Sorry to hear one of your babies is not doing well...follow the good advice given by RidgeRunner.
hirping is a normal thing and is a healthy sign as noted by others. I wanted to point out that chirping can also occur when they are cold or hot. Baby chicks can't hardly regulate their body temperatures with Mama hen to help...hence we provide artificial heat. Below is some info we teach to our club member regarding temperature.
I still recommend following the 5 degree per week program...regardless of feathering. Some chicks feather quicker than others...but that doesn't mean they can effectively regulate their body temperature. By providing them with a heat source of 95 degrees for the first week of life and drop that by 5 degrees each week until the provided heat is at least 10 degrees below the ambient temperature is just a safe and natural progression. This also slowly acclimates them the same way they would if Mama hen was there to provide warmth and wind protection.
Week 1, 95 degrees
Week 2, 90 degrees
Week 3, 85 degrees
Week 4, 80 degrees
Week 5, 75 degrees
Week 6, 70 degrees
Week 7, 65 degrees
Week 8, 60 degrees
Week 9, 55 degrees
Week 10, 50 degrees