Black Sun: Millions of Birds Block Out the Sun.

From the website:

During spring in Denmark, at approximately one half an hour before sunset, flocks of more than a million European starlings (sturnus vulgaris) gather from all corners to join in the incredible formations shown above. This phenomenon is called Black Sun (in Denmark), and can be witnessed in early spring throughout the marshlands of western Denmark, from March through to the middle of April. The starlings migrate from the south and spend the day in the meadows gathering food, sleeping in the reeds during the night. The best place to view this amazing aerial dance is in the place called "Tøndermarsken," where these pictures were taken (on April 5 from 19.30 to 20.30 local time).

Link – via Militant Platypus