Birds do not drown often, but it can happen. If a young bird lands in deep water and its feathers get wet, it may not manage to get out.

Over the last 100 years over 800,000 birds have been marked with monitoring rings. Drowning was only a cause of death for 2,901: that is less than 1%.

When reports emerged of young starlings drowning en masse, on more than one occasion, veterinarian Becki Lawson of the Zoological Society of London in the UK decided to investigate.

Relying on reports from the public, Lawson and colleagues identified 12 mass drownings in England and Wales over the course of 20 years. "These have all involved people finding dead starlings in water bodies in their gardens, most commonly garden ponds, also swimming pools, a well, and even a bucket," she says.

When they do drown, they do it in groups

The birds were all juveniles and had died only a few months after they were born, in May or June. In all but two of these cases, 10 or more juveniles were discovered drowned together.

Usually when many birds die at once, it can be attributed to disease or pollution. But these birds had clearly been healthy.

So something else had to be responsible. Lawson wondered if there was something about starlings that made them more susceptible to drowning. She particularly wondered why so many followed each other into the water.

Starlings always live together in flocks. They are famous for their beautiful "murmurations", in which hundreds of thousands of individuals take to the skies together.

The young birds are roaming around the countryside a bit like teenagers

This group mentality starts young and could be a key reason for their deaths by drowning.

Starlings are a gregarious species, says Lawson. "They flock, drink and bathe together. That may be part of the explanation, that when they do drown, they do it in groups."

Youngsters are obviously inexperienced. So if one goes into a dangerous water pool, the others may blindly follow.

To see if this was true, the team compared the incidences of drowning among gregarious starlings and solitary blackbirds.

"Starlings did seem to drown more frequently than blackbirds," says co-author Rob Robinson of the British Trust for Ornithology (BTO). "The key difference is they often go around in large quarrelsome flocks. The young birds in particular are roaming around the countryside a bit like teenagers."

If starlings fly to a pond or fountain to quench their thirst, they all jostle each other and get in each other’s way. "We think it’s the flocking nature of the starlings [that] seems to predispose them to these things," says Robinson.

While the drownings seem to be accidental, their exact causes remain a mystery.

However, they remain relatively rare, so Lawson say there is no need for members of the public to stop putting out water for birds in their gardens.

"All we are saying is that, particularly during the months of May and June, it might be a good idea to put some gentle slopes or ramps that might safely help them exit the water," says Lawson.

Although starlings are one of the UK’s most common garden birds, their population has fallen by 79% in the last 25 years, according to the BTO. The Royal Society for the Protection of Birds now lists them as a species of conservation concern.

The mystery drowning events are not frequent enough to pose an additional threat to the species. Nevertheless, the team are calling on the public to send them any new examples of such events in the UK, via the project website.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

Follow BBC Earth on Facebook, Twitter, and Instagram