It is good to learn on chickens. They are actually very hardy compared to other birds . . .
You get to understand bird body language just as well with a $4.00 chicken as an $800.00 African Grey. All birds hide any pain as best as they can. The rest of the flock will even kill a sick bird -- and predators kill the slowest or the sick in nature. Sometimes the first clue you get is a dead bird in the morning. So, watch them every morning. Are they hungry for their food? Running to get the treats? Who is the slow one? Pick her up, feel her breast. Is the breast bone sharp feeling? Are the feathers shiny and healthy or dry? Bugs? Drip from the beak? I could go on, but you get the idea. If anything is not normal, keep an eye on the whole flock, not just the weak sister. So sorry that you lost your bird, honor that by learning to see the others' in the flock's problem. You got a taste when the flock was sick, now some more might not have fully recovered.

Best wishes with the rest, monitor them closely!