A Macedonian police officer hits a stranded migrant attempting to cross the Greek-Macedonian border, near Gevgelija, Macedonia December 2, 2015. Picture taken from the Greek side of the border. (Photo by Alexandros Avramidis/Reuters)

Migrating starlings fly in formation near the southern Israeli town of Rahat February 2, 2015. The phenomenon, known as murmuration, is most common during the winter months. (Photo by Nir Elias/Reuters)

A Ukrainian serviceman wearing a new year's hat waves at his comrades while standing beside his tank near Debaltseve, eastern Ukraine, February 8, 2015. (Photo by Gleb Garanich/Reuters)

North Korean children wave to people on a Chinese tourist boat on the banks of Yalu River near the Chongsong county of North Korea, opposite the Chinese border city of Dandong, May 8, 2011. (Photo by Jacky Chen/Reuters)

A horse grazes on railway tracks near the old train station on the the Northeast railroad (Ferrovia do Nordeste) in the city of Missao Velha, Ceara state, northeastern Brazil, October 25, 2016. (Photo by Ueslei Marcelino/Reuters)

Participants fancy dressed go downhil in the XXVII Car Festival in a homemade cart in the Santa Elena Municipality, near Medellin, Antioquia department, Colombia, on December 18, 2016. (Photo by Raul Arboleda/AFP Photo)

A dog runs as Palestinian boys ride donkeys carrying vegetable after working in their field in the West Bank village of Nassariya near Nablus November 30, 2016. (Photo by Abed Omar Qusini/Reuters)

Anti-Heathrow expansion activists walk through the “Grow Heathrow” protest camp in the village of Sipson near to Heathrow Airport, west London Britain December 7, 2015. (Photo by Peter Nicholls/Reuters)