¡Míralo! ¡El Mirlo!

If you live in Chile, you’ve probably seen them, mirlos aka shiny cowbirds (Molothrus bonariensis), but what you might not know is that they depend on a trick: getting other birds to raise their young.

These medium-sized birds live throughout the Southern Cone, as reported by BioBioChile. The male is usually metallic black and the female is brown-gray. They are unremarkable in appearance, but extraordinary in behavior.

Extraordinary behavior: brood parasitism

As Patrick Monahan reported in Science, they are excellent “brood parasites”: birds that “lay their eggs in the nests of various unsuspecting songbirds, which then raise the foreign chicks as their own.” The female cowbird’s job is a “delicate” one, “and it requires precise timing. Too early, and her egg will stand out in an empty nest; too late, and her chick might not get the attention it needs from its surrogate parent.” To accomplish this feat, parasitic cowbirds “scope out nests beforehand to get the timing right.”

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Special cognitive and behavioral mechanisms

It is therefore no surprise that, as noted by the Global Invasive Species Database (GISD), parasitic cowbirds have developed cognitive and behavioral mechanisms to deal with “the spatial demands associated with brood parasitism. Neurological studies have revealed that parasitic cowbirds have larger hippocampal formations (the part of the brain associated with spatial memory acquisition) than non-parasitic cowbirds.”

Moreover, as also noted by the GISD, in “sexually dimorphic species,” such as mirlos, in which the female alone performs the parasitic behavior, “only the female has the larger hippocampus. This is believed to be important in patrolling the home range for appropriate nests, remembering the location and status of nests for subsequent identification, and updating the nest status after each egg has been laid in a nest.”

As Monohan noted, mirlos have also evolved a “grisly” behavioral mechanism for success: “poking holes in all of the eggs they come across.” This also likely requires a bigger hippocampus – to remember and avoid the nests they’ve already coopted, so that they don’t accidentally kill their own chicks.

Invasive species

Like many other bird species in Chile, the mirlo is an invasive one. In fact, it is believed that mirlos first debuted in Chile as caged birds from Argentina and subsequently escaped. A 105-year-old article from Revista Chilena de Historia Natural offers compelling evidence in this regard.

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Urban Invaders: Monk Parakeets

The next time you’re in a backyard, park, or meadow, and you see a sparrow feverishly hunting food with an outsized, squawking shadow, it’s probably the trick of the mirlo.

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