What's wrong with your bird?
A sick chicken hides trouble until it can't. Pick the symptom you're seeing and we'll lay out the likely causes and what to check first - a plain starting guide to point you in the right direction, not a diagnosis.
Likely causes: Often normal - short winter days, the annual moult, age, or a broody hen. Also stress, poor feed, or a hidden nest.
What to do: Check daylight hours, feed quality, and hunt for hidden eggs before worrying.
Isolate any sick bird from the flock to limit spread, and when in doubt an avian vet is worth the visit. This is a starting guide, not veterinary diagnosis.
This is a starting guide to help you spot what might be going on - it is not a veterinary diagnosis. Birds hide illness well and can go downhill fast. Isolate anything sick, keep it warm and watered, and if you're unsure or it worsens, an avian vet is worth the visit.
Several birds falling ill or dying at once is not a job for a checklist. In the United States, act quickly and report it: the USDA's Defend the Flock program gives a toll-free number for sick birds, 1-866-536-7593, or call your state veterinarian.