Every winter, people across towns and cities notice birds behaving in puzzling ways — a robin sitting motionless on a pavement, a blackbird wobbling on a branch, or waxwings crashing into windows. While the scenes may appear amusing at first, they often raise concern. In many cases, the explanation is surprisingly simple: fermented berries.
As temperatures fluctuate in winter, berries left on trees and shrubs can begin to ferment naturally. This process produces alcohol, which can temporarily affect birds that rely heavily on fruit when other food sources disappear.
How berries ferment in winter
Fermentation occurs easily because yeast is naturally present in the environment. During winter, mild daytime temperatures combined with cold nights accelerate the breakdown of sugars in overripe fruit. Without any obvious change in appearance, sugar quietly converts into ethanol.
For birds, these berries still look like a valuable food source. With insects scarce and seeds often buried or frozen, fruit becomes one of the few available sources of energy. Birds cannot afford to be selective, even if the berries are slightly fermented.
What alcohol does to birds
Birds are small and have extremely fast metabolisms. As a result, alcohol enters their bloodstream quickly and affects them more strongly than it would humans. Research published in The Auk: Ornithological Advances shows that ethanol from fermented fruit can disrupt balance, coordination and flight.
Affected birds may misjudge landings, fly into windows, appear unusually calm, or sit still for extended periods. While most recover within a few hours as the alcohol is metabolised, this temporary impairment makes them vulnerable to predators, cold stress and collisions.
Birds most commonly affected
Fruit-eating species are the most likely to show symptoms. Waxwings are particularly well known, as they feed in flocks and consume large quantities of berries at once. Thrushes, robins, starlings and blackbirds are also affected, especially younger or weaker birds. Cold weather worsens the risk, as intoxicated birds may struggle to maintain body heat if they remain inactive.
When strange behaviour has other causes
Not all unsteady birds are affected by fermented fruit. Head injuries from window strikes, pesticide exposure and certain diseases can produce similar symptoms. Time is often the key indicator — berry-related intoxication usually improves within hours. If a bird remains unwell or deteriorates, another cause is likely.
What people should and should not do
In most cases, restraint is best. If the bird is perched safely, it should be left alone. If it is on the ground in danger from traffic or pets, gently moving it to a quiet, sheltered spot nearby is appropriate. Feeding or forcing water should be avoided.
Gardeners can reduce risk by planting berry-producing shrubs that ripen at different times and clearing fallen, rotting fruit. In urban areas, window decals can help prevent collisions.
Drunk birds are not just a seasonal curiosity. They reflect how small environmental changes can significantly affect wildlife — a reminder that understanding is often the first step towards care.