Baikal is a male Siberian tiger brought from New York's Bronx Zoo in January to mate with Katja, one of the Calgary Zoo's female tigers. (Courtesy of Calgary Zoo)

A Siberian tiger at the Calgary Zoo has unexpectedly given birth to two cubs, one of which was found dead Tuesday morning.

Zoo staff members were taken aback when they found that Katja had given birth overnight, according to a zoo news release.

As keepers tried to secure a den for her so that she could care for her cub in comfort, they discovered another cub had not survived.

"Finding the surviving cub struggling, the staff took the cub to the zoo’s animal health centre to be examined, where staff are continuing to monitor it," the release said.

"It is hoped that should the cub recover well, it will be returned to Katja as soon as possible as it is preferable that the cub be mother-raised."

Katja is said to have shown no outward signs of pregnancy, although that's what was hoped for when the zoo brought nine-year-old Baikal, a male, from New York's Bronx Zoo to Calgary in January.

More cubs possible, but unlikely

This cub is the first offspring for Baikal and Katja, a 10-year-old zoo-born tiger.

“Both Katja and Baikal are considered very genetically valuable in the captive population, so the fact that breeding was successful is positive from a species-survival perspective,” said Tim Sinclair-Smith, a zoo curator.

“However, we are always aware things can still go wrong in spite of our best efforts. Inexperienced mothers can sometimes be too aggressive or cubs may have health issues that are not apparent immediately.”

One of the reasons the zoo was surprised at the birth was that while Baikal has expressed interest in breeding with Katja, he has had difficulty because of arthritic hips. The zoo said veterinarians are pleased that a surgery performed in May on Baikal to reduce the pain in his hips has proven successful.

Tigers can give birth over a period of several days. While it appears unlikely at this time, it is possible that there still could be more cubs born, the zoo said.

Siberian, or Amur, tigers are considered endangered and the population continues to decline.

The zoo currently has four adult tigers: Baikal, Katja, Kita and Vitali.

It's expected that Vitali will soon be moved to the Toronto Zoo.