Yellow-shouldered Blackbird Agelaius xanthomus

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

Behavior

Behavior

The following account is summarized from Post (1981):

LOCOMOTION


Usually walks; hops when traversing rough ground. Capable of sustained flight, except during heavy molt. During the molt period (7 September 1973), birds flying low over water to island roost were knocked into water by bursts of wind; three swam against the wind to mangrove roots. The first bird swam 40 m in 6 min 20 s; the second bird, 30 m in 2 min 31 s; the third bird, 20 m in 2 min 13 s. They swam partially submerged, propelled by wings and legs; only head, back and tail above water. On leaving winter roosts, birds in flocks moved to feeding sites at a rate of 6.8 km/hour.

Communicative interactions.

At least 12 visual displays are distinguishable:


Wing-raise. Initially, the head is pointed up (Fig. 11) After about 2 s the bird lowers its beak and begins to raise its wings, the tail becoming increasingly fanned and the body plumage fluffed. The wing elevation phase takes about 1 s, at the end of which the beak touches the breast, and the ventral plumage is ruffled (Fig.). The wings are held up 2-3 s, and lowered in about 1 s, then the head is again pointed up. During wing elevation, the carpus is rotated forward, providing maximum frontal exposure of the epaulets. Wing-raise is usually repeated every 3-5 s. Wing-raises are symmetrical or asymmetrical. Accompanying vocalizations are growl, pee-puu, queea, and cut-zee.


Bill-up. Head is rotated upward, near the vertical. Occurs at close quarters, between other Yellow-shoulders as well other species.


Bill-Down. Head is lowered and bill is pointed toward the abdomen. The head plumage may be ruffled. Yellow-shoulders give the head-down when near conspecifics in situations similar to those in which they give bill-ups, but the posture may indicate a greater tendency to escape or a greater conflict between escape and attack than does bill-up. Bill-down is often given by birds that are moving or about to move while in the presence of conspecifics. This posture, also called Head-down, may lead to the giver’s being allopreened by another Yellow-shoulder or by a cowbird (Post and Wiley 1992).


Head-Forward. The head is extended toward opponent, while the legs are flexed and plumage normal, fluffed, or sleeked (see illustrations in Post and Wiley 1976). Often seen at feeding sites during agonistic encounters.


Head-in. Bird crouches, retracts head, fluffs or ruffles plumage. Bill directed at nearby bird, may be gaped.


Solicitation (Wing-flutter). The body feathers are fluffed, belly feathers ruffled, wings are held out from body and vibrated. Legs are flexed, tail is spread, and may be elevated. Wings occasionally raised asymmetrically when fluttered. Females wing-fluttered when soliciting begging for food from their mates or soliciting, usually near their nests. Males copulate with soliciting females only at the nest.


Wing-Trail. As the bird walks slowly, the wings are lowered at the carpals and the remiges are spread so that the feathers may touch the ground. Body feathers are raised to varying degrees, but the rump feathers are usually ruffled. The rectrices are spread and may also drag on the ground. The beak is horizontal or pointed slightly down. The display was given by a female as she left her nest, in response to an approaching predator.


Moth Flight. This display takes the form of short flights in which the wings are moved slowly and with small amplitude. Females give moth flights when either approaching or leaving the nest and the male was near.


Male Nest Advertisement. Although females construct nests, during the period of early pairing, males stand in the cups of old nests and pull or jab at nest material. They also crouch inside the nest and push against the sides with their breasts. Sometimes males carry nest material, but usually drop it nearby. These activities occurred when the female was present.


Sleek. The body plumage is compressed, while the bird may flex its legs, as if to fly. Occasionally the bird assumes an erect posture and sleeks the neck and breast feathers, in which case sleek may indicate readiness to fly at an opponent.


Food-begging. Begging postures for young Yellow-shoulders are similar to those described for other icterids. The head is held in, the beak is pointed up and gaped. Wing tips are held into the body, while the wing is vibrated at carpus. Young 24 days post-fledging begged for food.


Sexual Chasing. On breeding areas, 2-4 birds occasionally chased females, ending in one bird’s pursuing her into the mangrove roots, occasionally into the water. These chases were accompanied by loud calls (queee, check, and cut-zee). On occasion, pursuers fought each other.


