Cat vs. Bird

I saw one of the kitties looking out the window intently this afternoon. I wondered why, and then I heard scratching in the rain gutters. I have seen this before - sparrows are down in the gutter, and they apparently like to eat some of the granules that come off the roofing material. Maybe they have it in their gullet like chickens do. Anyhow, I ran and grabbed my camera, and the kitty was still there absolutely rapt while watching the birds. I wonder what was in her mind. She clearly has built in instructions to hunt birds, but the window screen won't allow that, so I guess she just watches, because it is in her instruction manual!