My girlfriend has two cats who are about a year-and-a-half old. They left their mum at six weeks and, perhaps as a result of that, don't meow. They do make a plethora of other noises, mostly chirps and squeaks at the birds... however, every now and then they'll start yelping. Usually at mealtimes, but sometimes just for no apparent reason. Does anyone have any insights into this? They're very healthy, by the way; mental, but healthy.