The First Sight Record of Bronzed Cowbird (Molothrus aeneus) in British Columbia. By Rick Toochin. Published: June 3, 2023.

 

The Bronzed Cowbird (Molothrus aeneus) is a small passerine that is found breeding in North America in open habitats such as: open fields, pastures, and scrubby areas (Monroe 1968) from southern California, the Lower Colorado River Valley, south-western New Mexico, south-eastern Arizona, southernmost Nevada (Rosenberg et al. 1991), southern Texas, and eastward to southeastern Louisiana (Ellison and Lowther 2020), and into the extreme southern part of Mississippi (Goodman 2005). This species also breeds in open woodland, forest patches and scrub throughout Mexico (Howell and Webb 2010), south throughout Central America into Panama (Stiles and Skutch 1989, Howell and Webb 2010), and in coastal Colombia (Friedmann 1929, Kostecke et al. 2004). 

The Bronzed Cowbird withdraws in the winter from most of the species breeding areas in the southwestern United States. Small numbers are found in the winter in Southwestern Arizona (Monson and Phillips 1981), also found in small numbers wintering around New Orleans, Louisiana (Root 1988), are locally abundant around grain elevators in winter in southern Texas (Rappole and Blacklock 1985), is rare, but regular winter visitor in southern Alabama (Alabama Ornithological Society 2008) and western Florida mainland (Robertson and Woolfenden 1992a). South of North America, the Bronzed Cowbird winters from north-central Mexico (Howell and Webb 2010), south throughout Central America into Panama, and in coastal Colombia (Friedmann 1929, Kostecke et al. 2004).

 

The Bronzed Cowbird doesn’t wander frequently north of its limited range in North America. There are a few records of vagrancy from East Coast of North America from such states as: Maine, New York, North and South Carolina, and from Canada in Nova Scotia (e-bird database 2021). There are also a few records from the mid-section of North America from states such as: Tennessee, Missouri (Robbins and Easterla 1981), Kansas, Colorado, and Utah (e-bird database 2021). Outside of a recent sight record from British Columbia there are no other records north of California along the west coast of North America (e-bird database 2021).

 

There are 4 subspecies of which 2 occur in North America. The nominate subspecies (Molothrus aeneus aeneus) occurs from southern Texas to southern Mexico, Yucatán Peninsula, and central Panama (Clements et al. 2017). The second subspecies (Molothrus aeneus loyei) occurs from Arizona into northwestern Mexico in Sonora, Chihuahua, Durango, and Nayarit (Clements et al. 2017). The third subspecies of Bronzed Cowbird (Molothrus aeneus assimilis) occurs in southwestern Mexico in Jalisco to Colima, Guerrero, Puebla, Oaxaca, and Chiapas (Clements et al. 2017). The fourth subspecies (Molothrus aeneus armenti) is found along the Caribbean coast of northern Colombia (Clements et al. 2017). These 2 subspecies found outside of North America are not discussed any further as they fall well out of the scope of this article.

The identification of the Bronzed Cowbird is covered in all standard North American field guides. This is a small passerine, but large and stocky for a cowbird measuring 22 cm in length, with a wingspan of 35cm, and weighing 62 grams (Sibley 2000). This species, like other cowbirds lays its eggs in other species nests to raise and care for their young (Ellison and Lowther 2020). Bronzed Cowbirds have a stout bill that is shorter than the head (Dunn and Alderfer 2017). The iris is reddish, however the male in breeding season has a scarlet to crimson eye colour; males in the nonbreeding season males have eyes that are brownish orange to orange-brown to reddish (Dickey and van Rossem 1938, Sibley 2000). The bill is black in colour and heavy with an arched culmen (Ellison and Lowther 2020). Adult males have a conspicuous erectile ruff on the back and sides of the neck (Sibley 2000). The male is overall blackish with a bronzy iridescence on the back and bronzy or violaceous iridescence on the rump (Dunn and Alderfer 2017). The inner web of the 4 outer primaries is emarginated and the web widens beyond the middle portion (Ellison and Lowther 2020). The tail is about two-thirds as long as the wing and is slightly rounded (Ellison and Lowther 2020). The female is about 14% smaller than the male, with neck ruff essentially lacking. The overall color of the female is dull black in the subspecies (M. a. aeneus) of eastern Mexico and Texas; and gray brown in the subspecies (M. a. loyei) of western Mexico and in the southwestern United States. Eyes colour of females is a brownish orange to orange, brown to reddish in colour (Dickey and van Rossem 1938). Juvenile birds are similar to the female of respective subspecies, but the iris is chestnut, becoming mottled orange and greenish yellow as birds mature into adult females or sub-adult males, and not red as shown by adult birds (Ellison and Lowther 2020). The bill is brownish in colouration (Sibley 2000). Legs and feet in all ages are black (Dunn and Alderfer 2017). 

