5.2.5 Ageing Juveniles These are bats from the age of first flight until the loss of the characters described below. There is no widely accepted term other than baby to describe the period from birth to first flight, although the use of the term pup is becoming more widespread. Ossification At the time of first flight the bones are not completely ossified. This is most obvious in the joints of the digits (Figure 5.3). If held up to the light, the cartilaginous ends of the finger bones are apparent as pale bands either side of the joint. As cartilage is replaced by bone, the joint becomes more rounded or knuckle-like. The bones usually appear fully ossified by the autumn (60–75 days after birth).