Brood Parasites- Tropical Cowbirds

The Shiny Cowbird (Molothrus bonariensis) is a South American species increasing its presence in Florida probably as result of natural expansion from the Greater Antilles. Like other cowbirds, Shiny Cowbirds  choose agricultural lands, edges of human dwellings and other disturbed landscapes to breed, avoiding large expanses of unbroken forest, mangrove swamps, and other wetlands. In the West Indies where they are now common, they are especially attracted to dairies. This species reached the Greater Antilles in the 20th century apparently by island-hopping across the Lesser Antilles starting from northeastern South America. In recognition of its pleasant song, some birds also were introduced as caged birds in some islands and escaped. M. bonariensis has recently been confirmed to occur in central Florida along the Gulf of Mexico. It is believed to have reached the U.S. in 1985 in the Florida Keys and soon expanded its range to include mainland Florida and southeastern Texas. In Puerto Rico, the Shiny Cowbird leaves its  preferred dairy farm surroundings in the June-September period to move to the southwestern coastal zone where its preferred host, the critically endangered Yellow-shouldered Blackbird, nests (Perez-Rivera,  1986).

In South America, 93 species are known to have reared M. bonariensis young, while 250 species are known to have had their nests parasitized by this species. Unlike other cowbirds, M. bonariensis can parasitize species nesting in cavities or enclosed nests.  In Argentina, studies have found that shiny cowbirds that lay eggs in the cavity nests of wrens differ genetically from those that lay eggs in open-cup nests, suggesting that  specialization may be underway. Similarly, in the Caribbean, where some islands were colonized by only a few Shiny Cowbirds, females continued to lay eggs in nests of only 2 or 3 host species despite availability of many other species, and did so independently of the abundance of the chosen hosts.

The Shiny Cowbird’s incubation period of 11-12 days is shorter than that of most host species, giving the cowbird offspring a competitive advantage in addition to its larger size at hatching. The shiny cowbird female frequently removes the host egg from the nest and other times host chicks may starve if food is scarce. In addition, Shiny Cowbirds also are known to puncture eggs in nests that the parasite inspects but does not lay eggs in (Massoni and Reboreda 2002). Eggs of M. bonariensis vary from whitish and unspotted to blue or green and spotted but there is no recognizable relationship to host egg color as in the Old World cuckoos.

Range expansions as seen in M. bonariensis, expose species that did not previously co-exist with brood parasites and therefore did not evolve counter- or compensatory-responses. In Puerto Rican mangrove habitats, 42% of non-raptorial land bird species were parasitized (Wiley 1985). Some species suffered over 75% of nests parasitized, including the Black-whiskered Vireo and Yellow Warbler, two species vulnerable to cowbird inroads in North America, plus four other native Puerto Rican species. Other birds like the Northern Mockingbird and the Gray Kingbird, both found in the U.S., suffered only 2 to 17% of nests attacked. Overall, 67% fewer host chicks were fledged in parasitized nests than in non-parasitized ones. In another study conducted in the Lesser Antilles, where islands were colonized by only a few Shiny Cowbirds, females continued to lay eggs in nests of only 2 or 3 host species despite availability of many other species. Like in Texas, the cowbirds persist in choosing certain species despite the presence of other suitable species and independently of the abundance of the chosen hosts.

The population size and area inhabited by the Bronzed Cowbird (M. aeneus) in the U. S. have increased in the 20th century, a trend that may be continuing today as natural lands continue to be  converted to human-dominated ones. M. aeneus is native to most of Mexico and Central America as well as southern areas of Texas, New Mexico and Arizona. Oberholser (1974) attributed the increase of Bronzed Cowbirds in the Rio Grande Valley to changes in agriculture following a hard freeze in 1951- cotton fields and cattle feedlots, excellent foraging habitat for cowbirds and other blackbirds replaced the lower quality citrus groves killed by the freeze.

Breeding in Florida was first suspected in 1957 in Pensacola and birds were first collected near Gainesville in 1968. Prior to the late 1950s, it was known only as a winter visitor in Florida but by the 1980s was seen nearly annually in the peninsula. The first verifiable breeding record for the state and for the nation east of the Mississippi River was established in 2009 when a fledgling Bronzed Cowbird was photographed being fed by a Spot-breasted Oriole in south Florida. Interestingly the oriole, which is exotic in the U.S., has now been established as the only known species to be parasitized by a cowbird in both its natural range and its exotic range (Pranty and Nelson 2010)! M. aeneus also has established a local distribution in southern Louisiana and Mississippi.

The habitat of M. aeneus is  pasture, roadside thickets, woodland edges, etc.  In Mexico, it also occupies tropical deciduous and scrub forest. Four geographically distinct subspecies are recognized. Only one, M.a. aeneus, is involved in the U.S. range expansion. Like other cowbirds, the Bronzed Cowbird follows livestock, feeding on their ticks and on insects flushed from grass. Bronzed cowbirds withdraw south into Mexico from most of their U. S. range (except for the Rio Grande Valley) in the winter and join other blackbirds in winter-time flocks.

