Yellow-shouldered Blackbird Agelaius xanthomus

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

Demography and Populations

The following account largely is based on Post and Wiley (1975, 1976) and on Post (1981a, Post 1981b).


Age at First Breeding. Males were not found nesting the first summer after hatching year. Females banded as juveniles nested the next breeding season. One of these mated with a male banded as an adult 2 years earlier, successfully fledging young. The other paired with an unmarked male; their nest contained 3 blackbird and 4 cowbird eggs, and was abandoned.

Clutch. The average clutch size of 30 undisturbed nests at La Parguera was 3.03 ± 0.32. There was no significant difference in the average clutch sizes of birds nesting on cays and on the mainland, nor between those having cavity and open nests.

The distribution of clutch sizes was: 2 eggs, 1 nest; 3 eggs, 27 nests; 4 eggs, 2 nests. The average combined clutch size of 22 nests with cowbird and blackbird eggs was 5.59 ± 1.33. The most cowbird eggs contained in any Yellow-shoulder nest was six, in a nest with three blackbird eggs. All nine of these eggs hatched, the nest fledging two blackbirds and three cowbirds.

Annual reproductive success. In 1975, before blackbird management began, only 17% of 18 nests produced any fledglings. At La Parguera (salinas habitat), 40% of 35 nests produced at least one fledgling. Production in salinas was 0.17 fledglings per nest; in mangrove forest at Ceiba, 0.77. Relatively lower success at Ceiba was related to higher rates of cowbird parasitism there, and also to higher success of cavity-nesting birds at La Parguera.

At La Parguera in 1975, excluding offshore nests, 86% of seven cavity nests were successful, compared to 27% success rate for 22 open-cup nests. The cavity nests produced 1.71 fledglings per nest, while the latter produced 0.50.

Nests built on offshore cays rarely were parasitized, and were more successful than mainland nests: 58% of 19 cay nests produced young, compared to 40% for 35 mainland nests. Number of young fledged per nests was about same for the two habitats: offshore cays: 89% (15/19); mainland salinas: 0.77 (27/35). The relatively high success of the mainland nests, despite the fact that a higher proportion of them was parasitized, was because cavity nests had a lower parasitism rate than open-cup nests .


Annual survival: adult males (n=173): 83.2%; females (n=77): 80.5%; juveniles (n=32): 59.4%. Eight of 29 (27.6%) birds marked as nestlings survived to independence. Survival rates are based on observations of 250 birds color-marked during period December 1972-May 1974; censuses continued through 1975. Only residents of La Parguera area included; birds encountered only once were excluded; (Post 1981a).


Two forms of Mallophaga (feather lice), Philopterus agelaii and Machaerilaemus sp., were found in the plumage of 69% of 265 Yellow-shoulders. Significantly more males than females, and more adults than juveniles, had Mallophaga.

Nests had mites (Acarina: Onrniithonyssus bursa and Androlaelaps casalis). Infestation was particularly severe in cavity nests.

Of 215 Yellow-shoulders examined in hand, 41 (19%) were infected by avian pox virus. Possibly due to secondary bacterial infection and toxemia, 4% of 495 birds had lesions and tumors on tarsi, bend of wing, gape, and around eyes. Tumors on wings and legs interfered with movements. The eye of one individual was covered by a tumor. Deformed bills or feet hindered body maintenance, and some deformed individuals had 100-200 feather lice. Over a 15-month period, 26 infected birds had a survival rate of 23%, while the comparable figure for 38 apparently unaffected birds was 42%.


The causes of mortality for 81 eggs were: hatching failure, 16%; disappearance of egg, 23%; punctured egg, 3%; nest abandonment, 25%; faulty nest construction, 3%; predation 31%. Most (67%) of egg mortality was directly or indirectly related to brood parasitism. Female blackbirds may have been unable to properly turn, incubate, or shade excessive numbers of eggs, and some nests had as many as nine eggs. Blackbirds abandon nests that contain large numbers of eggs or eggs punctured by cowbirds

Mortality was higher during the egg stage than during the nestling stage. About half the egg losses were within broods: 58% of 81 blackbird egg losses and 56% of 52 cowbird egg losses fell in this category. Predation accounted for about 35% of the combined egg losses of blackbirds and cowbirds. For the most part, the remaining egg mortality was caused directly or indirectly by cowbird brood parasitism.

