Expansion can be due to a lot more than mere birthrates. First and foremost expansion will happen only when there is means to do so. And, 'means' can be driven

Expansion can be due to a lot more than mere birthrates. First and foremost expansion will happen only when there is means to do so. And, 'means' can be driven by a combination of curiosity and necessity. On earth people have gone somewhere far off (migrated), because there was the place to go and the means to get there. In many cases that (migration) was the option over having to fight (or find some form of accommodation) within the local social & political system. If this is why an intelligence can spread out on a planet, couldn't other intelligences (if they exist) spread out for similar reasons in the galaxy.