Figure 13. ONP for various birds.

The avian neck has a sigmoidal curve that is formed intrinsically by its osteology when the vertebrae are articulated in ONP. The alert resting head height for the ostrich Struthio camelus (top) is higher than predicted by ONP [15] (and the ostrich often further retracts the head during locomotion [98]). Many other birds, however, do assume a pose close to ONP as their characteristic alert resting posture: Cape Penguin Spheniscus demersus (bottom left), Flightless Cormorant Phalacrocorax harrisi (middle), and Kiwi Apteryx australis (bottom right). Note inflection points (arrows). Photographs by the author and John Martin; specimens at the Zoology Museum, University of Cambridge, access courtesy Matthew Lowe.