My cat loves to watch birds but leaves them be (probably because he was raised in a house where we had a pair of lovebirds in a cage and there would be hell to pay if he went for them); he chases, catches and eats mice. All that to say he loves sitting on the windowsill in the lounge - he watches the birds feeding with a lot of interest but doesn't make a sound HOWEVER a mouse has taken up residence behind the lounge shutter and when he comes out to eat Boots goes wild 'yakyakking' away...
My cats do this as well, but ive noticed if Moet (our 4 yr old female) is hunting a fly, and makes this noise then Lanson (2 yr old Tom) comes running to join the hunt.
Maybe its a hunting call to let other cats know their on to something?
Both my cats do this as well - my girlfriend and I do a rough impression of the sound whenever we see a moth or something in the house and both the cats come legging it in from wherever they are to hunt...