Pic of week: Is it a flock of birds ... is it a plane?
This stunning display of tens of thousands of starlings embarking on their murmuration is one of nature's greatest unexplained wonders.
The birds were captured on camera as they put on their magnificent aerial display, with Criffel mountain in the background and dusk falling near Gretna Green on the border between England and Scotland.
The photograph shows up to 50,000 starlings swooping and diving as one, making shapes in the autumn evening sky. It was taken by photographer Owen Humphrey near Gretna, which is a famous location for the gathering of starlings.
The reason for such murmurations remains unclear. One theory as to why they take place is as a defence mechanism against predators, like hawks or falcons; a case of safety in numbers.
Another theory is that they are signalling a large roost and are attempting to attract other birds to help build up a bigger flock.
Yet another mystery about murmurations is that it is only European starlings that undertake such breathtaking displays in the sky.
Dr Anne Goodenough, reader in applied ecology at Gloucestershire University, asked: "Why is it only European starlings that undertake massive aerial displays for sustained periods before roosting?
"We don't really know. But that makes this topic a really fascinating one to study. If we can work out exactly why starlings murmurate, maybe it will be a good way in to working out why many other species do not."
A large-scale survey of birds across the UK is now under way to help narrow down the theories, with members of the public encouraged to record any sightings.
The study, being conducted by the University of Gloucestershire and the Society of Biology, is also asking people who see murmurations to record details such as location, number of birds, habitat and duration. It has already seen some success, having received more than 600 reports from Cornwall to John O'Groats.
Starling numbers in the UK are actually declining, falling by 66% since the 1970s, and it is hoped the survey will provide important information about their habitat and behaviour.
To take part in the study visit societyofbiology.org/starlingsurvey for details.