Mass outbreak

Mass Outbreaks (Japanese: 大量発生 mass outbreak), also known as swarms and, in Generation V, Pokémon outbreaks, are a feature introduced in Generation II that allows the player to catch Pokémon that are usually rare or otherwise unobtainable.

My Pokémon Ranch explains the general mechanics of a Mass Outbreak.

Generation II

Mass Outbreaks originated in Generation II. In that generation, if the player registered a certain Trainer in the Pokégear, that Trainer would occassionally call to tell the player that he saw or heard about a large number of Pokémon in a certain area; for the next 30 minutes after such a call (in real time, not play time), the swarming Pokémon would appear in the given area with much greater frequency than the usual 1%.

Pokémon Gold, Silver, and Crystal

	Location	Notifier
Marill	Mt. Mortar	Hiker Parry (Route 45)
Yanma	Route 35	Bug Catcher Arnie (Route 35)
Dunsparce	Dark Cave	Hiker Anthony (Route 33)
Snubbull	Route 38	Schoolboy Chad (Route 38)
Qwilfish	Route 32*	Fisher Ralph (Route 32)
Remoraid	Route 44*	Fisher Wilton (Route 44)

Generation III

In Generation III this method allows the player to catch Pokémon that cannot be caught normally and make rare Pokémon temporarily more common. This was possible due to the record mixing feature of Pokémon Ruby and Sapphire. The outbreaks are advised by televisions in houses, where the Pokémon and its location is told.

Pokémon Ruby, Sapphire, and Emerald

	Location	Level	Game
Lotad	Route 114*	13	R	S	E
Seedot	Route 117*	13	R	S	E
Nuzleaf	Route 114	13	R	S	E
Surskit	Routes 102 and 114*	15	R	S	E
Skitty	Route 116	15	R	S	E

Generation IV

Mass Outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex, every day in a particular location, a "swarm of Pokémon" appears, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events. Information about Mass Outbreaks can be found in Sandgem Town, by watching Sinnoh television, or by listening to Professor Oak's Pokémon Talk.

When players all join the same group, they will all experience the same outbreak on any given day.

The species of an outbreaks seems to be determined using a formula, because it is not chosen randomly when the player starts the game that day. It is also possible that the species is determined the day before, though there is no additional evidence to support this. If the DS clock is changed or a different DS is used, then the game will detect this and will not change the species swarming until 12:00 AM.

Pokémon Diamond, Pearl, and Platinum

By also using the Poké Radar and Dual-slot mode in Pokémon Diamond and Pearl, players can get all Pokémon excluding Tangela, Tropius, the starter Pokémon, and legendary Pokémon of previous generations, while Tangela, Tropius and two Legendary Trios can be regularly acquired in Pokémon Platinum.

Many of the Pokémon obtained through Mass Outbreaks are outside of the Sinnoh Pokédex, and are usually exclusive to other regions. However, the player can obtain information about the current outbreak by talking to, depending on their own gender, Dawn's or Lucas's sister in Sandgem Town, or seeing the news report on a TV in Diamond, Pearl, and Platinum.

Due to higher catching rates for Pokémon that appear during outbreaks, they make longer and better chains when using Poké Radar.

