Migration, birth rate... immigration?

It's assumed that the player won't care because they'll all be given immigrants in time. Once the tiny place fills, the medium planet will be filled up. It's when you suddenly expand fast that there's a noticeable lag between new colonies forming and immigrants coming in.

The real purpose of immigration is to get alien races onto domed colonies that can breath the correct atmosphere. Then you can remove/scrap your own people from that colony and you have a fully functional colony with alien citizens loyal to you.

I just don't understand why players are complaining about migration. I've seen where my augmented growth rate is far beyond the migration rate and colonies fill up nice and quick. Instead of worrying about migration concentrate more on population growth to get those big new empty colonies full. I always use a cargo bay in my colony ships to augment taking population from my homeworld, or any world that reaches max population, to new colonies. A bigger starting population really helps get those first facilities built quicker. I like keeping near max populations as that helps keep production/research/intel high.

I also build up my medical bays to help increase population growth and they work well doing that and protecting from plagues.
Omnius

One thing I think people are missing is that full-population worlds help channel migration towards worlds that need population. That's good for getting new colonies growing faster.
Omnius

exactly, migration is coming from full pop world that have pop growth. If population is growing but has no place for the people to go they will go to another non-full colony.

I started up a 1.20 game with 7 hard max-bonus AI's. I started with all techs and 10 homeworlds and I picked the Organics Tech. At the one year mark I had grown my population from 40 billion to 49 billion! I had 5 systems to start. I decided to use all the population growth and happiness techs like Urban Pacification, Genetic Recoding, Gestation Vats, Replicant Centers and Climate Control. You might be wondering what the heck Climate Control has to do with population growth but I've noticed a 2% increase in population growth for every level of Conditions improvement.

With these items in place I'm getting 17% growth on one colony all by itself and before it has completed any of the magic growth items. In the 5 systems I've got all 5 complete I'm getting 33%-40% growth rates with a 7% migration rate. I managed to colonize 17 new colonies in my 5 systems consisting of huge and large domed colonies with 3 huge/large breathable worlds. The two huge breathable new colonies have grown from 241 million on turn 2 to over 1.3 billion on turn 10!

So stop fretting about migration and concentrate on growth tech items to build to boost your population growth. Don't worry about moving population around as while they're in transit they aren't growing.
Omnius

Migration is good for taking alien populations off of your usually full captured worlds, and depositing them on worlds they can breathe. Then you take all of the old colonists off the planet and use your growth techs to make it have more poulation.