Source Advanced Player’s Guide
A simple whistle used to send short, shrill audible signals. With a DC 5 Perform (wind instruments) check you can use a whistle to signal the same sorts of situations as signal horns. A whistle’s piercing report can be clearly heard (Perception DC 0) up to a quarter-mile away. For each quarter-mile beyond, Perception checks to hear a whistle suffer a -2 penalty.