A cuckoo is a military slang term for a sniper, disguised in the sprawling tree. This word has been particularly applied to the Finnish Winter War snipers and the World War II German snipers, who took pot-shots from hidden vantage points.
During World War II, before 1944, the German high command left many cuckoos behind as their armies retreated, in order to delay the Soviet rush.[1]
The 2002 Russian film The Cuckoo tells a story of Finnish cuckoo sniper.