Shiny Cowbird (Molothrus bonariensis)

[order] Passeriformes | [family] Icteridae | [latin] Molothrus bonariensis | [UK] Shiny Cowbird | [FR] Vacher luisant | [DE] Seidenkuhstarling | [ES] Vaquero Mirlo | [IT] Molotro splendente | [NL] Glanskoevogel

Subspecies

Physical charateristics

Recognized by their slender conical bill, a uniform dull blue-black plumage and squared-off tail, and a solid dark eye-colour. Males have a purplish shine on their head, neck, breast and upper back and a blue shine on their wings, while females are grey-brown with whitish eyebrows and throats. Nestlings have flesh-coloured skin with scattered tufts of blackish down. The oral flanges range from white to yellow and the mouth lining is reddish.

Listen to the sound of Shiny Cowbird

[audio:http://www.aviflevoland.nl/sounddb/S/Shiny Cowbird.mp3]

Copyright remark: Most sounds derived from xeno-canto

Range

A common bird throughout much of South America, the Shiny Cowbird only recently spread through the Caribbean and into the United States. A nest parasite like other cowbirds, its spread has imperiled a number of endangered bird species in the Caribbean and raised conservation concerns in the United States. The Shiny Cowbird first reached the United States in 1985 when a single male was seen in the Florida Keys. By the early 1990s they were considered permanent residents in southern Florida. In the next few years individual Shiny Cowbirds were seen as far away as Maine and Nova Scotia.

In Suriname is is a common bird in the coastal mangroves, swamps and plantations. The House Wren is very oiften parasitized by this species.

Habitat

Open or semiopen habitats, especially agricultural areas with patches of trees and shrubs. Suburbs, lawns, and at bird feeders.

Reproduction

Molothrus bonariensis is a brood parasite, that is, it has completely abandoned the task of building nests, incubating eggs, and feeding and rearing nestlings. It is an extreme generalist, with 60 to 100 eggs per breeding season. Shiny cowbirds monitor host nests in their territory but are sloppy at synchronising egg-laying with that of their host. There appears to be no specificity in terms of which host-species M. bonariensis prefers in any area. The breeding season of M. bonariensis is October to January in Argentina, but may be extended in the South American tropics. Shiny cowbirds have been known to synchronise breeding with that of their high quality hosts. The reproductive success of cowbirds depends on the size and life traits of the host; cowbird chicks in the nests of smaller hosts such as the house wren Troglodytes aedon have high survival rates due to cowbirds having a competitive edge over their ?siblings?. Cowbird chicks in the nests of large hosts such as the chalk-browed mockingbird Mimus saturninus have lower survival rates.
Some hosts reject unusual looking eggs or eggs laid before or after their own by pushing them out of the nest, building the nest over them or abandoning their nest. For example cowbird chicks hatch 1 to 4 days before brown-and-yellow marshbird chicks, which may give them up to a 4-day head-start on their nest-mates.

The incubation period of shiny cowbird is about 11 – 13 days. Eggs have an extraordinary diversity in the colour and markings and can be pure white or flesh coloured with sparsely or densely scattered pink or red flecks. Some may have fine marks like pen scratches while others may have large chocolate brown spots. There is no such thing as characteristic markings in the eggs of this species although the eggs of the same individual show a “family resemblance”. In general eggs may be white-immaculate or spotted; spotted eggs may have a white, pale gray or pale blue background with a variable pattern of gray and reddish-brown spots. In size they may vary from 20mm x 26mm to 18mm x 22mm.

Feeding habits

This abundant and gregarious bird feeds mainly on insects and some seeds, including rice, and forages on the ground or perches on cattle.

Conservation

This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be increasing, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.

Migration

Sedentary throughout range.

Distribution map

Literature

Title Parasitism by Shiny Cowbirds of Rufous-Bellied Thrushes

Author(s): Gabriela Lichtenstein

Abstract: I studied the interaction between a brood parasite..[more]..

Source: The Condor 100:680-687

Title Phylogenetic analysis of life-history adaptations
in parasitic cowbirds

Author(s): Myriam E. Mermoz and Juan Francisco Ornelas

Abstract: Parasitic cowbirds lay eggs in the nests of other ..[more]..

Source: Behav Ecol 15:109-119 (2004)

Title Conirostrum bicolor Parasitized by Molothrus bonariensis in Surinam

Author(s): F. HAVERSCHMIDT

Abstract: Hellmayr (Field Mus. Nat. Hist. Publ. 347, Zoo1. S..[more]..

Source: The Auk, Vol. 71, 328-329

Title THE SHINY COWBIRD (MOLOTHRUS BONARIENSIS) IN CHILE:
INTRODUCTION OR DISPERSION? ITS HOSTS AND PARASITIC
TRENDS

Author(s): Manuel Marín

Abstract: I analyzed the origin, distribution, and hosts of ..[more]..

Source: ORNITOLOGIA NEOTROPICAL 11: 285-296, 2000

Title THE RUFOUS-COLLARED SPARROW AS A HOST OF THE SHINY COWBIRD

Author(s): ROSENDO M. FRAGA

Abstract: The Rufous-collared Sparrow or Chingolo (Zonotrich..[more]..

Source: Wilson Bull., 90(2), 1978, pp. 271-284

Title SHINY COWBIRD PARASITISM IN CENTRAL BRAZIL

Author(s): ROBERTO B. CAVALCANTI AND THAIS MARTINS PIMENTEL

Abstract: Recent human settlement in Brasilia, Central Brazi..[more]..

Source: The Condor 90:40-43

Title Feeding associations between capybaras Hydrochoerus hydrochaeris (Linnaeus) (Mammalia, Hydrochaeridae) and birds in the Lami Biological Reserve, Porto Alegre, Rio Grande do Sul, Brazil.

Author(s): TOMAZZONI, Ana C., PEDO, Ezequiel and HARTZ, Sandra M.

Abstract: Feeding associations between capybaras Hydrochoeru..[more]..

Source: Rev. Bras. Zool. 2005, v.22, n.3, pp.712-716

Title BROOD PARASITISM IN A HOST GENERALIST, THE SHINY COWBIRD: I. THE QUALITY OF DIFFERENT SPECIES AS HOSTS

Author(s): PAUL MASON

Abstract: The Shiny Cowbird (Molothrus bonariensis) of South..[more]..

Source: The Auk 103: 52-60

Title BROOD PARASITISM IN A HOST GENERALIST, THE SHINY COWBIRD: II. HOST SELECTION

Author(s): PAUL MASON

Abstract: Host selection by brood parasitic Shiny Cowbirds (..[more]..

Source: The Auk 103: 61-69