The following account is based largely on Post (1981a):
Pair formation. In southwestern Puerto Rico, pairs form in nesting areas of previous years. Individuals and small groups visit these sites all year; in late December, males begin singing and intermittently defending sections of trees. The start of pairing coincides with spring rains (Post 1981a).
In 1975, in Boqueron, pairing began in late March, but egg-laying started in late May. On Mona Island, Barnés (1946) noted a long delay between the start of gonadal growth and deposition of eggs. Five males and 2 females he collected in late March and early April had enlarged gonads, but he found no eggs until early July. Perez-Rivera (1980) noted that breeding on Mona may begin in early February.
First brood. In southwestern Puerto Rico, nestlings were found throughout the summer dry period (June-August). Most young become independent in September, at the start of the rainy season.
Length of breeding season. In southwestern Puerto Rico, first clutch completed 2 June, last clutch completed 8 September. At San Germán, nested as early as 17 May (Post 1981). At Ceiba, breeding season coincides with spring rains; range of egg-laying dates was mid-April to mid-August; and peak of nesting was in 3rd week of May (Wiley 1988). Perez-Rivera (1980) stated that in northern Puerto Rico (San Juan and Cayey), breeding may last through November, depending on rainfall. On Mona Island, (Barnés 1946) found birds breeding from 23 March to 27 July.
Selection process. Males appear to choose nest sites. Both sexes visit nesting grounds in groups. The activities of individual males become localized: males visit nest sites of previous years, stand at the nest, alternately sing and pull old nest material. Nearby birds of both sexes sing repeatedly. Males begin following and defending females that visit nest site.
Microhabitat. In mangrove salinas, nests are placed on branches of isolated mangroves, also in crevices or hollows of dead trees. The average height of 10 open cup nests was 0.9 ± SD 0.7 m, and the average height of 8 nest trees was 1.8 ± 0.9 m.
Yellow-shoulders use two types of cavities, holes in the sides of dead trees and holes in the tops of stumps. Openings to cavities were 4-19 cm in width, and cavity depths were 5-30 cm. The average above-water height of the eggs was 0.61 ± 0.32 m.
On cays in southwestern Puerto Rico, nests were 0.2-4.0 m above water on branches or roots of mangroves. Mean above-water height of eggs in 15 nests was 2.14 ± 1.2 m. Tides around La Parguera are usually less than 0.6 m, and no nest was flooded. The average distance to the mainland of 21 offshore nests was 1153 m (range 880-1460 m).
The heights of 4 nests in royal palms were 10.7- 19.8 m. Nests were built on the midribs of palm leaves, 1-2 m from the axils. The nest material was woven under the leaf rib and around the leaflets that extended up on either side of the nest. Other nests were built in axils of coconut trees, usually only those protected by rat guards.
In upland pastures and coastal scrub, nests were built in the outer canopy of deciduous trees 10-12 m high. Nine pasture nests were on main branches and crotches at an average height of 5.6 m (range 4.3-7.6 m). Three nests were 1.5-4.0 m from the main trunk. Nests were usually screened from above by surrounding leaves, but were visible from below. Danforth (1926) reported blackbirds nesting in deciduous trees at Cartagena Lagoon.
On Mona Island, blackbirds nest on ledges or cavities in cliffs (Post and Wiley 1976). Barnés (1946) found 7 nests in cacti (Selenicerus) on the central limestone plateau of the island.
Construction process. Females build the nest alone, but occasionally are accompanied by males when gathering material. For one nest at San Germán, the female made 18 trips in 0.5 hr. One-way distances were 60 m (16 trips), 140 m (1 trip), for a total of 2680 m. Stage of nest-building are construction of platform, building of cup, lining of nest. Duration of nest-building (platform stage to deposition of first egg): 3-9 days.
Structure and composition. In mangroves, birds made nest platforms of leaves, grass, cotton, and occasionally paper, string, plastic bags, and twine. In addition to materials listed above, nest cups made of grass leaves and stems, cotton. In final stage, nests lined with fine grass leaves, stems.
Some cavity nests were used more than once during the same nesting season, and also repeatedly from year to year. Cavity nests were lined with grass; material from previous nests formed the platforms. All cavity nests were infested with mites (Post 1981, Wiley et al. 1991).
