Bonus whistles are items used to intiate special attacks during Moga battles. They appear to be small, strawberry-shaped fruits. Each Moga Tamer is given a bonus whistle tree at their ranch which produces a free bonus whistle every 20 hours. The tree must be watered to continue producing whistles; otherwise, it will wither.
Bonus whistles are used during battles to call neighbors to assist a tamer in battle. The attacks have a 100% accuracy rating and are generally more powerful than regular attacks. Each attack costs 1 whistle and can only be used once per turn. Using a whistle calls a neighbor (or Nova, if a tamer has not recruited any neighbors), whose randomly selected Moga appears and uses its Zodiac Attack (strengths/weaknesses are not calculated). Whistles give lower level Mogas an edge in battle.
In addition to the tree, there are numerous other ways to obtain bonus whistles. These include visiting neighbors' trees, completing quests and card collections, and sending and posting Facebook stories when prompted. They can also be purchased via the Sky Shop.