Daring air passengers have been known to smuggle many odd things in and out of airports.

But customs officials in Vietnam were surprised to discover what one passenger was hiding in his trousers on Monday.

The man was caught at Tan Son Nhat Airport in Ho Chi Minh City allegedly trying to illegally smuggle live birds — including endangered species — to Taiwan by strapping them to his legs inside his pants and attempting to board an aircraft.

The man also allegedly packed several live birds inside his luggage, bringing the total of live birds found in the man’s possession to 18, including 11 varieties listed as endangered species.

They included singing birds such as the white-rumped shama, oriental shama and melodious laughing thrush, Thanhnien News reported.

Photographs released by the airport show the man pulling up his trousers to reveal the live birds partially hidden in pouches and strapped to his legs.

The others were housed in boxes in his luggage.

Scientists told Vietnam news agency Hoanhapthe 11 protected species were strictly banned from export.

The man, who customs officials said was a Vietnamese national, is now being investigated by authorities.

The illegal trade of wildlife is a major international problem and is believed to be a $10 billion-a-year business.

A study by researchers from Indonesia and New Jersey’s Princeton University last year found the problem was most intense in south east Asia, where bird species are being threatened by a booming black market trade. Eagle-eyed customs officials have been working to arrest the trafficking of live birds across Asia’s borders.

In October, a Singaporean man was sentenced to nine weeks in jail for smuggling live birds into Singapore and an additional three weeks in jail for animal cruelty.

His collection of nine birds included the in-demand melodious laughing thrush and white-rumped shama varieties.

A 37-year-old man was arrested as he tried to board a passenger ship with the animals at the Port of Tanjung Perak in Surabaya.

The live birds, some of which were made very weak under the conditions, were freed and taken for medical treatment.

Had they made it to their intended destination, the critically endangered birds would have fetched up to a whopping $1,200 each on the black market.

A major investigation by the Australian Crime Commission in 2012 linked wildlife trafficking from Australia to organised crime groups in Asia, where Australian birds — among other animals — are illegally bred in captivity.