The following account is summarized from Post (1981):
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.
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.
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%.