Bill-Wiping. Bill-wiping serves to clean the bill, usually after feeding or preening. These movements also occurred in seemingly irrelevant circumstances, and more frequently than body maintenance alone seemed to warrant.


SELF-MAINTENANCE

Roosting: The Yellow-shoulder uses diurnal and nocturnal roosts all year (Post and Post 1987). Diurnal roosts are near feeding grounds, and are usually located in the inner canopy of large trees. It also roosts during the day under roofs of structures such as dairy sheds.


Nocturnal roosts are at sites isolated from ground predators. Yellow-shoulders roost with Greater Antillean Grackles (Quiscalus niger), Shiny Cowbirds (Molothrus bonariensis), Gray Kingbirds (Tyrannus dominicensis), and Mourning Doves (Zenaida macroura). On the coast, the blackbird roosts are over water on mangrove islands, also in the tops of coconut and royal palms. In inland roosts at San German and Lajas, blackbirds roosted on the superstructures of electric transformer stations.


Yellow-shoulders, often accompanied by cowbirds and grackles, gather at staging areas and then fly to roosts in flocks. Pattern of arrival at roosts was same for grackles and Yellow-shoulders; average time of first arrival was 104-105 min before sunset. The average light intensity at first arrival was 313 fc; flights ceased 1-2 min after sunset (79 fc). Mean duration of roost entry was 102 + 20 min (n=14 days).Due to slight seasonal variation in day length, little difference between seasons in arrival and departure times.


Yellow-shoulders leave roosts in flocks, often accompanied by grackles and cowbirds. Roost departure began 20 min before sunrise, and continued for 1 hr. These flocks first arrived at feeding grounds that were 4.5 km from the roost in 39.9 + 7.3 min (n=14 days). Rate= 6.8 km/hour.


Numbers using roosts vary between seasons. In southwestern Puerto Rico, average Yellow-shoulder roost sizes were: 413 in spring, 431 in summer, 737 in fall, and 955 in winter. The largest number at any roost was 3525 (in San Germán, 12 February). A fall roost (4 September) contained 1000 blackbirds, served 10-km area along coast. On Mona Island, site fidelity also varied with season (Hernandez-Prieto and Cruz 1989).


On Mona island, roosts are in cavities or ledges of cliffs, up to 45 m above water. The largest number roosting at one time was 433 birds, which occupied 8 cliffside sites (Hernandez-Prieto and Cruz 1989).

Sunning and bathing. Sunning birds orient the side of body to face the sun, tilt body to obtain maximum insolation. Plumage is ruffled, wings drooped; bird often gapes (pants). Bathes by entering breast-first, submerging body, but not head; quivers wings and tail; emerges from water, fluffs plumage, shakes body, and preens rectrices and remiges. Then moves to shade, where body feathers are fluffed and preened (WP).

Anting. Yellow-shoulders were recorded anting 6 March 1974 at La Parguera (Post and Browne 1982). Groups of 15-20 birds gathered on the ground and activity applied harvest ants (Pheidole sp.) to their remiges, breasts and upper tail coverts. Five anting individuals, which had positioned their wings under bodies, fell over ("tumbled;" Whitaker 1957, Post 1993).

Allopreening. In roosts, Yellow-shoulders occasionally interrupted bouts of autopreening to preen other Yellow-shoulder perched next to them (21 cases seen). Yelow-shoulders occasionally preened cowbirds (Post and Wiley 1992).

Daily time budget. Time budgets of adults nesting on offshore cay (1974): when eggs were in the nest, the female spent 72% of daylight period incubating; male did not incubate. Foraging (in nest vicinity): female 2%; male 11%; rest: female 3%; male 20%; body maintenance: female 1%; male 12%; aggression: female < 1%; male 2%; at nest: both 1%; flight: female 1%; male 2%; out of view (presumably foraging on mainland): female 17%; male 53% (Post 1981a).


Time budgets in salinas during nestling period: female brooded 45% of day, male did not brood. Foraging: female < 1%; male 11%; rest: female 1%; male 3%; body maintenance: both < 1%; aggression: female < 1%; male 5%; at nest (care of young): female 10%; male 6%; flight: both sexes 3%; out of view: female 41%; male 72%.

