Can you identify the crop, which is on the chick’s right upper chest? That is where food and water are stored before moving to the gizzard for digestion. Occasional they can get a leaking air sac from a bump or injury, that can feel like a small balloon under the skin. Feel the other chicks and their crops which may feel soft with some food inside, to decide if you are seeing the crop, or if it might be a leaking air sac. Is the chick having difficulty breathing?