	Location	Levels	Game
Pidgey	Route 229	51	D	P	Pt
Magnemite	Fuego Ironworks	28, 29	D	P	Pt
Farfetch'd	Route 221	28	D	P	Pt
Doduo	Route 201	2	D	P	Pt
Drowzee	Route 215	20, 21	D	P	Pt
Krabby	Route 226	49–52	D	P	Pt
Voltorb	Route 218	28, 29	D	P	Pt
Cubone	Route 203	4	D	P	Pt
Cubone	Route 202	3,4	D	P	Pt
Lickitung	Lake Valor	34–36	D	P	Pt
Pinsir	Route 229	48, 49	D	P	Pt
Natu	Route 224	53	D	P	Pt
Dunsparce	Route 208	18	D	P	Pt
Snubbull	Route 209	16	D	P	Pt
Swinub	Route 217	34	D	P	Pt
Corsola	Route 230	50	D	P	Pt
Delibird	Route 216	32	D	P	Pt
Delibird	Route 217	32, 33	D	P	Pt
Phanpy	Route 207	5	D	P	Pt
Smoochum	Lake Acuity	35	D	P	Pt
Larvitar	Route 206	14–16	D	P	Pt
Zigzagoon	Route 202	3	D	P	Pt
Zigzagoon	Route 201	3	D	P	Pt
Surskit	Lake Verity	2	D	P	Pt
Slakoth	Eterna Forest	10,11	D	P	Pt
Makuhita	Route 225	48-51	D	P	Pt
Nosepass	Route 206	14	D	P	Pt
Skitty	Route 222	40	D	P	Pt
Skitty	Route 222	36-40	D	P	Pt
Electrike	Valley Windworks	7,9,10	D	P	Pt
Spoink	Route 214	22, 23	D	P	Pt
Spinda	Route 227	55	D	P	Pt
Absol	Route 213	20	D	P	Pt
Beldum	Route 228	51-54	D	P	Pt

Pokémon HeartGold and SoulSilver

The feature also appears in HeartGold and SoulSilver, where all the wild Pokémon available include all those that were available in Generation II, Ditto, Chansey, and Pokémon from Hoenn and Sinnoh. Information on swarms can now be found by listening to Professor Oak's Pokémon Talk on the Pokégear radio.

	Location	Levels	Game
Chansey	Route 13	23	HG	SS
Ditto	Route 47	34	HG	SS
Marill	Mt. Mortar	15	HG	SS
Yanma	Route 35	12	HG	SS
Dunsparce	Dark Cave	2-3	HG	SS
Snubbull	Route 38	16	HG	SS
Qwilfish	Route 32	40	HG	SS
Remoraid	Route 44	20	HG	SS
Poochyena	Route 1	2	HG	SS
Wingull	Vermilion City*	35	HG	SS
Ralts	Route 34	10-11	HG	SS
Sableye	Route 9	13-15	HG	SS
Mawile	Route 9	13-15	HG	SS
Gulpin	Route 3	5	HG	SS
Swablu	Route 45	23	HG	SS
Whiscash	Violet City	40	HG	SS
Baltoy	Route 3	5	HG	SS
Clamperl	Route 19	35	HG	SS
Relicanth	Route 12*	40	HG	SS
Luvdisc	Route 27*	20	HG	SS
Kricketot	Viridian Forest	3	HG	SS
Buneary	Route 25	8-10	HG	SS

Generation V

This section is incomplete.
Please feel free to edit this section to add missing information and complete it.

Black and White

The gates in Unova have a monitor inside that will alert the player where an outbreak is taking place. During winter, the monitor may occasionally not report on outbreaks, which is due to parts of Route 8, where Croagunk can be found, becoming frozen during Winter and not allowing the Pokémon to be caught there. All have the level range of 15-55.

#	Pokémon	Location	Levels	Game
046	Paras	Route 11	15-55	B	W
056	Mankey	Route 15	15-55	B	W
083	Farfetch'd	Route 01Route 1	15-55	B	W
084	Doduo	Route 12	15-55	B	W
102	Exeggcute	Route 18	15-55	B	W
161	Sentret	Route 07Route 7	15-55	B	W
193	Yanma	Route 14	15-55	B	W
204	Pineco	Route 16	15-55	B	W
228	Houndour	Route 09Route 9	15-55	B	W
235	Smeargle	Route 05Route 5	15-55	B	W
236	Tyrogue	Route 10Route 10	15-55	B	W
261	Poochyena	Route 09Route 9	15-55	B	W
285	Shroomish	Route 11	15-55	B	W
311	Plusle	Route 06Route 6	15-55	B	W
312	Minun	Route 06Route 6	15-55	B	W
313	Volbeat	Route 03Route 3	15-55	B	W
314	Illumise	Route 03Route 3	15-55	B	W
353	Shuppet	Route 13	15-55	B	W
360	Wynaut	Route 02Route 2	15-55	B	W
449	Hippopotas	Route 04Route 4	15-55	B	W
453	Croagunk	Route 08Route 8	15-55	B	W