There is one adult male in the photo. The gray brown birds are females. I believe the mottled black bird to the left is an immature male.
This species does not build its own nest, but instead parasitizes the nests of other species. Over 200 host species have been identified. It has spread by island hopping from South America taking advantage of the conversion of land to cattle ranching. Its expansion in the Antilles has caused serious declines in some of its host species. In Puerto Rico, were it first nested in 1955, it has seriously endangered the endemic Yellow-shouldered Blackbird and Puerto Rican Vireo. An active program of cowbird trapping
continues to prevent the decline of those species in southwest Puerto Rico. Males can be very difficult to tell from Yellow-shouldered blackbirds which often have the yellow lesser coverts concealed by black scapulars, thus looking all black. In good light, the Shiny Cowbird males have shiny dark blue wings contrasting with a deep purple gloss on the body and head. These cowbirds also seem to have a somewhat shorter and thicker bill compared to Yellow-shouldered Blackbirds.
This is M. b. minimus which ranges through the Antilles to Florida. It is the smallest of the seven recognized subspecies.
Digiscoped with Panasonic DMC-LZ5 | Nikon FieldScope III | 30XWA | hand-held (no adapter)