largest subspecies. Male plumage similar to M. b. bonariensis; female slightly paler.

Molothrus bonariensis aequatorialis Chapman, 1915: Tropical zone of southwestern Colombia, south of Río Patia, and western Ecuador south to Guayaquil and Puná Island. Larger than M. b. bonariensis. Males with more violet and less blue iridescence; female darker and without buffy whitish postocular streak.

Molothrus bonariensis occidentalis Berlepsch and Stolzmann, 1892: Extreme southwestern Ecuador and western Peru east to province of Jaén in Cajamarca and south to Lima and to Ica (Schulenberg et al. 2006). Male possibly more bluish than M. b. bonariensis, similar to M. b. venezuelensis. Female differs from all other subspecies by pale upperparts, very pale and streaked underparts and conspicuous buffy postocular stripe.

Molothrus bonariensis venezuelensis Stone 1891: Tropical zone of eastern Colombia from Zulia Valley south to the eastern Llanos; northern Venezuela south in Llanos to Apure and Orinoco rivers and south of Orinoco in northwestern Amazonas and northern Bolívar. Male more richly glossed with purple than M. b. bonariensis, similar to M. b. occidentalis. Female darker than M. b. bonariensis, like M. b. minimus.

Molothrus bonariensis minimus, Dalmas 1900: Lesser Antilles north to Martinique; Tobago; Trinidad; Guianas and extreme northern Brazil in region of upper Rio Branco; expanded range through West Indies since about 1900 (Cruz et al. 1985, Post et al. 1993): Carriacou in 1899, Puerto Rico by 1955, Hispaniola by 1972, Cuba in 1982. First appeared in Florida Keys in 1985, and Florida mainland in 1987. Smallest subspecies. Male similar to M. b. bonariensis. Female with forehead and crown darker than M. b. bonariensis and with distinct streaks on scapulars and interscapulars.

Molothrus bonariensis riparius Griscom and Greenway, 1937: Lower Amazon Valley (to Obidos in northern Brazil on north bank of Amazon) west to the Río Ucayali, eastern Peru. Male similar to M. b. bonariensis. Female slightly blacker on upperparts and paler on underparts than M. b. bonariensis.

Molothrus bonariensis bonariensis Gmelin 1789: Eastern and southern Brazil, north to Mato Grosso, Maranhão, Piauí, and Ceará; eastern Bolivia, Paraguay, Uruguay and Argentina south to Chubut. Introduced into Chile and established from Coquimbo Province south to Valdivia in Los Lagos Province. Male with head, back, breast and upper abdomen silky violet-black; rest of body blue-black; wings glossy greenish black. Female with head and back grayish olive-brown; rump and tail coverts grayer; wings and tail dusky brown; underparts hair-brown.


Sequencing of mitochondrial DNA indicates that the five species of parasitic cowbirds - Bronzed (Molothrus aeneus), Brown-headed (M. ater), Shiny (M. bonariensis), Screaming (M. rufoaxillaris), and Giant (M. oryzivorus) - make up a natural (monophyletic) group (Lanyon 1992). Analysis of restrictive enzyme cleavage sites in mitochondrial DNA produces a tree in which Shiny Cowbird clusters with Brown-headed and Bronzed cowbirds (Freeman and Zink 1995). Combining restriction-site data with sequence data generates the phylogenetic topology of Lanyon 1992 (Freeman and Zink 1995). Data for presence or absence of 161 restriction sites for 49 icterids -- including all Molothrus species -- are included in Freeman and Zink 1995.

Recommended Citation

Lowther, Peter E. 2011. Shiny Cowbird (Molothrus bonariensis), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: