Yellow-shouldered blackbird (Agelaius xanthomus)

KingdomAnimalia
PhylumChordata
ClassAves
OrderPasseriformes
FamilyIcteridae
GenusAgelaius (1)
SizeLength: 20 - 23 cm (2)
Average weight: 38.4 g (2)
Average male wingspan: 10.3 cm (3)
Average female wingspan: 9.3 cm (3)

The yellow-shouldered blackbird is classified as Endangered (EN) on the IUCN Red List (1).

The yellow-shouldered blackbird (Agelaius xanthomus) is a glossy black bird, named for the conspicuous bright yellow patch on its shoulder. In sunlight, the black feathers of the yellow-shouldered blackbird shimmer with a faint blue-green colour (2) (3), while in some individuals there may be a tinge of orange on the feathers under the shoulder of the wing (3). There is no difference between the plumage of the male and female yellow-shouldered blackbird; however, the female is slightly smaller than the male (3).

The vocalisations of the yellow-shouldered blackbird are hugely varied (2), with many growls, rasps, ‘pee-puus’,‘chwips’,‘checks’, screams and ‘cut-zees’. The ‘cut-zee’ sound is mainly used during mobbing of predators, especially at nest sites, and acts as an alarm to other nearby blackbirds which might also be in danger (3).

Once found all over Puerto Rico, the yellow-shouldered blackbird is now mainly found in the south-west coastal areas, as well as on the islands of Mona and Monito (2) (3) (4) (5). There is reported to be a small population on the eastern coast, but there have been no breeding records from this area since 1986 (2).

The yellow-shouldered blackbird roosts, forages and nests in different locations. Roosts are further inland in mangrove forests and pastures, while this species increasingly nests on offshore cays (6). The nests are typically placed in cliff crevices or in stumps of dead or hollow mangroves (3) (4) (7).

The yellow-shouldered blackbird forages both in trees and on the ground (6), using separate techniques for the different locations (3). This species congregates at communal feeding sites during the non-breeding season (2) (7), and feeds mainly on arthropods, particularly moths and crickets, as well as seeds and nectar (3) (8). During the nesting season, the yellow-shouldered blackbird stays close to the nest and therefore typically forages in the sub-canopy of trees, in pastures alongside mangroves and in the mangroves themselves (3).

The male yellow-shouldered blackbird returns to the breeding site around six to ten weeks before the beginning of the breeding season. The yellow-shouldered blackbird is monogamous and will usually return to the same site each year in order to pair up with the same mate (3). The male stands on the edge of the previous year’s nest and displays to the female. Once paired, the male will follow the female persistently until the female leaves the nesting grounds (3). The female builds or repairs the cup-shaped nest in preparation for the egg laying (3) (4) (6). An average clutch contains three eggs, and incubation starts after the second egg is laid. The female will incubate the eggs for between 13 and 16 days, during which time the male will bring food back to the nest. The Males roost communally and forage during the day (3). Once the eggs hatch, the male and female yellow-shouldered blackbirds share feeding of the young, both in the nest and after fledging (3).

The yellow-shouldered blackbird will engage in communal mobbing to defend against predators. Most often the male birds do this in order to protect females and their eggs, with many individuals physically attacking intruders or predators (3).

The biggest threat to the yellow-shouldered blackbird is brood parasitism by the shiny cowbird (Molothrus bonariensis) (2) (4) (5), which impacts on the blackbird’s reproductive success rate. As a result of the shiny cowbird’s presence on the mainland, the yellow-shouldered blackbird is increasingly nesting on offshore cays (6).

Habitat loss, primarily due to the conversion of land for agriculture, is another major threat to the yellow-shouldered blackbird (2) (5). Nest predation by the pearly-eyed thrasher (Margarops fuscatus) and increased mortality due to introduced predators is also causing population declines (2).

The U.S. Fish and Wildlife Service (USFWS) have produced a recovery plan to help prevent further declines in the yellow-shouldered blackbird population. This involves creating artificial nests which are carefully monitored, as well as trying to control the impact of shiny cowbirds and pearly-eyed thrashers on this species’ breeding success (9).

More information on the yellow-shouldered blackbird and other bird species:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

  1. IUCN Red List (November, 2011)
    http://www.iucnredlist.org/
  2. BirdLife International (November, 2011)
    http://www.birdlife.org/datazone/speciesfactsheet.php?id=9714
  3. Post, W. (1981) Biology of the yellow-shouldered blackbird Agelaius on a tropical island. Bulletin of the Florida State Museum, 26(3): 125-202.
  4. Post, W. and Wiley, J.W. (1976) The yellow-shouldered blackbird - present and future. American Birds, 30(1): 13-20.
  5. Post, W. and Wiley, J.W. (1977) Reproductive interactions of the shiny cowbird and the yellow-shouldered blackbird. The Condor, 79(2): 176-184.
  6. Skutch, A.F. (1996) Orioles, blackbirds and their kin: a natural history. University of Arizona Press, Tucson.
  7. Jaramillo, A. and Burke, P. (1999) New World Blackbirds: The Icterids. Christopher Helm, London.
  8. Raffaele, H., Wiley, J., Garrido, O., Keith, A. and Raffaele, J. (1998) Birds of the West Indies. Christopher Helm, London.
  9. U. S. Fish and Wildlife Service (1996) Yellow-shouldered Blackbird (Agelaius xanthomus): Revised Recovery Plan. U.S. Fish and Wildlife Service, Atlanta.