I bought a bird feeder. It was expensive, but I figured in the long run it would save me money on cat food.

The line is a compact piece of dark, domestic humor built on a bait-and-switch. A bird feeder is normally justified as a wholesome purchase—inviting nature closer, offering pleasure and calm. The speaker instead frames it as a cost-saving measure on cat food, implying the cat will hunt the birds attracted by the feeder. The joke turns on the collision between sentimental intentions and predatory reality, and on the absurdity of “long-run savings” logic applied to an ethically dubious plan. It also gently satirizes consumer rationalizations: we often justify indulgent purchases with flimsy economic arguments.