The SOKOL movement began in pre-WWI Czechoslovakia.
It was a movement of renewed Slavic Patriotism
through gymnastics and physical fitness.
Its aim was to unite all Slavic Lands, from Poland to Russia.
As it was a uniting movement, its members wore red shirts like the Garibaldisti.
Giuseppe Garibaldi struggled to unite Italia, and wore a red shirt, like his troops did.
Hence the red shirts among the Sokols members,
calling each other Brat (Brother) or Sestra (Sister),
like some Slavic masonry.
When the initiators of this Idea were to name this movement,
they did it in remembrance of the Serbian Epic Poetry,
in which the Heroes are called "Sokol" ("Hawk").
I'll put a whole series of SOKOL posters:
