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Three species of African birds are slurping alcoholic palm wine from trees that have been tapped by local people. The behaviour may be socially learned and therefore evidence that the birds have their own unique cultures.

The birds all live on islands in the Bijagós archipelago, off the coast of Guinea-Bissau in West Africa.

There, as in other parts of Africa, one of the the most popular alcoholic drinks is palm wine, which is the partially fermented sap of oil palm trees (). To obtain it, people climb into the trees and cut into the central part, causing sugary sap to dribble out. They then use a folded leaf as a funnel to channel it into a container such as a plastic bottle.

In early 2019, researchers led by Jorge Gutiérrez, now at the University of Extremadura in Spain, noticed that birds were visiting the sap holes. They identified three species: village weavers (), common bulbuls () and mouse-brown sunbirds (). The bulbuls were the most frequent visitors and also spent the most time: 103 seconds on average.

The sap is rich in sugars and is also a source of water in the dry season. However, it is also alcoholic because it partially ferments. “There is about 4 per cent alcohol by volume, which is something like a beer,” says Gutiérrez.

It is unclear if the birds actually get drunk. “We don’t really know how much they drink,” says Gutiérrez, and the team saw no evidence of the birds being inebriated.

All three species eat nectar and ripe fruit, which are prone to ferment and become alcoholic. “From an evolutionary standpoint, they should be adapted to deal with alcohol,” says Gutiérrez.

In contrast, chimpanzees elsewhere in Africa are known to guzzle an entire litre of palm wine in one sitting, and there is tentative evidence that this affects their behaviour.

Nobody knows how the birds learned to find the palm wine, but Gutiérrez says it could be socially learned: perhaps one bird figured it out, after which others copied the behaviour. “There could be a role of social transmission,” he says. The researchers hope to test this, but it will be tricky because they will have to track individual birds.