Soldiers carry their rations slung across their back;
I have no need for food, yet I too have a sack.
I do not stir; I remain silent and still;
Watching, waiting, to hear the enemy trill.
Then I strike, warding off the warriors of the air,
But soldiers of the ground, me they do not fear.
The vulnerable about me, it is my job to defend.
I fight day after day, but see no end.
What am I?
I am not an urban dweller. I belong in the country.
When both Sides fight, my cousin's name is often invoked.
Portal 2 is the most evil computer game. I sympathise with GLaDOS.
The word "trill" is a big hint. Think about how this noun is typically used.
Fun fact: I am named after my enemy.