Linea Fusca

Linea fusca is a brown stripe on the belly of newborn Linaria great tits that appears about a week after hatching from the egg. It appears due to the accumulation of hemoglobin in the yolk sac, which is a mass of cells in the wall of the yolk sac and the auditory bladder, containing the yolk left over from the yellow-bellied squirrels. Over time, the yolk sac dissolves, and the brown stripe becomes lighter and eventually disappears completely.