Yellow-shouldered Blackbird Agelaius xanthomus

  • Order: Passeriformes
  • Family: Icteridae
  • Polytypic: 2 subspecies
  • Authors: William Post


  • Year-round
  • Migration
  • Breeding
  • Non-Breeding
Distribution of the Yellow-shouldered Blackbird
eBird range map for Yellow-shouldered Blackbird

Generated from eBird observations (Year-Round, 1900-present)

Distribution in the Americas

Uncommon, locally distributed resident of mainland Puerto Rico, and Mona Island; non-migratory.

Distribution outside the Americas

Endemic to the Americas.


Breeds in at least nine habitats (Post and Wiley 1976):

1. Open areas (pans and salinas) in mangrove zone, sparsely covered primarily by black mangroves (Avicenia nitida), red mangroves (Rhizophora mangle), and by dead trees and stumps.

2. Red mangrove cays (small islands, 100-1000 m2) that are 250-300 m offshore;
found on southwestern coast.

3) Black mangrove swamp-forest, in eastern Puerto Rico.

4) Savanna-like lowland pastures. Blackbirds nest in the canopy of large deciduous trees such as ucar (Bucida buceras), 11-14 m in height.

5) Suburban and urban areas, where birds nest in tops of tall ornamental trees
such as royal palms (Roystonea borinquena).

6) Dry upland forest with dense shrub layer and tall trees (ucar, white-cedar, gumbo-limbo); often grazed and timbered.

7) Mesquite woodlands.

8) Coconut palms (Cocos nucifera) planted on beaches or recreational areas. Birds use palms that have metal rat guards around trunks.

9) Coastal cliffs. Nest in caves or ledges of vertical cliffs, such habitats used on Mona Island.

In the non-breeding season, found in many habitats: suburbs, towns, mangroves, mesquite forest, dairy farms, cultivated fields, and habitats at higher elevations, such as coffee plantations (Post 1981a).
In the lowlands adjacent to southwestern mangroves, habitat use of post-breeding birds (n=133 observations) was as follows:

(1) 68% in heavily grazed mesquite woodlands (mean vegetation height 12.7 m) with sparse understory.

(2) 13% in dense, semi-deciduous scrub < 2.5 m high).

(3) 12% in dry upland forest (mean height 12.9 m) with dense shrub layer and tall trees (ucar, white-cedar, gumbo-limbo).

(4) 8% in ruinate, (cleared and overgrazed scrub), 2-4 m high). (T. Nakamura, unpublished).

Historical changes

Historical status poorly unknown. Puerto Rico originally was forested, and species occupying open habitats possibly had limited (lowland) distribution. By 1912, most of the island was clear-cut for agriculture; subsequently, the blackbird was reported breeding throughout lowland Puerto Rico (Wetmore 1916). Taylor (1864) visited San Juan area and described it as "excessively abundant." As late as 1935 species was still common in lowlands (Danforth 1936). On Mona Island, Barnés (1946) stated that it was "found in coastal littoral where it is abundant and well distributed." Data from museum specimens indicate it was still common until 1940 (Post and Wiley 1977).
Reduction in agriculture began in 1940s, leading to natural reforestation of abandoned farmlands. Half of the island was abandoned farm or pastureland; 40% of island had regenerated to secondary forest. The effects of these landscape alterations not known, as no censuses made during 1940-1970.
In 1975, the world population was about 2800 (2400 birds in southwestern Puerto Rico, 200 in eastern Puerto Rico and 200 on Mona Island; Post and Wiley 1976). Beginning in the 1970s, or earlier, the blackbird was the main host of a newly-arrived brood parasite, the Shiny Cowbird (Molothrus bonariensis; Post and Wiley 1977). Due primarily to brood parasitism, between 1975 and 1982, the southwestern population decreased by 80% (Cruz et al. 2005). In 1982, the total world population was 771-1212 birds (300 in southwestern Puerto Rico, 12 in eastern Puerto Rico; 771-1212 on Mona Island. (Wiley et al 1991).
Since mid-1980s, in response to cowbird control, blackbirds numbers began increasing. The total world population is now believed to be about 1000 birds, most of which are in the southwestern Puerto Rico municipality of Boquerón, and on Mona Island. The population that once existed in eastern Puerto Rico is extirpated. The status of the breeding population at San Germán, an interior site, is not known.

Fossil history

No information.

Recommended Citation

Post, W. (2011). Yellow-shouldered Blackbird (Agelaius xanthomus), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA.