Confirm the type and consistency of their dropping. From the picture however can predict white pasting at the vent. Before doing any other thing have the bird separated from the rest of the flock or transfer the rest to a separate temporary location spread lime(choka*) in the coop. Based on the confirmation of the dropping it could be basillary white disease, coccidiosis, fowl typhoid. Magical remedy is grinding charcoal mix with water and give the sick bird. If there is any diarrhea it should stop. Otherwise get special attention too from vet shop nearby for antibiotic prescriptions + multivitamin. Bottom line keep poultry unit disease free by observing strict biosecurity and hygiene.