For the birds is an idiom describing something as useless, meaningless, or only believed by the gullible. It is often used with the modifier strictly.
The phrase was coined in the US military during World War II. The original phrase included a swear word, talking about birds pecking at horse manure for seeds.
That’s for the birds hit its peak in popularity around the 1960s. It was used more often within the United States. However, now the positions have switched. Currently, the phrase, though rare, is used appropriately in other English-speaking countries. If it is used within the United States, it is almost always meant to be a parody of the original phrase, and is used to actually discuss something pertaining to birds.