Levels of sentience

I have a friend who has ants in her house. One day I put some plates with food on them in the ant zone. When we discovered the plates seething with ants I attempted to wash the ants down the drain. My friend stopped me. 'Let them eat,' she said. For several days I watched as the food congealed and hardened and the ants continued to feast.
During a recent snowstorm I found an Audubon Warbler floundering on the snow. I brought the beautiful bird into the house and attempted, unsuccessfully, to save it. At the same time I was trying to come to a decision about the phalanx of mice who had invaded my house. If they had stayed on the floor I wouldn't have cared, but their tiny feces were everywhere, especially on my kitchen counter. I know there is an allowed maximum of mouse feces in the flour that we eat, but I did not fancy mouse turds in my cheese sandwiches. Reluctantly I put out traps. The mice gorged merrily on the bait. Live traps seemed even more cruel and also ineffective. The mice are still here, but their population has declined at least for now. Procrastination has its uses.

Justifications can be made for killing.

Where do we draw the line?