The species engages in mobbing, a behavior in which a pack of birds, from one or more species, attack a known predator (usually to defend eggs or hatchlings). The yellow-shouldered blackbird was listed as an endangered species on November 19, 1976 by the U.S. Fish and Wildlife Service. Critical habitat was subsequently established for the entire Mona and Monito islands, the Roosevelt Roads Naval Station in Ceiba, an area expanding from Cabo Rojo to Guánica in the southwest region of Puerto Rico, and a small area in San Germán. Diseases such as avian pox have been associated with extinction of bird populations such as Hawaiian honeycreepers. For this species only one type of parasite has been observed, the chewing louse (formerly classified as "Mallophaga"), with three species being observed: "Philopterus agelaii"; "Machaerilaemus" species; and "Myrsidea" species. The parasites predominantly occur in the head area with adult males having the highest infestation percentage and juveniles having a lower infestation rate than adults. The mites "Ornithonyssus bursa" and "Androlaelaps casalis" have also been observed in nests, but the information presently available does not indicate they are a threat. Another disease commonly experienced by yellow-shouldered blackbirds is fowlpox, also known as avian pox. Lesions caused by this disease occur in exposed areas such as the legs and the bend of the wings. Nest predation has been an important contributor to the decline of the yellow-shouldered blackbird. Known terrestrial predators include rats ("Rattus rattus"), mongooses and feral cats, all introduced species to Puerto Rico. Rats constituted the main nest predator eating both eggs and hacthlings. A 1983 report concluded that rat predation accounted for 48% of nest failures that year. The majority of the activity coincided with the recession of water from July to August which exposed the mangrove nesting area to dry land. Since the middle 1980s artificial PVC nesting structures have been created in mudflats surrounding mangrove forests to reduce rat predation. These structures replaced old wooden nesting boxes and were readily accepted by the species. Presently, few (one or two) natural nests are observed each year in the area. The loss of feeding and breeding habitat and brood parasitism by the shiny cowbird are among other threats that limit and endanger the yellow-shouldered blackbird populations.