Staff said the month-old birds were hatched on June 22 and 25, and are growing fast.
In the wild, experts say Humboldt penguins usually raise only one chick per clutch. The female tends to lay two eggs, but only the stronger of the chicks survive.
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However, when one of the zoo's penguins laid two eggs, animal staff let her and her mate incubate one of the eggs, while the other was placed with an experienced foster pair to help ensure both chicks survive when hatched.
The zoo's indoor animal building, which includes The Living Coast, has been temporarily closed to guests per a mandate by the State of Illinois.
There are 432 Humboldt penguins in 20 North American facilities, including 29 at Brookfield Zoo, according to the Chicago Zoological Society.
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Humboldt penguins are considered a "red list" species, meaning they are vulnerable in their native habitat along the western coast of South America near Peru and Chile, according to the International Union for Conservation of Nature and Natural Resources. ICUN says the penguins' population, which researchers estimate to be around 32,000, continues to decline due to several factors, including climate change and severe weather, human encroachment, and pollutants.
The Brookfield Zoo reopend to the public at the beginning of July after being closed for months due to the coronavirus pandemic.