Every year in autumn, hundreds of thousands of migrating starlings gather in the marshes of the Wadden Sea National Park in Denmark looking for rest and for food on their way south for the winter.
When the birds attempt landing, they perform aerial ballets and formations to avoid or counter- attack birds of prey trying to enter the flock.
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These moving formations, known as murmurations, block the view to the sky and the phenomenon is therefore called "Black Sun."
"Black Sun is the biggest Danish nature event. It is when the starlings are going down in the vegetation to sleep and they'll be attacked by the hawk and they make marvellous formations," explained nature expert Iver Gram.
"So when the birds of prey are coming in, we see the biggest black sun," he added.
Every evening, up to a million birds can be seen landing in a chosen spot at sunset.
"Right now in the highlight from the end of September to the beginning of October, we have around 800,000 starlings in one sleep-over," said nature expert Anja Kjolby.
The Wadden Sea National Park, which is a World Heritage Site, is situated on the west coast of Denmark, stretching to the German border.
Around 100,000 visitors from near and far visit the national park every year to experience the birds' performance.
The concentration of birds is the largest from the end of September to mid-October. After eating and resting, the birds all rise at once and head south before returning in the spring.