Bronzed Cowbirds are the largest of the three three North American cowbirds (62 g vs. 44 g , Brown-headed, and 36 g, Shiny Cowbird) and typically parasitize larger host species than the others. M. aeneus most often lays one egg per nest and the cowbird adult may pierce the host eggs. Typically a cowbird female replaces a single host egg with one of its own, but more than one Bronzed Cowbird may leave an egg in a host’s nest. As in other cowbirds, Bronzed Cowbird nestlings are larger than most host nestlings thereby monopolizing the food delivered by the host adult. In most cases some of the host offspring do fledge although in fewer numbers. In a study in southern Texas, ½-2/3 of host nests contained one or two M. aeneus eggs, but a few had six or more ( Eighty-two species of birds are confirmed to have received a  M. aeneus egg in their nests and 32 species have “successfully” reared M. aeneus young.

Typical hosts in Texas are the four local oriole species, plus Red-winged Blackbirds, Northern Mockingbirds, Brown Thrashers and Northern Cardinals, with orioles apparently favored. Parasitism by the Bronzed Cowbird is believed to be contributing to the decline of the Altamira and Audubon’s Orioles in southern Texas.

Expansion of M. aeneus may have slowed recently based on 49 Breeding Bird Survey routes monitored annually within its Texas range ( Over the period from 1966 to 2003, year-to-year changes in annual breeding bird estimates  varied from  -3.0% per year to +4.5% per year – i.e., only slightly above  a 0% change.

Tropical Molothrus– other species of Molothrus are strictly tropical having never been recorded in the U. S., however they do overlap in distribution with the Shiny Cowbird and with each other. The screaming cowbird (M. rufoaxillaris) differs from other cowbirds in parasitizing only one host species over most of its range, the Baywing  (Agelaioides badius). The Baywing is a fellow icterid of central and southern South America that is closely related to Molothrus cowbirds and until recently was included in that genus. It is also known as the Bay-winged Cowbird, but is not a brood parasite.

Pair of Screaming Cowbirds. Photo by Claudio Dias Timm in Brazil.

Pair of Screaming Cowbirds. Photo by Claudio Dias Timm in Brazil.

In a comparative study of 3 host species, the Baywing and 2 suitable but unparasitized species, the House Wren and the Chalk-browned Mockingbird, Screaming Cowbird egg survival and hatching success were similarly high in all hosts but survival of cowbird chicks was significantly lower in  the wren and mockingbird  hosts. Results of the study suggest that M. rufoaxillaris can parasitize other hosts than the Baywing but has higher reproductive success relying on A.badius. This greater success is no doubt due at least partly to the great similarity exhibited between eggs and nestlings of M. rufoaxillaris and its preferred host-yet another example of brood parasite mimicry. In adulthood, the two species diverge in plumage color.

In a study in Argentina, Baywing adults were observed to remove ectoparasites (botfly larvae and mites) from its own nestlings and those of the Screaming Cowbird (Fraga 1984). This behavior may explain why screamers select Baywings as foster parents over most of their range as ectoparasites can reach lethal levels in nestlings.

The Giant Cowbird (M. oryzivorus) ranges from southern Mexico to northern Argentina, Trinidad and Tobago. This species is much larger than the other cowbirds, approaching a crow in size. Unlike other cowbirds, the Giant Cowbird doesn’t perch on cattle but does perch on capybaras, removing horse flies. It is the only Molothrus species to nest in deep forests. Giant cowbirds select much larger host species, choosing to  lay eggs in nests of oropendolas, the largest birds in the Icteridae, and caciques, grackle-sized  blackbirds. Species of both host genera build long, pendant basket nests. Ordinarily the Giant Cowbird does not destroy the eggs and young of  the hosts but does compete with them for food. As its hosts are colonial breeders, so is the cowbird.

Colony of oropendola with their basket nests.Photo by charlesjsharp.

Colony of oropendolas with their basket nests.Photo by charlesjsharp.

The oropendolas often remove the cowbird eggs and the male oropendola may attack the cowbird near the nest. In addition there is some indication that Yellow-rumped Caciques and Russet-backed Oropendolas may work synergistically to repel Giant Cowbird attacks. But not always! There is documented benefit from brood parasitism under some circumstances because cowbird young will feed within the nest on botflies which are a major cause of oropendola nestling mortality (!  In a striking example of multiple species interaction, oropendolas that nest near bee and wasp nests have no tolerance for cowbirds apparently because the bees and wasps attack botflies, therefore eliminating the benefit oropendolas would have gained from tolerating  cowbirds in their nests!

Giant Cowbird. Photo by Tom Davis.

Giant Cowbird. Photo by Tom Davis.

Explaining the diverse reproductive patterns implicit in the brood parasite strategy remains a challenge. Coevolutionary theory predicts increasing host specificity as time passes within a particular community as hosts evolve defenses that the parasite can’t overcome. Cowbirds would then, under pressure of natural selection, gradually shift to using mainly hosts that were more vulnerable to parasitism. If so, host specialization would be derived through selection from generalist species similar to the present North American cowbirds, especially M. ater. The Bronzed Cowbird’s status could be hypothesized to be intermediate in terms of host specialization because although they are capable of parasitizing many species, they often choose from only a few, such as orioles, even if other species are available. However another concept suggests that species generalism is the derived condition because natural selection would favor parasites that had wider choices of host species and numbers of nests in appropriate habitats, leading to a  higher than average success rate over time than the “putting all your eggs in one basket” approach of host specialists.

References  cited or reviewed: See “Brood Parasites-Never An Empty Nest?

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