Trivial; worthless; only of interest to gullible people.
This phrase is of American origin and, while still in use there, has never been commonly used elsewhere. It is US Army slang and originated towards the end of WWII. An early example of its use is this piece from The Lowell Sun, October 1944, in an interview with a Sergt. Buck Erickson, of Camp Ellis, Illinois:
"Don't take too seriously this belief that we have football at Camp Ellis solely for the entertainment of the personnel - that's strictly for the birds. The army is a winner... the army likes to win - that's the most fortunate thing in the world for America."
'Strictly' is frequently used as an intensifier, as in the example above.
'That's for the birds' is a shortened form of the vulgar version 'that's shit for the birds'. That suggests the derivation of the phrase which is the habit of some birds of pecking at horse droppings (a.k.a. road apples) in order to find seeds. Both versions were defined in an edition ofAmerican Speech from 1944:
That's for the birds. It's meaningless
Shit for the birds. Nonsense, drivel, irrelevant matter.
This is only about half the pictures I had to share -- for some reason blogger thinks I don't have a license to load my own photos...