Territoriality

The following account is summarized from Post (1981):

In March and April, when small groups of blackbirds visit nesting areas, pairs localize their activities at nest sites, which males defend from other Yellow-shoulders. Males also begin defending females they accompany.


Yellow-shouldered Blackbirds nest in loose aggregations, which appear to result from active attraction among birds. In salinas, shortage of suitable nest sites, particularly cavities or artificial nest sites, may contribute to aggregation. But in pastures, pairs nest in the same tree, although other, seemingly identical, trees are available. In San Germán, royal palms were common, but pairs nested in the same palm or in adjacent trees, leaving many widely-spaced sites unoccupied.


The average distance between 15 occupied nests in salinas was 15.9 +7.6 m, with a range of 5-35 m. Two pairs nested simultaneously in different holes in the same dead tree. On nesting cays, distances between three nests were 11.0-12.5 m. In pastures, two nests were in the same tree, 3 m apart. In San Germán, two nests were in the same royal palm, 3.5 m apart.
By supplanting, chasing and displaying the male Yellow-shoulder defends a small area, usually 3-5 m in radius around the nest. The female is rarely involved in territorial aggression, and then only when a intruder is immediately at the nest. Degree of nest defense varies during different stages of nesting. The male is most aggressive during the courtship and egg-laying periods. As nesting advances, he is progressively tolerant of the intrusion of other Yellow-shoulders. When young are in the nest, it is not uncommon for intruding Yellow-shoulders to approach within a meter of the nest, and in the male’s absence, to the nest edge. Intruders appear to show interest in the young, but do not help. Birds occasionally stole materials from new nests, but beyond this, did not interfere.


When females were away from the nest, males remained nearby: one nest with eggs was left unguarded only 12% of the time, although the female was away 37% of the day. After the eggs hatch, on average the nests were left unguarded 44-78% of the day.


Individual distance: In roosts, Yellow-shoulders may sit in contact, and are occasionally preened by conspecifics, rarely by cowbirds. During sexual chases or fights, some individuals are physically forced to the ground or into the water.

Sexual Behavior

Mating system and sex ratio The social system of the Yellow-shouldered Blackbirds is characterized by social monogamy, mate fidelity, and high male investment in reproduction. While pair formation occurs at nest sites, pair maintenance is also based on site-independent interactions of the pair.
Based on 25 marked pairs (La Parguera, 1974-1976) involved in 43 nest attempts, no cases of polygamy were seen. Only one instance of mate-switching was noted, a case in which the mates of both birds had disappeared.


Pairs stayed together despite repeated failures. In several cases, pairs moved together to new sites, both within and between years In 17 cases of renesting within the same year, 16 involved retention of the previous mate: 9 cases of one renesting and 4 cases of 2 renestings. One pair in 1974 remained together on same cay although unsuccessful in three nest attempts; the next year pair nested on different cay 275 m away.
In winter, adult males consistently outnumbered females; e.g., on 15 February 1975, the sex ratio of all marked birds (n=432) was 1.57 male:1.00 female. At the same time, the sex ratio of 71 marked birds at one feeding site was 3.73 male:1.00 female. At another station,, sex ratio among 53 birds was 1.9: 1.00. Differences between local sex ratios and overall population sex ratio implies segregation of sexes in winter, which is possibly related to the dominance of the larger male
In the breeding season, the sex ratio was close to unity: 48 males and 43 females (1.12 M: 1 F) were recorded at feeding sites outside the nesting areas.

Social and interspecific behavior

The following account is summarized from Post (1981):

Nest sites are defended by both sexes; most interspecific intruders are met by the male. On cays, males challenged Green Herons (Butorides striata) and Yellow Warbler (Dendroica petechia), both of which nested nearby. At inland locality (San Germán), during 14 hr at one nest, Yellow-shoulders challenged Canary-winged Parakeets (Brotogeris versicolurus) 17 times (14 challenges by male, 3 by female); Shiny Cowbirds (Molothrus bonariensis), 13 times (all by the male); Greater Antillean Grackles (Quiscalus niger), 5 times (4 by male, 1 by female).


