A feature that was introduced in Past games is the implementation of Swarms. These are where Pokémon in certain routes have their appearence Percentage raised so that they are more common.
In Diamond & Pearl however, after you get the National Pokédex, if you go to your Other Rival's house in Sandgem Town and talk to their sister, they will tell you that there is an increase of Pokémon in a certain Route or área.
If you go to the route or area that the girl states, within one or two wild Pokémon battles you will find the Pokémon at the same level as the other Pokémon.
The Pokémon and Route change every day and all of the Pokémon that you find and their evolutions are not obtainable by any other means in Diamond & Pearl so be sure to capture them while you have the chance.
Below are all the Swarming Pokémon and the routes/areas they swarm in:
Doduo - Route 201
Zigzagoon - Route 202
Cubone - Route 203
Nosepass - Route 206
Phanpy - Route 207
Dunsparce - Route 208
Snubbull - Route 209
Absol - Route 213
Spoink - Route 214
Drowzee - Route 215
Delibird - Route 216
Swinub - Route 217
Voltorb - Route 218
Farfetch'd - Route 221
Skitty - Route 222
Natu - Route 224
Makuhita - Route 225
Krabby - Route 226
Spinda - Route 227
Beldum - Route 228
Pidgey - Route 229
Corsola - Route 230
Surskit - Lake Verity
Smoochum - Lake Acuity
Lickitung - Lake Valor
Magnemite - Fuego Ironworks
Electrike - Valley Windworks
Slakoth - Eterna Forest
Source - Serebii
Hope this helps!
