Yellow-shouldered Blackbird

Agelaius xanthomus

Description

The yellow-shouldered blackbird, similar to the better-known red-winged blackbird (Agelaius phoenicus ) of North America, is about 7-9 in (18-23 cm) in length. Adults are predominately dark to neutral gray, with a yellow shoulder patch, usually edged with a narrow white margin.

There are two subspecies of the yellow-shouldered blackbird: A. xanthomus xanthomus, known only from Puerto Rico and Vieques islands, and A. x. monensis, which is restricted to Mona and Monito islands.

Behavior

Yellow-shouldered blackbirds feed primarily on insects but will eat some plant material such as cactus fruits. They are monogamous and pair six to 10 weeks before breeding. Males establish and defend limited territories around nesting sites, which the females also defend after nests are constructed.

Blackbirds at the eastern end of Puerto Rico also use cavities or hollows in dead mangroves, and those on Mona Island place their nests on the ledges or in crevices of the sheer coastal cliffs. Nesting pairs often aggregate, and in several cases, nests have been located in the same tree as close together as 11.5 ft (3.5 m).

Clutch sizes are two or three. Only the female incubates and broods, although both sexes bring food and clean the nest. Incubation lasts 12-13 days and the nestling period ranges from 13-16 days. All incubation and breeding is handled by the female, with the pair sharing equally with feeding the young. Usually only one nesting is attempted per year. The females are known to reach breeding age during their first year; sexual maturity of the male remains unknown.

Habitat

The yellow-shouldered blackbird is known to nest in eight different habitat types: mangrove pannes and salinas (coastal mangrove zone), offshore red mangrove cays, black mangrove forest, lowland pastures, suburbs (a university campus), coconut and royal palm plantations, cactus-scrub, and coastal cliffs. On Mona Island, it nests on pinnacles along the coast.

Distribution

During the mid-1800s the yellow-shouldered blackbird was abundant in the San Juan area of Puerto Rico, and as late as the 1930s the bird was still common in lowland areas. After this time, there was no available information until 1972, when numbers were estimated at about 2,400 individuals in three principal population centers: coastal southwestern, coastal eastern Puerto Rico, and Mona Island.

Present distribution of the species includes the Boqueron Commonwealth Forest in southwestern Puerto Rico, the Roosevelt Roads Naval Station in eastern Puerto Rico, Salinas in southern Puerto Rico, and Mona and Monito Islands.

In southwestern Puerto Rico, a mean of 258 and 352 yellow-shouldered blackbirds have been counted during pre-and post-reproductive seasons, respectively. Approximately 400 birds are known from Mona Island, 20 have been sighted in Salinas, and approximately 14 were observed at Roosevelt Roads Naval Station. Migration reports indicate that the source of the blackbirds on Monito Island may be western Mona Island, since the blackbirds reportedly fly to Monito from the Cabo Barrio Nuevo area of Mona.

Estimated populations in 1976 were 200 in eastern Puerto Rico; 2,000 in southwestern Puerto Rico; and 200 on Mona Island. At the time, it was estimated that the population in the Southwest was declining at a rate of approximately 20% per year.

Threats

Since the turn of the century sugar cane cultivation and housing development have transformed extensive areas of Puerto Rico into unsuitable habitat for the blackbird. By 1968 only about 25% of the blackbird's original feeding and nesting habitat remained undisturbed.

The introduced rat (Rattus rattus ) and the mongoose (Herpestes auropunctatus ) are widespread in lowland areas, and as a result blackbirds have been forced to nest on small islands, in cactus and palm fronds, or on steep cliffs, out of reach of the predators. The aggressive, cavity-nesting pearly-eyed thrasher (Margarops fuscatus ) displaces the blackbird in some suitable habitat. Nest parasitism by shiny cowbirds is the most critical limiting factor for the blackbird. The shiny cowbird has become one of lowland Puerto Rico's most common birds and is now found spreading throughout the West Indies.

Conservation and Recovery

The Commonwealth Department of Natural and Environmental Resources, through a cooperative agreement with the U. S. Fish and Wildlife Service (FWS), has been conducting a program for the control of shiny cowbirds and the monitoring of yellow-shouldered blackbird reproduction in southwestern Puerto Rico (specifically, in the Boqueron Commonwealth Forest) since 1983. In 1984, the Natural and Environmental Resources Department established an office in the Cabo Rojo Natinal Wildlife Refuge to monitor yellow-shouldered blackbird reproduction and conduct a cowbird control program.

A zoning plan was established for the Roosevelt Roads Naval Station in 1980 to minimize the impact of base activities on the blackbird, and since 1977, 96 nest boxes have been placed in the Boqueron Commonwealth Forest. The Cabo Rojo National Wildlife Refuge Youth Conservation Corps has been active in the recovery effort, building nest boxes and cowbird traps. These programs appear successful. The Mona Island race receives complete protection within the Department of Natural Resources' Mona Island Refuge.

The 1996 revised Recovery Plan for the yellow-shouldered blackbird, from the FWS, notes that downlisting of the species could be initiated in 2020, if interim recovery criteria are met. The interim criteria call for the assurance of a self-sustaining population in the Boqueron Commonwealth Forest by enhancing the species reproductive success by at least 0.96% daily survival for eggs and chicks, and a reduction in parasitism rates by at least 20%. These criteria should be maintained for at least five years in the artificial structures. The criteria for delisting will be developed after modeling data obtained from natural nests in the Boqueron Commonwealth Forest and at least two additional areas in Puerto Rico, including Mona Island.

Among the actions needed to meet downlisting goals, according to the revised Recovery Plan, are the protection and management of yellow-shouldered blackbird habitat and populations; the monitoring of reproductive success in existing artificial nest structures; and the development of education programs.

Contacts

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

Boqueron Ecological Services Field Office
U.S. Fish and Wildlife Service
P.O. Box 491
Boqueron, Puerto Rico 00622-0491
Telephone: (787) 851-7297
Fax: (787) 851-7440

References

Post, W. 1981. "Biology of the Yellow-Shouldered Blackbird (Agelaius ) on a Tropical Island." Bulletin of the Florida State Museum, Biological Sciences 26(3):125-202.

U.S. Fish and Wildlife Service. 1983. "Yellow-Shouldered Blackbird Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.

U.S. Fish and Wildlife Service. 1996. "Yellow-Shouldered Blackbird Revised Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.