‘Sharing’ Other Birds’ Nests

Author: Fernando Fuentes


n this Argentina Salvaje not all birds are as industrious as an ovenbird. Some of them don’t have the patience to build a nest twig by twig for their offspring. They directly swindle other birds or have attitudes worthy of the mafia when securing a place to leave their eggs. They bring into play evolutionary strategies that have been of great help to their survival as a species. Eventually they have forgotten what it was like to build their own homes.

Vanina Fiorini, who likes to delve into these issues, is a scientific at the Ecology and Animal Behavior Laboratory in the Faculty of Natural Sciences of the University of Buenos Aires. Thanks to patient observation and filmic records she has managed to study the behavior of a bird often sighted in Argentine skies. We are talking about the shiny cowbird, a generalist parasite that is very good at its job. So much so that these birds manage to ignore and leave their eggs in the care of more than 250 different bird species.

After ten to twelve days of incubation—usually requiring less time to hatch than the eggs from the parasitized species—juveniles generally bigger than those of their adoptive parents start to hatch.

The Cowboy Way


The shiny cowbird, or Molothrus bonariensis by its scientific name, is a widespread species in South America. Males can grow to 8 inches (20cm) long and weigh 45 grams, and their plumage is all black with an iridescent purple-blue gloss. Meanwhile, females have a brownish plumage and a smaller size. Both have a long beak and slender legs. They usually feed on insects and forage scattered on the ground, and they lay their eggs—which can be immaculate white or pale blue with dots—in other birds’ nests.


After ten to twelve days of incubation—usually requiring less time to hatch than the eggs from the parasitized species—juveniles generally bigger than those of their adoptive parents start to hatch. They spend fifteen days in the nest and are then fed out of it until three months of age, time at which it is unclear why they decide to leave the territory. During that time span, they engage in direct competition for food with biological nestlings of the affected species. And they generally succeed.

That whole process has been documented by the eyes and cameras of Fiorini. She has seen how the female usually approaches the nests to be received aggressively by the host: a creamy-bellied thrush or a chalk-browed mockingbird, for example. Despite the welcoming pecks, the shiny cowbird almost always manages to place its egg. Sometimes the unfortunate owners try to remove or peck the egg fruitlessly; at other times they don’t even notice the presence of a foreign egg in their nest.


In addition, scientists have very curious photos where the asymmetry of sizes between the hungry juvenile and the self-sacrificing adoptive parent raises questions about who is who. “Why, after birth, the nestlings are still fed by the adoptive parents?” they wonder. Answers still don´t abound. But in the case of other birds, the mafia-like behavior of parasites, including aggressive regular visits to the nest, is enough for the adoptive parents not to give up their task.

Like the Cowbird but ‘Santiagueño’


In Santiago del Estero—province north of Argentina—it is said that in order to calm the longing for this land, there is nothing better than singing a good chacarera. Far from their home turf, people miss Santiago del Estero nights, and their skies like grapevines with clusters of stars; but you can always be a beating drum if you think of Santiago.


The striped cuckoo—Tapera naevia for scientists—must have heard about it since after migration they sing day and night to prevent cicadas so filled with nostalgia from getting into their throat. This species can inhabit a wide area, ranging from Mexico to central Argentina, but they know it is in Santiago del Estero where their balladeer hearts beat better.


It happens that this bird is a source of native legends and is present in popular culture. That’s why every October they return with the mission to leave offspring. They are shy birds who prefer to remain hidden in the forest of Santiago del Estero, which is becoming less extensive. Progressive deforestation forced them to adapt and survive in other environments, which they have managed to do successfully since according to records of the International Union for Conservation of Nature, far from rolling back, in recent years the species has increased its population. At the moment, survival is a minor concern for the striped cuckoo.

Tapera naevia is a species that doesn’t build its own nest either. The female is also in the habit of laying eggs in other birds’ nests. She preferably chooses those enclosed and with little visibility. She waits for the slightest slip of the inhabitants to leave the eggs in the care of adoptive parents. After hatching, striped cuckoos are no longer responsible for the care of the nestlings. With a more limited range than the shiny cowbird, there are about twenty species of birds that can be parasitized in this way by the striped cuckoo. Among them are the firewood-gatherer, the groove-billed ani and the always kind ovenbird.


Eggs take fifteen days to hatch. A newborn striped cuckoo nestling weighs around 3 grams. They are very aggressive and compete fiercely with their adoptive brothers for food and parental care. They usually succeed and leave the nest after eighteen days.

World Heritage


Besides the striped cuckoo, there are 90 species worldwide that have the ingrained habit of ‘sharing’ other birds’ nests: equal to 1% of all birds. Such species are not a unique heritage of Argentina. Examples include the brown-headed cowbird (Molothrus ater) in North America or the great spotted cuckoo in Europe. Birds like these inhabit every continent. They don’t build their nests; they parasitize and leave their offspring even if it means swindling others or using mafia-like strategies.


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