Recorder

A Warp Whistle is a rare item from Super Mario Bros. 3. Using it sends Mario (or Luigi) to the Warp Zone. From here, he can skip up from the middle of a world to the start of a later world. Using it in World 1 will allow Mario to warp to World 2, 3, or 4. Using it in a world from 2 through 6 will allow him to warp to World 5, 6, or 7. Using it World 7 or 8 will allow him to warp to World 8. If a second whistle is used while in the warp zone, then the player is taken directly to the World 8 Pipe.

There are only 3 whistles in the North American/European Super Mario Bros. 3. In the Japanese version of Super Mario Bros. 3, there are 4 whistles, as there is a glitch where one is located in World 5-1.

World 1-3
Obtained pressing the down button the D-Pad on a white block, and running behind a black scenery.

World 1-Fortress
Obtained by running so the P-Meter is full, and flying up to a secret platform, where there is a door. The player needs to press up on the D-Pad once the player has gone as far as they can, and a Warp Whistle in a treasure box is inside.

World 2-Fire Bros.
The player must defeat the last Boomerang Bro.. The player then obtains a hammer, which must be used to break a block and go to a secret passage. In the passage, there is a Toad House that contains better items than normal, and a Fire Bro..

World 5-1 (Japanese version only)
In the Japanese version of Super Mario Bros 3, there's a glitch in 5-1 which makes random item appear at the end of the level. It can be done by flying above the stack of blocks at end of the level. After that behind the roulette block the player will notice glitched graphics. By hitting it from far left corner it's transforming into either 5-Up Mushroom or Warp Whistle.

Trivia

 * Mystical Whistle, the name of a Sammer Guy in Super Paper Mario, is a pun on Warp Whistle.
 * The song played by the Warp Whistle is from The Legend of Zelda series - in fact, this is based on the Recorder from the original game, complete with a Whirlwind taking the player to another location.
 * In the 2006 version of Nintendo Monopoly, the Warp Whistle is referrenced in the Coin Block "Advance to Go" card.
 * Another flute is found in the game Mario is Missing!, stolen from the Teatro Colon by Koopa Troopas. Luigi had to return it to the opera house.