Composition of five cay nests averaged 70% Sargassum, 12% turtle grass (Thalassia), 13.6% wrack (plastic bags, nylon twine, bamboo roots, pelican feathers, burlap), and 4% fine grass. Cay nests often had material hanging under them; 2 nests had sargassum hanging on a branch above the nest, where the females had tangled the material during nest construction. The weight of one freshly collected nest was 123.4 g. The same nest weighed 99.3 g when dry.
Barnés (1946) described 7 nests on Mona island as a "global structure, somewhat elongated, built of dry grass, small twigs, and other vegetable matter."
Dimensions. In southwestern Puerto Rico, nest dimensions of open nests were: inside width: 8.1 ± SD 1.1 cm (n=9); outside width: 5.9 ± 1.6 cm (n=9); inside depth: 5.6 ± 2.5 cm (n=8); outside depth of cup only: 11.6 ± 3.1 cm (n=10); outside depth of nest, including material hanging below or draped above the nest: 25.0 ± 6.7 cm (n=8). All nests lacked domes, and adults did not appear to manipulate the vegetation to improve cover. Surrounding foliage provides cover and shade. Degree of cover at mid-day, indicated by ratio of light (fc) in nest to light in open was 89%.
Mainentance and reuse of nests. Both cavity and open nests were reused within and between years. On offshore cays, when sargassum is scarce, birds steal material from each others’ nests. Pairs nesting on cays reused their own nests and those of others in same season. On mainland and cay habitats, if old nests are gone, blackbirds build new nests in previous site.
Coloration. Eggs are purplish-brown, speckled more heavily at one end; background color light blue.
Size (n=59). In eastern Puerto Rico: length: 22.69 ± 0.14 mm; breadth: 16.75 ± 0.10 mm (Wiley 1988). In southwestern Puerto Rico: (n=20) length: 22.8 ± 1.7 mm; breadth: 17.1 ± 0.6 mm.Mass: (n=10; 5 nests): mean 3.80 ± 0.40 g; range 3.3-4.5 g.
Only female incubates. Incubation begins after the second egg is laid; hatching is asynchronous. One blackbird egg hatched 13 days after it was laid, or 12 days after incubation began, and another hatched 13 days after it was laid and incubation had begun.
Early in nest cycle, female stays on nest at night, while male join a communal roost. One female at San Germán spent 77% of the daylight period incubating, while another nesting on cay incubated 72% of the daylight period. The average duration of 37 daytime incubation bouts was 23.5 + 16.7 min, (range 2.5-99.5 min; two nests). The average time that the two females spent off their nests during these periods was 9.6 ± 8 min, range 1-36 min (n=32).
The longest continuous incubation time, 99.5 min, was recorded on 19 July, between 11:16 and 12:55 h; the eggs hatched on 21 July. At San Germán, the maximum percentage of any diurnal interval that was spent incubating was 86% (14:33-16:44 h), while the minimum percentage was 57% (10:11-12:22 h). No significant diurnal variation in time spent on the nest or in the length of incubation bouts, but a slight reduction in time spent incubating during midday, probably depending on degree of shading at nest.
Growth and development. The average nestling period is 14.6 ± 1.3 d, range 13-16 d (n=10 young in 5 nests). Growth pattern described by logistic curve; growth rate (K; Ricklefs 1967) is 0.458. Asymptotic weight is 28.5 g (89% of adult weight). Time to grow from 10% to 90% of asymptotic weight is 9.6 d. Nestlings remain in nest for 4-5 days after attaining asymptotic weight.
Brooding. Only female broods. At one inland nest (San Germán) the female brooded young (< 3 days old) 45% of the daylight period, while at a cay nest on coast, the female brooded young < 3 days old 16% of the daylight period. The difference in brooding times may be related to the greater exposure of San Germán nest: most brooding occurred in middle of day. While females are on the nest, males deliver food to them, although this behavior was variable.
Females regularly brood young through day 5 (counting day of hatching as day 0). When females stop brooding regularly during the day, they begin leaving the nest sites at night, flying with the males to communal roosts.
Feeding. Sexes deliver food at equal rates. On occasion, males feed incubating or brooding females. Males sometimes deliver food to females at nest for transfer to young. Males begin feeding young at the same time as females. On the mainland, average rate of delivery per nest per h for 11 nests (n=73 h) was 12.7 ± SD 5.8. The average delivery rate per young per h was 5.4 ± 2.0. There was no significant diurnal variation in food delivery rates. Pairs nesting on offshore cays delivered food at lower rates than those on the mainland. During 2.5 h at 2 cay nests, the female made 17 trips, totaling a minimum of 32 km; male made 24 trips (45 km) (Post 1981a).
