Whistling is a skill and game mechanic from The Legend of Zelda: Breath of the Wild. It is available from the start of the game.
Link can whistle by pressing down on the control pad and is available from the start. When performed, Link will cup his hands and whistle the notes to Epona's Song. Whistling is primarily used to summon Link's horses once registered and regardless of their temperament or bond with him. However he can only call a horse if it has been taken out of the stable. Also unlike the Horse Call, Horse Grass, or Epona's Song used to summon Epona in past games, all registered horses can only be summoned if they are nearby and can reach Link's location as it simply calls them and does not make them warp to Link's location if they are too far away. If too far away, Link will receive a message stating that the horse cannot hear Link's whistle, and the message will occasionally state that the horse cannot reach him. If the horse is nearby when Link whistles, the horse's mini-map icon will pulsate to indicate that it has heard him and is traveling to his location.
In addition to calling horses, Link can also use whistling to call Wolf Link, who is summoned by using the amiibo Rune with the Wolf Link amiibo, to him after commanding him to stay. During the Quest to intercept Vah Rudania, Link can also whistle to signal to Yunobo when to stop and when to follow him. While whistling is useful in these instances, it creates noise which can alert enemies to Link's presence. However, Link can use it to lure enemies to him which can be useful for setting traps or initiating combat.