The ones that can’t reach the lower branches make nests on the ground. Those can reach the lower branches make nests there. That makes sense - if it’s relatively safe in the brances and dangerous on the ground, then stay in the branches.
A better point would be to consider the dogs. Which don’t climb very well. So eventually, if all the chooks end up on the lower branches, the dogs would become extinct.
But let’s say that a few dogs are slightly better climbers and now and then can get to those lower branches. They would survive whilst the dogs that were not such good climbers would die off. Pretty straightforward?
So what do we have now? It’s like an arms race. The dogs that can climb to the lower branches survive and the chooks that can get to a slightly higher branch also survive.
If the dogs can climb no higher, they become extinct and the chooks remain as they are. There is nothing that is taking the least able flyers out of the gene pool. But if a greater proportion of the dogs being born have a genetic tendency passed on to be good climbers, then we still have this ‘arms race’.
Sound reasonable? It’s like poachers shooting elephants with long tusks and taking them out of the gene pool so we are left with elephants that are genetically inclined to have short tusks. If the dogs which are good climbers take out the chooks which can’t fly very well, then we are left with ones that can.