neossoptile, neoptile, nessoptile 1. Regarding young birds, a down feather which forms the natal plumage. 2. Feathers of nestlings; down feathers, or the down of newly hatched birds. The plumage of the newborn chick is downy, called neossoptile; the development which follows is termed teleoptile. Juvenal plumage, is frequently distinct from that ...