Photograph by RIA Novosti, Alamy

Read Caption

A waxwing feeds on the berry of a rowan tree.

Photograph by RIA Novosti, Alamy

Why Backyard Birds Are Getting Drunk on Fermented Berries

'Tis the hangover season for many birds—some of which are ending up in "drunk tanks."

If birds that feed on winter berries in the Northern Hemisphere seem to be painting the town red, it could be because they're intoxicated.

Alcohol forms in berries as they ferment with the first frosts, and the birds that gorge on these winter fruits may get drunk more often than we think, scientists say. (See National Geographic's backyard bird identifier.)

"Most birds likely just get a bit tipsy, and very few people would be able to pick them out as intoxicated," said Meghan Larivee, laboratory coordinator at the government agency Environment Yukon in Canada. "However, every now and then, some birds just overdo it."

Such was the case with several Bohemian waxwings (Bombycilla garrulus) that flew into Whitehorse, Yukon, this fall, and ended up in "drunk tanks" after bingeing on fermented berries of the rowan tree.

The birds were admitted to the territory's Animal Health Unit, a wildlife facility equipped with modified hamster cages in readiness for the influx of berry-seeking migrants.

While there's no Breathalyzer for birds, it's pretty clear the waxwings were flying under the influence, according to Larivee.

"They cannot coordinate their flight movements properly or at all, and they are unable to walk in a coordinated way," she said in an email.

The birds, who come in with juice-stained beaks, are checked for illnesses that might otherwise explain their groggy condition. They generally recover after a few hours, she said.

Less fortunate are those intoxicated birds that die in collisions with buildings—two such fatalities were recorded in Whitehorse this fall.

An Alcohol Problem

While there are numerous anecdotal reports of such incidents in North America and Europe, there have been few studies to show that alcohol is indeed the cause.

One problem is that there's no routine test for diagnosing alcohol poisoning in animals, said Paul Duff, a veterinary scientist with the U.K.'s Animal and Plant Health Agency. The tests are "relatively expensive, and alcohol can disappear quickly. And toxicity leaves no characteristic lesions," he said. (Also see "Boozing Mammal Drinks 'Beer' Every Night, Study Finds.")

In 2011, Duff was called in as part of a police investigation into the suspicious deaths of 12 common blackbirds (Turdus merula) at an elementary school in the county of Cumbria in northern England.

Postmortem results, which included the detection of significant levels of alcohol in a liver sample, suggested the birds succumbed after eating fermenting rowan berries.

The same agency made a similar diagnosis for a group of redwings (Turdus iliacus) in 1999. The birds sustained fatal falls onto concrete from a berry-laden holly bush.

High Tolerance

The relatively large livers of waxwings and other species that rely on berries to get through winter are thought to help the birds handle alcohol. Young birds, however, may be more vulnerable.

Duff noted that the blackbirds he investigated in 2011 were all immature. "It is possible that adults learn to avoid toxic berries," he added.

A more obvious risk factor for drunk birds, certainly for those that flock to urban areas, are buildings and hard surfaces.

Bohemian waxwings that make their way south to Britain from Scandinavia in winter often make a beeline for ornamental berry trees in shopping center parking lots and residential areas. (Also see "'Drunk' Bats Fly Right—Discovery Surprises Scientists.")

In such an environment, being unable to fly straight may be lethal, said Ben Andrews, a wildlife advisor with the U.K.'s Royal Society for the Protection of Birds.

"We've had the odd report from people phoning us up and saying they've had a waxwing or waxwings dead under their window and things like that," Andrews said.

If you come across a live bird that appears drunk but uninjured, Andrews advises putting the animal in a cardboard box with some air holes for a few hours until they sober up and can be released.

Climate Change Effect?

But if outbreaks of insobriety aren't so unusual among berry-eating birds, might climate change make them even more frequent?

Though not an unusual fall phenomenon, freezing causes the berries to convert starches into sugars, while subsequent thawing makes it possible for yeast to get in and speed up the fermentation process, Larivee explained.

Larivee's recent waxwing patients were admitted to her Yukon animal unit following several frosts and thaws due to warmer temperatures. Average global surface temperature increased 1.5 degrees Fahrenheit (0.85 Celsius) from 1880 to 2012, according to the Intergovernmental Panel on Climate Change.

"We have to look at longer term trends when speaking about climate change, but we are seeing some changes," Larivee said. (Read more about the effects of global warming.)

While these effects are most pronounced in winter, "we also likely have longer autumns, which gives more time for berries to ferment, but still have early frost that allow sugars to be produced in berries early in the fall," she said.