It's just nature ... which can (to our eyes) seem terribly violent and horrific at times, but it is all just in the natural order of things .... little things get preyed on by bigger things and so on and so on .... and we all know nature is a wonderfully balanced thing most of the time, everything having its own little niche and coexisting within the overall framework .... as usual, the only time this is put under threat is when man gets involved!
Upsetting, of course, for the cat owner, but, as WW pointed out, her little moggie has already dispatched many mice and birds off this mortal coil, and, in my experience, not in any humane way ... I've seen a cat literally "playing" with a young bird until the bird is so badly shocked or injured (or both) that it stops moving, and then the cat loses interest ....
