sacrifice for a worthy cause

The bird's nest hanging
Strong even with gushing wind
It sways yet it never falls

The little birdies tweetering
Short distance calls for food
As I stand listening to its sound
The windy breeze rustling through

In the beginning there were 4
One came two days later
In time the little birdies grow
With feathers maturing quickly

Yet this morning I look
The youngest one died in the nest
It could be too many to hold
One has to go so the youngest one died
Stepped on its stomach dark blue black
It was dead in the early morning

Which one killed it?
I guess it could be the mother bird
One has to sacrifice to hold the nest
When they have grown
The nest may not take the weight

The nature of elimination
For the better good to the specis
So the natural law of equation
Sacrifice for a worthy cause
The survival can populate