Before Viagra, there was duck embryos, with feathers and a beak, and considered a Filipino delicacy believed to boost male fertility and libido.

An Auckland, New Zealand restaurant serving duck eggs with developing embryo has been cleared by the SPCA for possible cruelty against unhatched ducklings but its suppliers are now being investigated.

The society said it received complaints following a Herald report that Island Joe’s Hawaiian Barbecue in Onehunga had put balut on the menu, and an investigator was sent to the premises.

The restaurant said that with the ongoing investigation, it would be serving balut as just a “blackboard special”.

Ms Kalin said the restaurant had not breached the Animal Welfare Act and the society was satisfied with the practices employed there, and was now turning its investigation to the suppliers. The fertilized duck egg is boiled and served hot, and diners consume the broth, yolk and young chick with salt or a chilli, garlic and vinegar sauce.

“The eggs arrive at the restaurant in a chilled state and are placed in the refrigerator and accordingly the embryos would not be alive at the time of boiling,” she said.

Island Joe’s owner Cecilia Tan said staff had also received calls accusing the restaurant of being “cruel” and “heartless”, with some using profanities. She said the SPCA investigator had thought that “maybe we drop the egg with a live duck squirming in boiling water”.

Massey University sociologist Paul Spoonley, who has done studies on food introduced by migrant communities, believed the complaints were due to “a lack of cultural understanding”.