Survial of the Fittest

It was shocking to me this morning to see how much the little cardinals had grown in just one day, but I have to admit it was a bit disturbing to see that only 2 babies are now in the nest. Yesterday there was 4. One had been rather lethargic and there was another that seemed a bit less developed than the two strong ones. Those two are gone.
I am not sure if the mom knocked out the two weaker babies or if the stronger babies knocked them out of the nest, whatever, it was a reminder that in nature, the rule regarding the survival of the fittest is more than just a phrase.
When I came up to the nest this morning, the two remaining babies were sleeping, the slightest touch to the pine needles stirred one immediately and he/she was wide mouthed, ready for food.
I hope these two make it to the stage where they will fly out of the nest. I have seen so many nests on our land be invaded by some sort of creature.