In the days before automobiles, the streets were filled with horse-drawn carriages, and these animals quite naturally left behind deposits from their digestive systems.
These emissions contained half-digested oats that attracted swarms of birds, which took nourishment from the deposits.
The people of the time coined the expression “for the birds” as meaning anything of the same value as these horse-droppings.
We say that something that is worthless is “for the birds”.
“For the Birds” is also a short story written by Isaac Asimov in 1978.