...repeatedly makes PrrrrppPPP! noises, you know something's going on that it's proud of.
I have just rescued a wren from one* of ours. This was unusual, because by the time I'm close enough to do anything the captive is usually beyond saving. However on this occasion the 'guest' was able to get away through sheer speed when it was put down for playing, and found a corner too small for the cat to access. Fortunately I don't have to stand on my hands, and so could get one either side to extract it. It flew away, emitting 3 cross sounding chirrups, much to the cat's disgust and my pleasure.
*This particular cat is a highly effective hunter, as the daily piles of mouse-guts in the garden demonstrate. The other is mostly too lazy, apart from the rabbit it brought messily in through a window a couple of years back, and dismantled more messily in the bathroom. :P