Mortality in the nestling stage was relatively low, and due mainly to death in the nest, possibly due to starvation (69% of 16 young).The average age of 12 young that disappeared was 4.3 ± 3.0 days. Faulty nest construction was related to 19% of nestling loses; 13% of losses were attributed to predation.

All 35 mainland nests, but only 3 of 19 cay nests were parasitized by Shiny Cowbirds (Molothrus bonariensis). Of the 35 mainland nests, 31% were destroyed by predators, in comparison to 26% of 19 cay nests.

The success of unparasitized nests was significantly higher than that of parasitized nests. A contrast between the nesting success of open nests that were parasitized and not parasitized, making adjustment for the slightly larger number of eggs in unparasitized nests, indicates that Shiny Cowbirds reduced Yellow-shouldered Blackbird production by about 0.39 fledglings per nest.

Hurricane Hugo (winds 140-150 mph) passed over Puerto Rico in mid- September 1989. At Ceiba, only two Yellow-shouldered Blackbirds were found afterwards. Haney et al. (1991) stated that Ceiba population was directly affected by hurricane, but the population already had declined, and no census had been taken before the hurricane. Habitats in western Puerto Rico were not affected by the hurricane.


Initial dispersal from natal site. The average distance between hatching places and winter feeding grounds was 1549 m, range 980-2680 m (n=7).

Fidelity to breeding site. When their nests failed, pairs moved together to new sites. Mate fidelity overrode site fidelity: 25 marked pairs remained together even if new sites were far from previous ones. 1) A pair remained on same cay although unsuccessful in 3 nest attempts; next year the pair reformed, nesting together on another cay 275 m away. 2) A pair nested successfully, then in same season moved together to another cay 1250 m away, and the next year nested together at that site. 3) A pair nested unsuccessfully on 3 different cays, the birds remaining together during successive moves of 670 and 1250 m.

Home range. Total winter home ranges same between years. The average home range of 26 males was 2.56 ± SD 0.50 km2 (range 0.27-13.34 km2); 12 female home ranges averaged 1.73 ± 0.36 km2 (range 0.39-3.70 km2). Distances between successive sightings or recaptures on same day: males (n=200 movements): 1325 ± SD 95.2 m, range 25-5440 m; females: (n=200): 1630 ± 131.5 m, range 25-7260 m.

Individual birds ranged widely, but the sizes and shapes of home ranges remained the same during breeding and nonbreeding seasons. Many individuals used the same winter feeding site repeatedly. Out of 198 marked males, 32 (16%) used only one site; out of 90 females 24 (27%) used only one site.

Marked birds at La Parguera were permanent residents: no significant seasonal were detected. Some individuals banded on the coast dispersed inland during the nonbreeding season: 11 birds (6 males and 5 females) appeared in flocks at cattle barns near Lajas, 7.3 km north of La Parguera, but these birds returned nightly to the coastal roosts.


Density. Based on transects, winter density in eastern Puerto Rico mangroves was 1.0 individuals/1 km2 (Wiley 1988). Breeding season density in southwestern mangroves and adjacent uplands was 66.9 individuals/km2 (Post et al.6 1990). Post-breeding density in mesquite woodlands: 126.0 individuals/km2 (T. Nakamura, unpubl.).

Numbers. The world population was estimated at 2400 birds in 1975-1976. By 1983, blackbird numbers had declined significantly. The world population was estimated to be 771 to 1212 birds (300 in southwestern Puerto Rico, 12 in eastern Puerto Rico, 771-1212 on Mona Island; Wiley et al. 1991, Cruz et al. 2005). In 1982, the blackbird population in southwestern Puerto Rico was thought to be > 800 individuals (Cruz et al. 2005).

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.