Many intrusions, particularly by Shiny Cowbirds, were ignored. At San German, a male was present during 46 cowbird intrusions, but challenged only 13 (28%); in contrast, 78% of 18 Canary-winged Parakeets intrusions were challenged. At Ceiba, Wiley (1988) also found that blackbirds on nests did not always react to cowbirds, although some cowbirds approached within 3-5 cm.


Male Yellow-shoulders dominated females. In 20 aggressive interactions at winter feeding stations, males supplanted females 17 of 20 times. At these feeding sites, blackbirds and cowbirds were equally matched: Yellow-shoulders won 47% of 129 aggressive encounters. When entering roosts, Yellow-shoulders are occasionally chased and hit in flight by cowbirds Gray Kingbirds (Tyrannus dominicensis) occasionally chased Yellow-shoulders when they were roosting together (Post 1982).


In aggressive encounters (supplants and attacks), Yellow-shoulders and Shiny Cowbirds are equally matched; blackbirds won 60 of 119 dyadic interactions at a feeding tray. Cowbirds frequently gave a head-down display to Yellow-shoulders: a cowbird approaches another bird, lowers its head and ruffles the feathers of its nape> Head-down was seen 128 times in the blackbird’s breeding season, and 100 times at other times. The display resembles a preening invitation posture, and often results in the recipient’s preening the giver. Yellow-shoulders preened cowbirds in 34% of 170 incidents. The head-down most often occurred in roosts; Post and Wiley (1992) consider the display to be a form of aggression, related to maintenance of dominance.
Play. A Yellow-shoulder hung upside-down from a branch for 4-5 sec, 30 cm above a bird feeder. Another bird hung upside-down when it was supplanted by another blackbird. In the latter case, context suggests that this behavior may be an escape maneuver. Juveniles rapidly utter a short, muted growl accompanied by wing-raise. This may be examples practice singing, which may be classified as play.

Predation

Introduced black rat (Rattus rattus) is arboreal and depredates nests, also may kill female on nest. In 1977-1982 at Boqueron, 20% of 295 blackbird-occupied cavities and nest boxes were appropriated by rats. In 1975-1986 at Ceiba 12% of 262 cavities were used by rats. In settled areas, house cats (Felis domesticus) kill fledglings and nestlings (Post and Wiley

Near Caguas, Perez-Rivera (1978) saw 2 Short-eared Owls (Asio flammeus) attack a group of roosting Yellow-shoulders, cowbirds, and grackles. In another instance, at Adjuntas, Perez-Rivera (1977) saw an owl, possibly a Barn Owl (Tyto alba), enter another blackbird roost. In southwestern Puerto Rico, diurnal avian predators probably are rare, as smaller raptors (accipiters and falcons) are uncommon visitors to coastal lowlands. In southwestern salinas habitat, Caribbean Martins (Progne dominicensis) take over artificial nest sites, occasionally building nests over blackbird progeny. Martin interference was most severe when wooden bluebird-type boxes were provided. Fewer martins occupied nests made of PVC. When PVC pipes were furnished with an elbow, the martin problem was eliminated (Cruz et al. 2005).

In their nest vicinities, birds mob, and also dive at, humans. Diving birds come within a meter of intruder but do not make contact. Birds nesting close to each other respond in concert when mobbing predators (humans, monkeys, Little Blue Herons (Egretta caerulea), and Smooth-billed Anis (Crotophaga ani). When a predator is detected, an incubating female often remains on the nest, but as predator nears, leaves nest, perches 4-5 m away, and give checks and chwips. In salinas, birds leave nests when Black-necked Stilts (Himantopus mexicanus) mob predators (Post 1981a).

The intensity of predator-mobbing by both sexes increases as young became older. Adults seldom dive at predators when nests contain eggs, but do so when young are in nest. Although Yellow-shoulders mob ground predators, when hawks or vultures approach, they become silent, often moving to dense vegetation.

Yellow-shoulders on mainland Puerto Rico maintain distances of > 3 m from humans, but on Mona island, 30 birds foraging together in scrub: "continued foraging and displaying within three feet of observers" (S. Silander).

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