Giant Cowbird Molothrus oryzivorus

  • Order: Passeriformes
  • Family: Icteridae
  • Polytypic: 2 subspecies
  • Authors: Peter E. Lowther
Sections

Systematics

Geographic Variation

The slight variation in size and in plumage iridescence of males has been represented by 3 or by 2 subspecies; or, the variation observed has been considered insufficient for subspecific recognition. The pattern of variation intended to be described by subspecies is that populations of largest the Giant Cowbirds are centrally located within the species' distribution (in Panama and Colombia), with smaller forms to the north and to south; males of more northern populations had been described as more violaceous while central and southern populations more bronzy. Boundaries for subspecies distributions were in the areas of Belize - s. Mexico and s. Panama - Colombia; this second boundary is, perhaps, more real.
Three subspecies, if recognized (e.g., Ridgway 1902, Friedmann 1929), consist of (from north to south): M. o. impacifus (Peters 1929), M. o. oryzivorus (Gmelin 1788), and M. o. violeus (Bangs 1900). With more and better material available, Hellmayr (1937) and Blake (1968) saw no distinction between M. o. oryzivorus and M. o. violeus. Wetmore et al. (1984) argued that the amount of overlap between different populations was too great to justify any subspecific distinctions [e.g., wing measures of males: São Paulo, Brazil, 176 - 200 mm (n = 10); Santa Marta, Colombia, and w. Ecuador, 186 - 210 mm (n = 10; Hellmayr 1937)].
The genus Cassidix had been mis-applied to the Giant Cowbird until Peters (1929) supplied the name Psomocolax for the genus; however, that generic name was shown to be unnecessary since Scaphidura had been valid and available (Parkes 1954). Genetic work by Lanyon (1992; see Related species), indicate that generic allocation for Giant Cowbird should be as part of the genus Molothrus defined as brood parasitic cowbirds (see Am. Ornithol. Union 2000).

Related Species

The five species of "true" cowbirds (Molothrus) are brood parasitic; the Bay-winged Cowbird (Agelaioides badius) of southern Brazil, Bolivia, and northern Argentina is non-parasitic. The Giant Cowbird has been described as a larger version of the Bronzed Cowbird (Molothrus aeneus; Lack 1968). Mitochondrial DNA sequencing indicate that the 5 parasitic cowbirds comprise a natural (= monophyletic or holophyletic) group not including the Bay-winged Cowbird (Lanyon 1992), a finding further supported by additional phylogenetic analyses of the Icteridae (Lanyon 1994, Lanyon and Omland 1999, Johnson and Lanyon 1999). The Giant Cowbird, which formerly had been classified in a separate, monotypic genus (Scaphidura; earlier known as Psomocolax) is nested within Molothrus: the basal Molothrus is the Screaming Cowbird (M. rufoaxilaris), which is sister to Giant and all other Molothrus (Lanyon 1992, Lanyon and Omland 1999, Johnson and Lanyon 1999). (Data for Giant Cowbird along with 36 other icterid species are presented in Lanyon 1994.) Analysis of restrictive enzyme cleavage sites in mitochondrial DNA produces a tree in which Giant Cowbird is not far removed from most other Molothrus (Freeman and Zink 1995). Combining restriction site data with sequence data generates a phylogenetic topology matching that given by Lanyon's 1992 results (Freeman and Zink 1995). (Data for presence/absence of 161 restriction sites for 49 icterids – including all Molothrus species – are included in Freeman and Zink 1995.)

Recommended Citation

Lowther, P. E. (2010). Giant Cowbird (Molothrus oryzivorus), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.giacow.01