Despite an "exhaustive investigation," the Calgary Zoo says it still doesn't know why seven penguins drowned last month.

Seven Humboldt penguins were found dead at the back of the Penguin Plunge exhibit at the Calgary Zoo on Dec. 8. The animals drowned in a back holding area of the popular Penguin Plunge exhibit.

In a release issued Thursday morning, the zoo said there is "no conclusive evidence" as to what led to the drownings.

It appears something spooked the penguins, they panicked, and took shelter in the pool, said Jamie Dorgan, director of animal care for the zoo.

"So most likely there were birds on top of each other, and a bird might try to surface, and maybe another bird was there at the time. And it doesn't take very much water for an animal — and even more so for a bird — to inhale before they succumb to drowning."

Experts 'baffled'

The zoo conducted detailed interviews with staff and outside penguin behaviour experts, but haven't found a similar case, said Dorgan.

Rob Laidlaw, the executive director of Zoocheck, said the deaths at the zoo are the first time he's heard of this many penguins dying at once in a zoo. What was especially surprising was the fact that the penguins drowned, he said.

"That was something that baffled us when we heard about it and we still find it baffling now," Laidlaw said.

The zoo also pledged to make what it calls "precautionary changes" to how the penguins are handled. The penguins will have restricted access to the pools in the back holding areas when staff are not present, and there will be a maximum capacity for the number of birds in the holding areas at one time.

"I guess [the zoo is] doing what they feel might help in a variety of situations," said Laidlaw. "Whether or not it will address the particular circumstances that led to the death of these seven penguins, I think, is anybody's guess."

Laidlaw said Humboldt penguins are "highly gregarious" and are used to roaming huge distances in the wild — making them unfit to be kept in zoos.

"If you look at their biology and their behaviour and their lifestyle, I think any reasonable person would say that putting them in this confined situation of a zoo or an aquarium really isn't the best thing for them," he said.

Additionally, staff will investigate making physical changes to the holding pools, as well as ensure there is continuous ambient lighting in the area at night.

"This was an extraordinary and tragic situation," the zoo's statement said.

"The remaining 15 Humboldt penguins are in good health and remain in Penguin Plunge alongside three other penguin species."