That's...not what Darwin's theory says at all.
The strongest don't survive, the fittest survive.
Strongest in terms of the fit traits they have. If the creature doesn't have the traits that are suited to their environment, they die. Their beak isn't efficient enough, etc. Therefore, not 'strong' in terms of physical strength or hardiness, but strong in that the creature has what it takes, genetically, to survive in their given environment.
I was dumbing it down for my earlier post. :>