Shiny Cowbird in Puerto Rico

How to identify the Shiny Cowbird

The male is entirely black with a purple-blue gloss that lights up in the sun; it measures 18 to 20 centimeters and weighs about 43 grams. The female is smaller, dull brown, sometimes paler below, with no striking marks. They walk on the ground, often in small groups and near livestock.

Compared with Greater Antillean Grackle, the cowbird is much smaller — about 43 grams versus 80 — and lacks the yellow eyes and V-shaped tail. Compared with Yellow-shouldered Blackbird, it lacks the yellow shoulder patch. Brown-headed Cowbird, a vagrant on the island, has a brown-headed male; its female is very similar to the female Shiny Cowbird, which differs by its longer, finer bill and flatter head.

How it arrived: a natural colonization, not an introduction

Since 1900, Shiny Cowbird's range has shifted north through the Lesser and Greater Antilles. The U.S. Fish and Wildlife Service places the first Puerto Rico record in 1955, and the atlas notes it is believed to have arrived before that year. It then colonized Culebra, and on Vieques it is an uncommon resident in spring, summer, and fall and rare in winter.

The expansion continued to Florida: one male in June 1985, three in 1986, a pair in the Everglades in 1987, and permanent residence in southern Florida by 1991. Because it arrived without human help, the catalog classifies it as native, although agencies call it invasive for its effect on species with no defenses.

What brood parasitism is

An obligate brood parasite never builds a nest or raises its young. The female cowbird lays eggs in other species' nests, which incubate them and feed the intruding chick, often at the cost of their own eggs and young. Audubon notes the female sometimes punctures eggs already in the nest; the chick leaves the nest after 10 to 12 days.

Shiny Cowbird is a generalist: known hosts grew from 14 in 1929 to 74 hosts, among 232 recorded victim species. In Puerto Rico it parasitized 27 species, and Wiley found that the cowbird breeds in the same months as its highest-quality hosts, those that fledge more than 55% of the cowbird chicks that hatch.

The victims: from Yellow-shouldered Blackbird to Puerto Rican Oriole

In his study of two bird communities, Wiley (1985) found that in mangroves 42% of resident non-raptorial land bird species suffered parasitism. Parasitized-nest rates were extremely high: 76% for Yellow Warbler, 82% for Black-whiskered Vireo, 85% for Puerto Rican Flycatcher, 95% for Yellow-shouldered Blackbird, and 100% for Venezuelan Troupial and for the oriole then treated as part of Black-cowled Oriole. Other species, such as Gray Kingbird, Red-legged Thrush, and Bronze Mannikin, were parasitized at 2 to 17%.

The harm is measurable: parasitism lowered nest success by an average of 41%, and parasitized pairs fledged 67% fewer of their own young. The atlas adds that parasitism also affects the endemic Puerto Rican Vireo, and the Puerto Rican Oriole, now its own endemic species, is considered threatened by cowbirds and is listed by the DNER as Data Deficient.

Yellow-shouldered Blackbird: collapse and management

The endemic, endangered Yellow-shouldered Blackbird fell from about 2,400 birds in 1975 to 770–1,200 in 1982–1986, with 470–900 of them on Mona. Wiley, Post, and Cruz (1991) documented that nearly all nests in most of its habitats were parasitized, because the species had no evolved defenses against the cowbird.

The fixes worked: cowbird removal improved breeding, and the blackbird accepted nest boxes, where it fledged more young than in open nests. Today the Service installs PVC nest structures, checks nests weekly to remove cowbird eggs, and traps cowbirds during the nesting season. In 2011–2016 the southwestern population ranged from about 400 to 1,000 birds, Mona held about 150, the south 20 to 80, and at least 8 to 10 were counted in the east in 2018.

Song, season, and how to help

The male sings a melodious warble, and both sexes give harsh metallic call notes. In Puerto Rico, the atlas found breeding year-round with most activity from March to June and a peak in May, at the onset of the rains; it breeds mostly in moist lowlands but also in the mountains and dry plains. Hearing that warble in a cattle pasture is the best clue.

Don't touch or remove eggs from wild nests: Yellow-shouldered Blackbird nest management is done by permitted biologists. What helps is reporting cowbirds and blackbirds on eBird, avoiding exposed dog or livestock feed in coastal areas, and supporting protection of the mangroves and palms where the blackbird nests.

What is the Shiny Cowbird?

It is a South American icterid (Molothrus bonariensis) that reached Puerto Rico on its own and was first recorded in 1955. The male is black with purple-blue gloss and the female brown; it is a brood parasite that lays its eggs in other birds' nests.

Why is the Shiny Cowbird a problem for the Yellow-shouldered Blackbird?

Because the blackbird has no defenses against parasitism: Wiley (1985) found cowbird eggs in 95% of its nests. The population fell from about 2,400 birds in 1975 to 770–1,200 in the 1980s, and it is now protected with nest boxes, egg removal, and cowbird trapping.

Does the Shiny Cowbird build a nest?

No. It is an obligate brood parasite: the female lays her eggs in other species' nests, and those foster parents incubate and feed the chick. Twenty-seven host species have been documented in Puerto Rico.

What is the difference between Shiny Cowbird and Brown-headed Cowbird?

The male Shiny Cowbird is all glossy black; the male Brown-headed Cowbird has a brown head. Brown-headed is North American and a vagrant in Puerto Rico, while Shiny is common. Females are very similar, though the Shiny female has a finer bill and flatter head.

Is the Shiny Cowbird native or introduced in Puerto Rico?

It arrived naturally, spreading island by island from South America, so the catalog classifies it as native. Agencies describe it as invasive because of the harm its parasitism does to endemic species.

Where can I see the Shiny Cowbird?

In open fields and cattle pastures, gardens, open woodland, and farmland with grain, and also in the mountains where open habitat exists. The atlas found it in 66% of the island's hexagons.