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If you visit Lucas/Dawn's house in Sandgem Town after getting the National Pokedex, his/her sister will tell you about a daily swarm of Pokemon. These pokemon only appear at these locations for 24 hours, so you should go to the location and catch some as quick as possible!

Diamond/Pearl

Absol - Route 213

Beldum - Route 228

Corsola - Route 230

Cubone - Route 203

Delibird - Route 216

Doduo - Route 201

Drowzee - Route 215

Dunsparce - Route 208

Electrike - Valley Windworks

Farfetch'd - Route 221

Krabby - Route 226

Lickitung - Lake Valor

Magnemite - Feugo Ironworks

Makuhita - Route 225

Natu - Route 224

Nosepass - Route 206

Phanpy - Route 207

Pidgey - Route 229

Skitty - Route 222

Slakoth - Eterna Forest

Smoochum - Lake Acuity

Snubbull - Route 209

Spinda - Route 227

Spoink - Route 214

Surskit -Lake Verity

Swinub - Route 217

Voltorb - Route 218

Zigzagoon - Route 202


Platinum

Beldum - Route 228

Corsola - Route 230

Cubone - Route 203

Delibird - Route 217

Doduo - Route 201

Drowzee - Route 215

Dunsparce - Route 208

Electrike - Valley Windworks

Farfetch'd - Route 221

Krabby - Route 226

Makuhita - Route 225

Natu - Route 224

Larvitar - Route 206

Phanpy - Route 207

Pinsir - Route 229

Skitty - Route 222

Slakoth - Eterna Forest

Snubbull - Route 209

Spinda - Route 227

Spoink - Route 214

Voltorb - Route 218

Zigzagoon - Route 202

Every 24 hours a different swarm appears, but at the end of the day they go away. So one day she might tell you that Electrike are swarming at Valley Windworks, but if you check back at that same location the next day, they'll be gone.

Also, the swarms seem pretty random. Sometimes the same type of pokemon will swarm twice in one week. Since I got the National Pokedex in Platinum, I've seen Electrike swarms at least 3 times...and I've only had the National Pokedex for about a month. So if you aren't able to catch one the first time around, you'll get another chance...you might just have to wait a while!