The population and range of the Yellow-shouldered Blackbird have declined drastically since the middle of the 20th century. It now is listed as Endangered by the U.S. Fish and Wildlife Service, and as Endangered on the IUCN Red List. The Yellow-shouldered Blackbird currently is confined to a small region in southwestern Puerto Rico, and on Mona Island.
Factors affecting population maintenance are:
1. Loss of natural feeding habitat. Since 1900, large areas of the lowlands have been converted to agriculture; e.g., since Danforth’s (1924) research, Cartagena Lagoon, an inland freshwater wetland that extended from Guanica to Boqueron, was drained, primarily for sugar cane production.
2. Reduction in nesting habitat, primarily mangroves (Holdridge 1940). Mangroves are being destroyed at ~1.7% per yr. In 1980, Puerto Rico had 80 km2 of mangroves; in 1990, 65 km2 remained (Ellison and Farnsworth 1996).
3. Introduced predators. Rats (Rattus rattus and R. norvegicus) accompanied Europeans that arrived in the16th century or earlier. To control rats, an Indian mongoose (H. auropunctatus) was released in 1877.
4. Disease. Fowl pox, transmitted mainly by mosquitoes, is prevalent in southwestern Puerto Rico. Tumors and lesions resulting from fowl pox impair blackbirds’ movements, and reduce survival rates. Other studies (Amadon 1950) have implicated fowl pox as a factor involved in the decrease of island bird species.
5. Pollution. Oil spills kill mangroves. A large oil spill (1.4 x 107 l) occurred off the southwestern coast in 1962 (Ellison and Farnsworth 1996).
6. Brood parasitism. Shiny Cowbird (Molothrus bonariensis) first appeared in eastern Puerto Rico about 1955 (Grayce 1957). The Shiny Cowbird was found parasitizing the Yellow-shoulder in 1972 (Post and Wiley 1976). Parasitism is believed to have occurred earlier, because of the large number of cowbirds seen initially (Grayce 1957, Biaggi 1963, Lowther and Post 1999). Brood parasitism reduces the reproductive output of Yellow-shoulders, and is correlated with a >50% reduction of the blackbird’s world population.
Effects of human activity on populations
Cowbird trapping has resulted in Yellow-shoulder mortality in eastern and southwestern Puerto Rico. Birds left in traps die from dehydration and predation, and perhaps vandalism. Traps must be checked every day, and this is often not possible if manpower is limited. In eastern Puerto Rico (Ceiba) in 1986, 17 traps were spread over ha of mangrove forest, and Yellow-shoulders were found dead in some traps (Heisterberg and Nunez-Garcia 1988).
In southwestern Puerto Rico during 1973-1975, Post and Wiley (1977) determined that brood parasitism reduced the reproductive success of open-nesting Yellow-shouldered Blackbirds by 72%. In 1976, the U.S. Fish and Wildlife Service asked for a review of the species’ status, and recommendations for its management. Several management strategies were recommended (Post 1976, Post and Wiley 1976): (1) protection of nesting cays; (2) cowbird control in southwestern Puerto Rico and on Mona Island; (3) provision of artificial nest sites. In 1976 the species was listed as Endangered, and critical habitat was delineated (U.S. Fish and Wildlife Service 1976).
In southwestern Puerto Rico, blackbirds that nest on cays are rarely parasitized by cowbirds (Post and Wiley 1977b), and these small islands are important refugia. Unfortunately, in the La Parguera area, shacks have been built on the cays, which now harbor rats. Although the cays are within the Boqueron State Forest, ordinances to protect them have not been fully enforced, and some nesting cays are no longer used by the Yellow-shoulders.
In 1980, the U.S. Fish and Wildlife Service began cowbird removal in the Boqueron Commonwealth Forest. From 1985 to 2003, managers destroyed 29,981 cowbirds (Cruz et al. 2005). In an experimental area, where 53 female cowbirds were removed, only 30% of nests were parasitized, and 80% of them produced blackbird young. In a control area (cowbirds not trapped), 92% of nests were parasitized, and only 42% of them produced young blackbirds. According to Wiley et al. (1991) only female cowbirds need to be removed, and trapping is effective only if it is conducted within the blackbird nesting areas.
Cowbird removal should be considered a stop-gap measure, useful in buying time for endangered hosts (Ortega 1998); the long-term solution is reducing conditions that favor cowbirds. The occurrence of cowbirds on West Indian islands appears to be more related to availability of food than to host abundance (Post et al. 1990). Mangrove nesting habitats in southwestern Puerto Rico adjoin areas devoted to cattle production. Cowbird food and water is readily available in pastures, feedlots and troughs, but also in suburban developments, mesquite woodlands and other disturbed habitats.
A nest box (artificial nest structure, ANS) program began in 1977 (Wiley et al 1991; Cruz et al. 2005). This program enables managers to increase blackbird nesting success, and to concentrate nesting blackbirds in proscribed areas where nests can be more easily be protected from cowbirds and predators. Initially, wooden boxes were placed on poles in salinas. Blackbird production was relatively low, however, due to competition from Caribbean Martins (Progne domimicensis), rat predation, and mite infestation. Later, ANS were made of polyvinyl chloride pipes, and production improved. In 2002, 97% of 200 ANS were used by blackbirds; 50 nests contained eggs, and 57% of the ANS produced young.
In southwestern Puerto Rico, cowbird control and the ANS program has reduced cowbird parasitism rates. In 2001-2003, the parasitism rate in managed areas was 3%, compared to 52%, in non-managed areas. By 2004, the blackbird population in southwestern Puerto Rico was estimated to be > 800, in contrast to 300 individuals in 1982 (Cruz et al. 2005).
Mona Island has a significant breeding population of Yellow-shouldered Blackbirds Although Shiny Cowbirds were seen on Mona Island in 1972 (Post and Wiley 1976), brood parasitism has not yet been reported. Assuming low levels of cowbird immigration to this isolated island, trapping could possibly forestall cowbird parasitism. In addition to cowbird removal, management objectives on Mona Island should be: (1) ensuring that foods (grains, garbage, pet food) and water are not available to cowbirds; 2) elimination of feral goats and dogs; 3) control of rats, cats, and other predators.