How do birds "find" the bird feeder?

I have a bird feeder hanging outside the doors to my patio.

I do my best to remember to keep it filled, but the truth is, I run out of bird food, or forget to fill it up, sometimes for a couple of weeks.

When I finally do fill it, it hangs there for a while and no birds come around.

But then, a few hours later, no more than a day later, the birds are all over it.

This tells me that for a while the birds are not aware that the bird food is there. Then, somehow, they become aware of it, and not only that, a whole lot of them become aware of it. And not just one type, who certainly flock together, but all sorts of types of birds.

So how is it that the birds “find” the food? Is it by some coincidence that they fly by and see it? So they smell it? When one type of bird arrives, do the other ones see the action and join in? Or do they chirp at each other? Or what?