Today I Learned that...I need to learn how birds find food

I have a small garden outside my first floor...and I put a bird bath in the middle of it. I expected a passing bird to stop in and have a bath.

But it was under-used until my dad put a bird feeder adjacent to it.

First there were ten birds there, then twenty. Today when I looked out I swear there were a hundred birds clustered around that feeder.

How do they find out? Birds don't have a sense of smell...so I guess they must be flying by and see some other birds clustered around something and descend to investigate...and once they've found a source of food they don't leave it..

My mom's got bird feeders on her side of the house (top floor, in the opposite direction from where my bird feeder is) and she gets hundreds of birds too. I wonder if they are the same birds that fly to each feeder, or if each bird has staked out their own territory.