A cedar waxwing partaking of berries from a mountain ash tree in Alberta, Canada (Image: Mike Sturk/Rex Features)

After overindulging in berries, flocks of cedar waxwings flew drunkenly to their doom. That’s the conclusion of a new report in the Journal of Ornithology.

Cedar waxwings have evolved to live on a diet that averages 84 per cent fruit. But those evolutionary innovations backfired on several occasions between 2005 and 2007 when flocks of them died after crashing into windows and fences in broad daylight in the Los Angeles area.

Residents puzzled by the deaths sent the bodies to the California Animal Health and Food Safety Laboratory in San Bernardino. Necropsies performed by Hailu Kinde and colleagues at the lab showed the birds had been healthy when they gorged on berries from the Brazilian pepper tree, then died from ruptured livers or other traumas caused by the collisions.


Flocking hell

Cedar waxwings have the most fruit-rich diet of all North American birds, and dining on overripe berries can leave them visibly tipsy. Their short intestines can process large volumes of fruit, and their large livers – about 5 per cent of body mass – can break down toxic ethanol before it causes serious damage.

Yet although earlier studies had found isolated birds that had died from collisions when flying drunk, no one had seen whole flocks careen to their doom.

Now Kinde and colleagues report that the downfall of the flocks came from eating too many berries. Lacking a crop – the expandable pouch near the throat used to store food – waxwings stow meals in a distensible oesophagus. “When the fruit stays in the oesophagus, it ferments and produces ethanol,” says Kinde.

The waxwings had ingested so many berries that their large livers “could not keep up with the alcohol produced by the fermentation”, with ethanol concentration as high as 1000 parts per million. By the time they finished stuffing themselves with berries, they were too drunk to fly safely.

Journal reference: Journal of Ornithology (DOI: 10.1007/s10336-012-0858-7).