Page 84 - Hand rearing birds second

Incubation 67

Figure­3.2­ California Condor embryo in correct
hatching position with head under right wing.
Source: drawing credit: Mike Clark, courtesy of the
Los Angeles Zoo.

The first indication that the hatching process has begun is air cell “draw down” seen on candling
as the inner and outer shell membranes begin to separate. This occurs at about 85% of the incuba-
tion term. The inner membrane is then draped over embryo rather than being held taut and the air
cell margin appears irregular. Internal pip occurs as contraction of the hatching muscle causes the
egg tooth to pierce the inner membrane. Pulmonary respiration is then initiated. This may be audi-
ble, including vocalization in response to stimulation by the parent or surrogate. The contractions
subside as gas exchange improves and the embryo rests. With exposure to air and the rubbing
movements of the embryo, the chorioallantoic vasculature begins to recede, initially around the air
cell, and finally at the umbilical seal just prior to hatch. External pip occurs as hatching contrac-
tions resume in response to decreasing O 2 and increasing CO 2 and the egg tooth pierces the shell.
The contractions again subside as gas exchange improves and the embryo rests.
The yolk sac is gradually drawn into the abdomen, facilitated by embryonic movement. By
hatching, the yolk sac is fully internalized behind a tight umbilical seal. The remaining yolk con-
tinues to be a food and water source for the chick for up to three days as it transitions to feeding or
being fed. It also continues to provide maternal antibodies while the chick’s immune system gains
competence.
The pip site is broken up as hatching contractions resume in response to decreasing O 2 and
increasing CO 2 . The embryo rests only briefly at this stage. Rotation occurs with sustained hatch-
ing contractions combined with the embryo pushing with its legs. The embryo rotates in counter-
clockwise direction (viewed from air cell end), from one half to more than a full circumference of
egg. Hatching occurs as the embryo pushes the cap off and frees itself from the shell (Figure 3.3).
The chick then rests and dries off. Residual chorioallantoic vessels protrude from the seal but nor-
mally dry quickly. The chick may remain on its side for some time, even continuing to push as if