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In addition to changing routes (something that happens every time the player moves to a new area), roaming Pokémon will also attempt to flee from battle.
When encountering a roaming Pokémon for the first time, their HP and DVs are initialized. Those stats remain static once generated. This means that Shiny Pokémon will remain Shiny, and any damage that has been inflicted to them will remain across battles. However, their PP and status conditions are reset each time they flee.
In addition to moving around each time the player changes map, all currently roaming Pokémon will also move after each battle with a 1/16 chance (always, if the player just encountered a roaming Pokémon, not merely the one they have just encountered).
There is a 1/8N chance that a roaming Pokémon will move to a completely random map other than the one the player is on, where N is the amount of possible connections to where the roaming Pokémon is at the moment. Otherwise, a roaming Pokémon will move to an adjacent route. A roaming Pokémon will never move past Olivine, meaning that if it is currently on Route 39, it will always move to Route 38.
Each time the player steps in tall grass that is currently in the same area as a roaming Pokémon, and the game attempts to generate a wild Pokémon, the player has a 25/256 (or roughly 10%) chance of randomly encountering it.
A roaming Pokémon will flee on any turn it is able to, immediately before it would use its move; if it is unable to flee, it will instead use its prepared move. Since the roaming Pokémon in Generation II all know moves with different priorities, depending on the move they are prepared to use, the player's Pokémon may sometimes act before it flees and sometimes not.
If a roaming Pokémon is defeated, it will not reappear under any conditions.
The chance of encountering a roaming Pokémon is now 25%, given that a wild encounter has successfully been generated in tall grass or water in the same area as a roaming Pokémon. Roaming Pokémon are affected by Repels but not Abilities that affect appearance of wild Pokémon.
From Generation III onward, if a roaming Pokémon is afflicted by a non-volatile status condition, it will retain those effects in subsequent encounters.
If a roaming Pokémon is not already trapped, each turn it will attempt to flee at an effective priority of 0 instead of using a move. If it becomes trapped in the battle during the turn, it will attempt to flee but fail. If it is already trapped at the start of a turn, it will use a move instead of attempting to flee.
A roaming Pokémon can become trapped if it is hit by a trapping move before it is able to act. Trapping Abilities (such as Shadow Tag) can only prevent the Pokémon from attempting to flee if in place at the start of the turn; if a Pokémon with a trapping Ability is switched in, its Ability will not prevent the roaming Pokémon from fleeing on that turn if it is able. Status conditions do not prevent a roaming Pokémon from fleeing.
Starting in Platinum, if the player defeats a roaming Pokémon, it will reappear when the player next enters the Hall of Fame, but the player must retrigger its roaming. For example, if the player defeats Mesprit and then enters the Hall of Fame, Mesprit will only begin roaming again once the player visits Verity Cavern.
If a roaming Pokémon is not already trapped, each turn it will attempt to flee at an effective priority of -7 instead of using a move. If it becomes trapped (either by a trapping move or Ability) before it is able to act, it will attempt to flee but fail.
If the player defeats a roaming Pokémon, it will reappear when the player next enters the Hall of Fame.
A roaming Pokémon will flee unconditionally after the player sends out their Pokémon. It will become stationary in a specific area after the player encounters it 11 times. Not even Shadow Tag will prevent it from fleeing.
If a roaming Pokémon is defeated, it will reappear in its stationary position the next time the player enters the Hall of Fame.
Unlike in previous games, roaming Pokémon no longer flee once encountered. They appear in the overworld and must be caught up to in order to be encountered.
In Generations II, III, and VI, players can use their Pokédex to check the current location of a roaming Pokémon (after they have seen it). In Generation IV, the Pokétch's Marking MapDPPt application and the Pokégear's map cardHGSS serve this purpose. In Generation V, the player can track their locations using the notifications of stormy weather on electric bulletin boards in gates.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
In Pokémon Black and White, after obtaining the Legend Badge, when entering the gate leading to Route 10, the woman behind the desk will alert the player about the big storm on Route 7. TornadusB or ThundurusW will make the weather more extreme in Route 7 when entering the route. Heading towards the second house on the route will cause an old lady to come out and pull the player in. She will then recite the story of either TornadusB or ThundurusW. Upon exiting the house, the player will be approached by the respective Pokémon, which will then fly off and will roam around Unova until being caught or defeated. Wherever these Pokémon appear will be subject to weather identified as "stormy" by the bulletin boards in gates. Only these two Pokémon can cause stormy weather (which manifests as rain in-battle), so they can be tracked down using the bulletin boards.
Unlike other generations, the roaming Pokémon's location varies depending on the time of day. At night, between midnight and morning, the Pokémon can only be encountered in Routes12, 13, 14 and 15, in the morning it can only be encountered in Routes 2, 3 and 18, during the daytime it can only be encountered in Routes 4, 5 and 16, in the evening it can only be encountered in Routes 6, 7 and 8 and at night before midnight it can only be encountered in Routes 9, 10 and 11.
Bulletin boards can be considered somewhat inferior to the tracking methods of previous generations, as, unlike Generation IV, Tornadus or Thundurus's location changes when the player exits the gate. However, as they simply cycle between three or four routes, one may continuously re-enter and exit the same gate until the route displays stormy weather.
If the player uses Fly, the roaming Pokémon will relocate to a random location. Traveling normally, on the other hand, will only provide a small chance for the roaming Pokémon to change routes, and even then it will move to an adjacent route. Furthermore, entering any of the connecting gates between cities (or Victory Road) may cause the roaming Pokémon to relocate randomly. This appears to be on a timer, however, as immediately pursuing the roaming Pokémon does not seem to cause it to change location.
As the bird will automatically flee upon encounter, it cannot be caught until after it has been encountered an additional 10 times (not counting the first encounter), and has subsequently retreated to the Sea Spirit's Den in Azure Bay, where it remains as a stationary Legendary Pokémon. However, if the battle in the Sea Spirit's Den is fled from, the bird will begin roaming Kalos again as before.