Yellow-shoulders bring arthropods as well as vegetable matter to young. Adults often carry more than one item at a time when delivering food. They also regurgitate plant material and fragments of arthropods. The bulk of 25 food samples taken from nestlings consisted of wood-boring beetles (Buprestidae), average length 11 mm; tree crickets (Gryllidae), 20.6 mm; larvae and pupae of moths (Olethreutidae and Noctuidae), larvae averaging 11.9 mm and pupae averaging 14.1 mm; arboreal spiders (Anyphaenidae), 9.1 mm. The larvae of a moth, Ecdytopha, which feed on leaf buds of red mangrove, is a common food of nestlings on cays. The larvae appear at time of new leaf growth, which coincides with rains occurring in April-May (Post 1981a).
Vegetable matter occurred in 15 of 25 food samples. Rice and grain were in 7 samples); bread and flour products (5), and monkey chow (4). On average, vegetable material composed 29% of food volume (range, 5-100%; n=15) (Post 1981a).
On Mona Island, food brought to nestlings was composed primarily of arthropods (44% of 52 records) fruits (29%) and flowers (17%) (Hernandez-Prieto and Cruz 1989).
Nest sanitation. Both sexes clean the nest cup and preen young. When standing on the nest rims, adults peck at their legs and nest surface, apparently to remove mites. Both sexes remove egg shells and fecal sacs; the latter are occasionally eaten. Adults peck cloacal region of nestlings to induce defecation.
The Yellow-shouldered Blackbirds is the main host of a brood parasite, the Shiny Cowbird (Molothrus bonariensis). The cowbird had arrived in western Puerto Rico by 1969, and in eastern Puerto Rico, probably 10 years earlier. In 1972-1976, in eastern Puerto Rico all of 26 Yellow-shouldered Blackbird nests were parasitized; in southwestern Puerto Rico, 38 of 54 nests had cowbird eggs. In southwestern Puerto Rico, parasitism reduced blackbird reproductive success by 0.39 fledglings per nest. Effects of parasitism more severe in eastern Puerto Rico: 18 nest produced only 3 blackbirds, but 17 cowbirds. In the Southwest, 35 nests produced 27 blackbirds and 24 cowbirds.
In 1975-1983, mean number of blackbird chicks per parasitized nest: 0.25 ± 0.65 (n=152); non- parasitized nests: 0.33 ± 0.78 (n=12). Multiple parasitism (> 2 cowbird eggs per nest) occurred in 78.1% of 114 nests (Post and Wiley 1977).
Egg-puncturing by cowbirds was the main source of egg loss: 88.2% of nests with punctured eggs failed to hatch any young, while 40.5 % nests without punctured eggs had hatchlings. Blackbirds often abandoned nests that had damaged eggs. Abandonment occurred within 3 d of cowbird visit; 92% of abandoned nests had damaged or removed eggs, as opposed to a 30% abandonment rate for unparasitized nests. The puncture rate is possibly related to cowbird density (Post and Wiley 1976, 1977, Nakamura and Cruz 2000).
Cowbirds locate blackbird nests by cryptically monitoring blackbirds’ behavior. Cowbirds also find nests by flushing females, and by watching nest-building and territorial defense. To determine when to parasitize, cowbirds remain vigilant, and repeatedly visit nests to determine their status. At Ceiba, where parasitism rate was 85%, Wiley watched 10 blackbird nests during 42 h, recording 111 cowbird visits (2.71 visits/hour). Visitation rate peaked (9.8 visits/hour) when first blackbird egg laid. Black-cowled Oriole (Icterus prosthemelas) nests had the next highest cowbird visitation rate (1.38 visits/hour).Yellow-shouldered Blackbird was preferred host: the nests of 17 other potential host species were watched; only 5 of 110 nests were parasitized (Wiley 1988).
The average nestling period was 14.6 ± 1.3 days, range 13-16 days (n=10 young in five nests).
Young follow and beg from parents for extended periods. The peak of the fruit crop occurred in October-November, and adults fed bananas and granulated sugar to young that follow them. On a nesting cay, two young, 7-day old post-fledgings were 5-6 m from their nest; after 24 d, they were within 50 m of the nest (Post 1981a).