What are the enemies of a cockatiel?

I've read many an article where Budgies are said to be a natural enemy to the Cockatiel, however; I have three Budgies and two Cockatiels who live in separate cages but daily are allowed out to roam around the livingroom together. They all get along quite well and most times explore each others cages while sampling each others food dish. Occasionally the male Cockatiel will make a ruckus if one of the male Budgies happens to be in the cage with the female Cockatiel, but none of them have ever fought or shown any display of aggression where I have had to intervene. They get along (as a family).