It seems to be related with the Peyote Plant:
One new feature in the updated version of that Rockstar has kept quiet about is the addition of peyote plant collectibles, which when consumed send you through a hallucinatory trip into the body of a random animal! Some peyote plants turn you into a bird, whereas others transform you into four-legged or aquatic creatures, but until you complete your collection you only get one shot with each plant - so make a save beforehand if you want more than a single attempt at unleashing your inner beast.
Once the player finds a Peyote plant, an on-screen prompt will tell the player to press right on the D-pad in order to eat it. If pressed, a short cutscene will play, during which the player will be seen kneeling down and breaking a chunk off of the Peyote plant, before popping it in his mouth. After this, he will faint and fall on the ground, and the screen will shimmer and display bright colors (similar to the effects of Weed), and then the screen will suddenly go black. When the player comes to, he will seemingly find himself in the body of a random animal. While this is obviously part of his hallucination sequence, the player will be able to move the animal around and (for birds) even fly it around the gameworld (using controls similar to those of flying normal fixed-wing aircraft), interacting with pedestrians and other animals. The player is unable to pick up any objects while in the hallucinogenic state, so exploring underwater Wrecks can be done but no weapons or briefcases can be collected.
However, you can get a wanted level (Wikia again):
If you fly as a bird over any restricted areas, expect a wanted level.