(CNN) At least 21 cockatoos have been discovered stuffed into 1,500 ml plastic water bottles at an Indonesian port during an anti-smuggling operation.

The cockatoos, which are described as critically endangered, were cut free of their plastic confines by Indonesian customs officials at Tanjung Perak port in Surabaya, Indonesia, after they spotted the consignment of illegally-trafficked birds.

Harbor police caught a passenger disembarking with two birds packed in jerry cans. Police searched the boat and found 21 more birds, stuffed into water bottles and packed in travel bags.

"The birds were still alive then but some were already very weak," Lily Djafar, spokeswoman for the Tanjung Perak police, told CNN.

It was not the first case of its kind, she said. In April, there were two cases of animal smuggling at the port and more than 200 rare and endangered animals, including birds of paradise, reptiles, sugar gliders -- known locally as tupai loncat -- and cockatoos were seized. In both cases, the suspects were boat crew members, Djafar said.

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