I remember back in 10th grade I learned that birds that had beaks that were easier to break the seeds carried those traits to their babies and the whole area had that type of beak since it was necessary to break some seeds. So the beaks that were a disadvantage no longer existed and the beaks that became an advantage passed along to all the other birds.
Side note: if they all had the same beak they could all fight for the same type of seeds and probably have some sort of intense competition but idk if that’s necessary