Factorio Forums

The wiki states that "Every 4-60 minutes, a group of 5-20 biters/spitters will leave their base to create a new base which will consist of as many worms/nests as there are members in the group. This group will search for a suitable spot that's 3-7 chunks away from existing bases." (emphasis mine)

My question is, What happens if there are no suitable spots within the 3-7 chunk radius? Will that base just never spawn any migrators?

Or will they migrate to the "best available" chunk even though that chunk may have a .0001 suitability rating?

They will always pick one of the available spots, a high suitability rating just means it is more likely to be picked.