Today's Earth Science Picture of the Day highlights 16 shots of these flocks of European Sparrows in Denmark. I've highlighted only these two so as to claim fair use, or something.
Flocks of millions of these birds make formations... but only during a few weeks of the year, and only a half hour before sunset. So many of the birds are in the sky that, at certain times, they block out the sun... thus the name of the phenomenon: "Black Sun".
