The Calgary Zoo has implemented changes to the care of its penguins in the Penguin Plunge building following the inexplicable drowning deaths of seven birds in December.

According to zoo officials, an exhaustive investigation into the incident found no conclusive cause.

“We conducted detailed interviews with everyone relevant to the situation and consulted with outside penguin behaviour experts, but we could find no definitive evidence as to what caused this incident,” said Jamie Dorgan, the Calgary Zoo’s director of animal care. “Penguins normally roost quietly throughout the night; it appears that something, perhaps unusual behaviour within the colony, frightened the penguins during the night which caused them to suddenly jump into one of the two pools in the holding area; unfortunately seven were unable to get out safely.”

“It was extraordinary and tragic.”

Seven dead Humboldt penguins were located in a pool on December 8, 2016 in the back area of the building. Zoo officials say the remaining 15 Humboldt penguins are in good health.

To improve the safety of its penguins, the Calgary Zoo has altered its procedures in the holding areas of Penguin Plunge including restricting penguin access to holding areas when staff members are not present and enforcing a maximum capacity for birds in holding areas.

The zoo has added continuous ambient lighting to the building and officials are considering physically altering to the holding pool to improve safety.