Why do Pokemon like Staraptor have a second type?

Question

They can easily be only Flying-type.

Answer 1

Game Freak Logic and Roost
Well, first of all Birds are Common and Normal, and Game Freak just wanted it that way as already mentioned. They may also want it to be Balanced Gameplay,however Roost is another Main Factor

Before Gen VI any Pure Flying type(Tornadus) would turn into a ??? type
That would be A Problem. Game Freak thought of it and Fixed it in Gen VI, so now if they decide to make any other Pokemon like that they'll have no Problem. In Gen VI if Tornadus uses Roost, he will become a Normal type for the Rest of the Turn

Comments

Good explanation

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What is this "fixing" you speak of, Tornadus is the only pure Flying type, and cannot learn Roost.

Answer 2

Because Gamefreak.

There are some explainations to this one though. A normal type is generally regarded as a Pokemon that is not too much out of the ordinary (with exceptions of course). Most Pokemon that get a normal typing are those that are part of the general and simpler Pokemon.
Staraptor and many other Normal/Flying types are so most probably because they are not very much out of the ordinary, and use very simple tactics and moves. This is also the reason Tornadus is not a Normal type, being the only pure Flying type.

Hope I helped!

Comments

Roost is probably a big factor too.

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That is also there, but other pokemon like Pidgeot and Fearow had been Normal/Flying much before Roost was introduced though. That's the only problem, otherwise it's a good explaination.