A critically endangered black rhinoceros was born in Chicago's Lincoln Park Zoo after more than a year of waiting, the zoo announced Sunday. The unnamed, eastern black rhino's 13-year-old mother Kapuki was 15 months pregnant.

The zoo announced the birth Sunday on Twitter after Kapuki went into labor. "After 15 months of pregnancy and a relatively quick labor, we're excited to announce Kapuki gave birth! Kapuki's maternal instincts kicked right in and she has been seen tending to the calf," the zoo said.

After 15 months of pregnancy and a relatively quick labor, we’re excited to announce Kapuki gave birth! Kapuki’s maternal instincts kicked right in and she has been seen tending to the calf. The next big milestones will be for the calf to stand and begin to nurse. #rhinowatch pic.twitter.com/IffKhuxFN1 — Lincoln Park Zoo (@lincolnparkzoo) May 20, 2019

The photo shows the calf on the ground as its mother tended to it. The calf stood up on its own after being alive for "53 minutes of age," according to the zoo.

#RhinoWatch continues with the first of many important milestones – the calf stood up last night at only 53 minutes of age! Stay tuned for updates on the milestones the calf will go through in the critical first year of its life. #rhinowatch #lincolnparkzoo #forwildlifeforall pic.twitter.com/INKDfQzJVi — Lincoln Park Zoo (@lincolnparkzoo) May 20, 2019

In a Monday update, the zoo said the calf had been nursing from its mother, what the zoo called a milestone for the young animal,

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"The first 48 hours of a calf's life are critical and we remain cautiously optimistic," the zoo tweeted, along with a photo of the unnamed rhino and Kapuki.

The calf continues to surpass milestones! Animal care continues to monitor from afar as Kapuki cares for the calf. Since last night, the calf has been observed nursing several times. The first 48 hours of a calf’s life are critical and we remain cautiously optimistic. #rhinowatch pic.twitter.com/3X7JxxEoxm — Lincoln Park Zoo (@lincolnparkzoo) May 20, 2019

Veterinary staff will continue to monitor Kapuki and her calf with remote cameras to give the rhinos "privacy," the zoo said. For now, the animals will not be on public display, a zoo spokesperson told CBS Chicago.

"As #RhinoWatch continues we will post a timeline and updates about the calf as it begins its journey at Lincoln Park Zoo," the zoo said in a blog post.

Eastern black rhinos are on the International Union for Conservation of Nature's red list of threatened species. The World Wildlife Fund estimates the species, a target of hunters, is now between 5,000 and 5,500 today after it plummeted to less than 2,500 in 1968 and 1995.