Bonus Population(officially called Population Upgrade) increases population of existing residences.
When collecting rents from houses, there's a chance that population truck will drop, provided that your citizens are happy. How this chance is calculated is unknown.
On March 12, 2012 a +20 Population option was added to the Gifts menu.
There are four types of Population Trucks distinguished on the basis of the amount of bonus population each gives.
During Halloween (2011), monster housing dropped purple population trucks. > > > > > > > > > >
From December 22-25, 2011, CityVille increased the chances to earn more Population by 50%.
Chances and Amount of Bonus Population[]
These are only a few examples. To see the Bonus Population of a specific house, go to that item's page.
Initial Population - Starting population of a residence (lower number in housing menu).
Bonus Population amount - The amount of bonus population you can get upon collecting rent if Happy or Unhappy, but not when Very Sad. % chance of getting bonus is unknown.