Every day waxwings are found dead or dying in the street in Baotou City
The battered corpses of cute, but timid, waxwings are piling up in Baotou City, Inner Mongolia, say media reports.
Gruesome video shows the birds lying on the ground, dying or dead, says HK News.
It reports that most of the birds have smashed straight into transparent glass windows before plummeting to their deaths onto concrete at the base of the brick residential building.
Some windows still have blood stains as a result of the carnage.
But scientists in China are puzzled about why the birds are flying into the glass.
The mass deaths were reported to the Wildlife Protection Office of Baotou Ecological Wetland Protection and Management Centre.
Experts who have taken samples from the birds' bodies to work out what is causing the odd behaviour have already ruled out disease.
Local officials told reporters that the area has sufficient food and water.
But, one theory is that the shy birds are so easily spooked by noise that they panic and fly straight into the glass to escape any potential threat.
Or, that their sight is affected by glare reflected from the glass.
Upset residents have closed their curtains or put newspapers inside the windows to try and prevent the birds from being killed.
Yahoo! reports that the little birds eat fruit from trees in the area.
So, one theory is that after drinking water, the fruit swells and makes the birds feel uncomfortable.
Another suggestion is that the sociable birds move in large numbers, and are simply flying too fast to be fully aware of potential dangers such as glass windows.
Experts also told Yahoo! that the glass reflects the sky and nearby woods, which could easily confuse the skittish birds.
In 2018, Bird Watching Daily said that "drunk birds" were reportedly flying into windows, acting confused, and being hit by cars in America.
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