Two starlings bathing
© blickwinkel/Alamy
Two starlings bathing: the songbirds are a stocky species that bathe and drink together in groups.

Behaviour could be one cause of the unusual drownings of the birds in large groups in England and Wales

Starlings have been consistently drowning in large groups in a phenomenon yet to be fully explained by scientists, according to new research led by the Zoological Society of London (ZSL).

In 12 separate incidents recorded between 1993 and 2013 in England and Wales, starlings were found drowned in groups of two to 80. In 10 cases, at least 10 starlings were found drowned at a time, the research published in the journal Scientific Reports on Wednesday shows.

One expert said that the mass mortalities were "really unusual", with drowning considered a rare cause of death among wild bird populations and normally only recorded as affecting individual birds.

Records since 1909 of 800,000 ringed birds from 79 species reveal that drowning was more commonly recorded as a probable cause of death in starlings than in any other species.

Post mortems revealed no evidence that underlying disease had been a factor in the incidents which all occurred during the summer and spring months and concerned juvenile birds in most cases.

Dr Becki Lawson, lead author and wildlife veterinarian at ZSL told the Guardian that the cause is therefore likely to be behavioural. The stocky songbirds are a flocking species that bathe and drink together in groups, where the starlings may then be unable to exit a confined space or their plumage may become waterlogged. The inexperience of juveniles in recognising water hazards could also be a factor, she added.

"It does seem to be something related to that species being vulnerable or predisposed to these drowning events. It is really unusual," she said.

Starlings are listed as a threatened bird species in the UK, with numbers falling by 45 million since the 1980s and the current population recorded as 3.4 million during the breeding season. It is thought that a lack of nesting sites and and insect food sources is to blame.

Although Lawson said that the incidents are likely to be only a proportion of those that have occurred, she said there is no evidence to suggest the drownings are happening on a very wide scale. The researchers do not consider the incidents to be a conservation issue.

Scientists ask members of the public to report incidents at Garden Wildlife Health, a project that monitors the health of British wildlife.

"Members of the public from around Great Britain have been instrumental in bringing this unexpected cause of starling mortality to our attention by reporting these incidents," said Lawson.