if a bird stops chirping, what does that mean?

Philippines
March 20, 2009 6:17am CST
i am taking care of 4 little hatchlings which we found on the ground, probably pushed off the nest by their parent sparrows. the first time i saw them, they were chirping nonstop. even while asleep, they would make little chirping noises as they were breathing. i fed them according to the schedule and recipe i found on the net, guided by their hunger, of course. the three look healthy, but i think one is dying because of gapeworms. i'm wondering why they've stopped chirping. is this something i should be worrid about? i play them sparrow songs (the website recommended it so that they will learn those songs as they are growing up without parents to teach them). so i don't know what's wrong. it's hard to be a momma bird when you're not a bird at all. :(