This is done by letting such matter flow out of the iliac arteries out of a partly sealed lower anastomosis.
This, when fully developed, consists of two parts, but inserted by a single ribbon-like tendon upon the hinder surface of the femur, near the end of its first third; the caudal part, femoro-caudalis, expressed by Garrod by the symbol A, arises from transverse processes of the tail; the iliac part (accessorofemoro-caudal of Garrod, with the symbol B), arises mostly from the outer surface of the postacetabular ilium.
Of course this doubleheaded condition is the more primitive, and as such exists in most nidifugous birds, but in many of these, as well as in many nidicolous birds, either the caudal or the iliac head is absent, and in a very few (Cancroma, Dicholophus, Steatornis and some Cathartes) the whole muscle is absent.
The young when in the nest are clad in down of a dirty white, varied with grey on the head and neck, and with ochraceous in the iliac region.
Squeeze the left thigh muscles to force blood from branches of the iliac artery up the main vessel.