 

In flight, the thick neck makes bird look front heavy (Ellison and Lowther 2020). Wings are short and round in shape when in flight (Sibley 2000).

 

The Whistle call of males differs regionally, helping to distinguish races (Sibley 2000). Eastern birds give a clear, level whistle about a second long “pseeeeeee” (Sibley 2000). Birds in West Texas give a slow rising trill up to one second long “brrrrreet”; with birds from this region through Arizona also giving a short up slurred whistle “wink” (Sibley 2000). 

The following molt sequences for the Bronzed Cowbird are taken from Ellison and Lowther (2020).

 

Juvenile Plumage is held from May-September. Birds of this age appear deep dull sooty black without a metallic gloss. The underparts are paler and dusky. The female is slightly paler than male. 

Formative Plumage is held from September-March. Plumage differs by sex. The male is primarily dull black; the underparts, especially on the throat, are dark sooty-brownish; the back and scapulars are faintly glossed with bluish green; the wings, upper tail coverts, and tail brownish, slightly glossed with bluish green; some to all underwing primary coverts have retained juvenile feathers, that appear contrastingly pale brown and worn. The males possess a “ruff” (Ridgway 1902) or “cape” (Friedmann 1929) comprised of elongated feathers on the back and sides of the neck. The feathers of ruff are erectile and cover the upper back. The ruff averages less extensive than in Definitive Basic Plumage. The female is dull black with a less pronounced metallic sheen, is more brownish below and has much less-developed ruff. Retained underwing coverts and brownish primaries of both sexes is the best means of differentiating formative from basic plumage.

First Alternate Plumage is held from March-August. May not always occur, but if present, plumage aspects are like that of Formative Plumage; but the head and neck of the males achieves a glossier dull bronzy color. Females are like birds in Formative Plumage.

Definitive Basic Plumage is held from September to March or possibly August. Color aspect differs by sex. The male is primarily black with a glossed with greenish-bronze head, neck, body, rump, and upper tail coverts; the wings are glossy dark greenish blue; the tail is dark a metallic greenish blue. The underwing primary coverts are uniformly a dull blackish, contrasting with glossier underwing of the secondary coverts, but not brown and worn as in Formative Plumage. The female is dull black; the underparts are lighter with a faint bluish-green gloss on the back and scapulars. The females have a darker gloss on the wings, upper tail coverts and tail; a bit brighter and more metallic than in Formative Plumage (Dickey and van Rossem 1938).

The uniform underwing coverts and blackish primaries of both sexes is the best means of differentiating basic and formative plumages.

Definitive Alternate Plumage is held from March through till August. May not always occur in some birds, but if present, colouration is like birds in Definitive Basic Plumage in both males and females; perhaps with males averaging slightly glossier when there are new feathers on the head and neck.

In British Columbia, the only the widespread Brown-headed Cowbird (Molothrus ater) might cause confusion a Bronzed Cowbird. The Brown-headed Cowbird is smaller measuring 19 cm in length, with a wingspan of 30 cm, and weighing 44 grams (Sibley 2000). Adult males have brown heads with green glossy-black body, wings, and tail. The eyes are dark, and the bill is small and black (Dunn and Alderfer 2017). Females are dull gray-brown overall, with a whitish throat, a stout bill and dark eyes and feet (Sibley 2000). Call notes a harsh rattle and squeaky whistles (Dunn and Alderfer 2017). This species is obvious and should not pose any identification issues with any potential Bronzed Cowbird.

 

The Bronzed Cowbird is a new addition to the avifauna of British Columbia with the observation of a female bird coming to a compost area along 5 Road at Sumas Slough in Sumas Prairie on November 10, 2019 (R. Toochin Pers. Comm.). This bird was well scene in a telescope and was in the company of a large flock of 1100 Red-winged Blackbirds, 200 Brewer’s Blackbirds, 6 Rusty Blackbirds, and 300 Brown-headed Cowbird (R. Toochin Pers. Comm.). The bird was obviously stocky in shape, larger to the nearby Brown-headed Cowbirds, looking all black with a stubby black bill, black legs, short tail, and obvious reddish eyes (R. Toochin Pers. Obs.). Unfortunately, the bird was a bit far from the road, so photos taken where not good enough to show small details even though these were obvious in optics. This bird was not seen again after the flock was flushed by a passing Cooper’s Hawk (Accipiter cooperii) and was not relocated despite a large effort by several observers. This bird by appearance looked to be of the subspecies M. m. aenus which has been found as a vagrant in parts of Eastern North America and there a very few records for western North America away from established breeding areas(e-bird database 2021).

 

I wish to thank Don Cecile for editing the original manuscript.

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