Yellow-shouldered Blackbird Agelaius xanthomus

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

Appearance

Distinguishing Characteristics

A black bird with yellow "shoulders" (wing coverts). The sexes are similar.

Similar Species

Two other black birds of similar size co-occur on Puerto Rica: male Shiny Cowbird (Molothrus bonariensis) and the Greater Antillean Grackle (Quiscalus niger). Neither of these two species have yellow shoulder patches. The cowbird also is smaller, while the grackle is larger, with a longer, keeled tail.

Detailed Description

Plumages of males and females are similar; the male (A. x. xanthomus) is significantly larger than the female. Body feathers of adults are blackish neutral gray (Smithe 1974), and in the sunlight have a faint bluish green reflection (Ridgway 1902). In nominate xanthomus, the shoulder patches are "deep, rich, golden yellow color," close to spectrum yellow (Smithe 1974), The humeral patch is usually edged with a narrow white margin, and in adults under-wing humeral feathers are tinged with orange.


Juvenile plumage. When they appear in nestlings, the feathers of the humeral tract are buff yellow to spectrum yellow, tipped with grayish brown. In juvenile A. x. xanthomus, yellow of shoulder patch is lighter than in the adults; lower edge infused with black spots. Most juveniles retain their dark tipped humeral feathers until the first postnuptual molt (in the fall of the year after hatching). Other regions of the nestling and juvenilel plumage are dark grayish brown.

Molts

Post-nuptial (pre-basic) molt of Yellow-shouldered Blackbirds in southwestern Puerto Rico occurs after fall rains start; first replacement of remiges seen on 3 August, and the last molt of wing feathers on 20 December. Molt is heaviest during October and November. Some birds in August were reproductively active and were also undergoing molt. For example, a female captured on 9 August had a hard egg in her oviduct and was starting to replace her primaries (primaries 1-2 were half grown). Another female caught on 16 August had a vascularized brood patch, while her primaries 1-2 were new. Pre-Basic I molt occurs in January-February; apparently only involves flight feathers.

Bare Parts

Data from Ridgway (1902):

Iris: dark brown.

Bill: black.

Tarsi and toes: black.

Measurements

A. x. xanthomus, southwestern Puerto Rico (Post 1981): wing length (chord): male, 102.8 ± S.D. 2.0 mm (n= 366); female (n=216), 93.3 ± 2.0 mm.


A. x. monenesis (Barnés 1945): male (n=9): wing mean 104.4 mm (range 102.2-106.4 mm); tail mean 81.4 mm (range 77.0-88.4 mm); culmen from base: mean 21.6 mm (range 20.7-22.8 mm); tarsus: mean 24.6 mm (range 23.8-24.9 mm); female (n=4): wing mean 96.4 mm (range 95.5-98.0 mm); tail mean 74.6 mm (range 73.4-77.6 mm); culmen from base: mean 19.8 mm (range 19.7-20.1 mm) tarsus: mean 24.6 mm (range 23.8-24.9 mm).

Mass: Male (n=270): 41.4 ± SD 2.3 g; female (n=157): 35.5 ± 2.8 g (Post 1981).

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. https://doi.org/10.2173/nb.yesbla1.01