Why do headlines drop their a's and the's?

Why do some say walkin, talkin,watchin etc? Is it slang?

Is this grammatically correct? The war had ended. The people were rejoicing in the streets.

Is it correct to say? Is it correct to say Sky is grey. instead of The sky is grey. in poetry or in an informal situation?

Tenses, what is the difference? (a) The Beatles have influenced a lot of musicians. (b) The Beatles influenced a lot of musicians. (c) The Beatles had influenced a lot of musicians.

Are these correct? (a) A bird flew under the bridge. (b) A bird flew underneath the bridge. (c) A bird flew below the bridge. (d) A bird flew beneath the bridge.