Archived 2016 topics: Bronzed Cowbird (Molothrus aeneus) is being split: list M. aeneus as Least Concern and M. armenti as Near Threatened or Vulnerable?

This is part of a consultation on the Red List implications of extensive changes to BirdLife’s taxonomy for passerines

Lynx Edicions and BirdLife International will soon publish the second volume of the HBW-BirdLife Illustrated Checklist of the Birds of the World, building off the Handbook of the Birds of the World series, and BirdLife’s annually updated taxonomic checklist.

The new Checklist will be based on the application of criteria for recognising species limits described by Tobias et al. (2010). Full details of the specific scores and the basis of these for each new taxonomic revision will be provided in the Checklist.

Following publication, an open and transparent mechanism will be established to allow people to comment on the taxonomic revisions or suggest new ones, and provide new information of relevance in order to inform regular updates. We are also actively seeking input via a discussion topic here regarding some potential taxonomic revisions that currently lack sufficient information.

The new Checklist will form the taxonomic basis of BirdLife’s assessments of the status of the world’s birds for the IUCN Red List. The taxonomic changes that will appear in volume 2 of the checklist (for passerines) will begin to be incorporated into the 2016 Red List update, with the remainder to be incorporated into subsequent Red List updates.

Preliminary Red List assessments have been carried out for the newly split or lumped taxa. We are now requesting comments and feedback on these preliminary assessments.

Bronzed Cowbird Molothrus aeneus is being split into M. aeneus and M. armenti, following the application of criteria set out by Tobias et al. (2010).

Prior to this taxonomic change, M. aeneus was listed as Least Concern, on the basis that it did not approach the threshold for Vulnerable under any criterion. M. aeneus (as now defined following the taxonomic change) is known from southern USA (California, New Mexico, Texas and Louisiana) and northern Mexico where it is a short-distant migrant, and elsewhere in Mexico and throughout Central America where it is resident (Fraga 2016a). It is common to fairly common, inhabiting open areas, avoiding densely wooded areas (Fraga 2016a). It is not thought to approach the threshold for Vulnerable under any criterion and so it is proposed that it be listed as Least Concern.

M. armenti is found only on the northern coast of Colombia in northern Bolivar, Atlántico and Magdalena (Rada Quintero 2002). It inhabits a range of arid tropical lowland habitats including dry woodland, second-growth shrubland, agricultural land and roadsides; as well as mangroves (Fraga 2016b). The population size has not been quantified directly, but from population densities of congeners and assuming that not all of its range is inhabited, the population is unlikely to exceed 10,000 mature individuals. Deforestation and land clearance has affected the species’s range, and habitat loss had been estimated at 30% in 2002 (Rada Quintero 2002). More recent estimates of habitat loss may be much less than this (see Global Forest Watch webpage:, and since this species does inhabit disturbed habitats such as roadsides, population declines may not have been that large. However, the population and available habitat are likely to have declined.

In Colombia this species has been considered Vulnerable under criteria B1ab(i,ii,iii)+B2ab(ii,iii); but the number of locations* where the species is found may be greater than 10, and it is not known whether the population may be considered severely fragmented (see IUCN 2001, 2012). Therefore, it would not warrant listing as Vulnerable under those criteria, though we welcome any further information regarding to what degree this species has become fragmented as a result of habitat loss. The suspected small population size, however, means the species likely qualifies as Vulnerable under criterion C2a(ii), but we welcome any information regarding population size in case the population may in fact be larger and so not meet the threshold for listing as Vulnerable.

Comments are invited on these proposed categories and further information would be welcomed.

*Note that the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).


Fraga, R. 2016a. Bronzed Cowbird (Molothrus aeneus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from on 3 October 2016).

Fraga, R. 2016b. Bronze-brown Cowbird (Molothrus armenti). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from on 3 October 2016).

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.

IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.

Rada Quintero, E. (2002) Molothrus armenti. Pp. 453-457 in: Renjifo L. M, Franco AM, Amaya JD, Catan GH, Lopez B. (eds) (2002). Libro rojo de aves de Colombia. Serie Libros rojos de especies amenazadas. Instituto de Investigación de Recursos Biológicos Alexander von Humboldt y Ministerio del Medio Ambiente. Bogotá, Colombia.

Tobias, J. A., Seddon, N., Spottiswoode, C. N., Pilgrim, J. D., Fishpool, L. D. C. and Collar, N. J. 2010. Quantitative criteria for species delimitation. Ibis 152: 724–746.

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3 Responses to Archived 2016 topics: Bronzed Cowbird (Molothrus aeneus) is being split: list M. aeneus as Least Concern and M. armenti as Near Threatened or Vulnerable?

  1. Thomas Donegan says:

    I believe there is a real lack of attention to and awareness of the situation with this population in Colombian ornithology and birding. There are few records of this species in northern Colombia, but I am not sure if this is genuine or because people don’t pay much attention to cowbirds (which are otherwise very widespread nest parasites).

  2. I agree with Thomas. The people confunded Bronzed Cowbird with Shiny Cowbird (Poorly records)
    I think that Continued forest degradation (mangroves) and clearance for construction, agriculture and commercial plantations n this region (Isla Salamanca) are having profound and long-term environmental impacts on populations the Bronzed Cowbird
    I advance listing as Vulnerable

  3. James Westrip (BirdLife) says:

    Based on available information, our preliminary proposal for the 2016 Red List would be to list:

    M. aeneus as Least Concern.

    M. armenti as Vulnerable under criterion C2a(ii).

    There is now a period for further comments until the final deadline of 28 October, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from the initial proposal.

    The final 2016 Red List categories will be published on the BirdLife and IUCN websites in early December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Comments are closed.