Page 80 - Hand rearing birds second

Incubation 63

a room that can be completely darkened and to use a candler with the strongest light available. The
brighter the candler, the hotter the light, so great caution must be used to avoid damaging eggs.
Eggs should not be held against the light for more than a few seconds at a time and their surface
should be felt to ensure they do not overheat. Even strong LED lights generate heat when used for
longer periods.
Before eggs are placed in the incubator, they should be candled to evaluate shell quality,
check for cracks and flaws, note yolk quality and mobility, and determine air cell formation and
position. During incubation, development of the embryo and its extraembryonic membranes
should be monitored to the extent possible, based on visibility. The progression of the air cell
can be tracked by tracing its margin on the shell with a blunt pencil at regular intervals, usually
every 3–7 days.
The first observable evidence of development is often the shadow of the blastoderm, appearing
as a faint crescent along one edge of the yolk, although this is not visible in all fertile eggs. The yolk
will also begin to increase in size and appear more fluid as water from the albumen moves into the
yolk material. By 15–20% of the incubation term (3 days of 21 for the chicken) the embryo and yolk
sac vessels should be clearly visible in even the smallest eggs. Practiced eyes will be able to discern
this even in the most heavily pigmented eggs, although candling will quickly become impossible
as vessels line the shell in such eggs. Note that while it is possible to confirm fertility by candling
once the embryo has reached the stage of blood development, it is not possible to confirm infertil-
ity by candling alone. In eggs that remain “clear” on candling, fertility can only be determined by
breaking them open and carefully examining the contents.
It is useful to record landmarks of development, such as heartbeat, eye pigment, membrane
progression, and embryonic movement, and other detailed comments on candling observations,
including sketches. Review of past records can help determine whether eggs are progressing nor-
mally, but only if notes are descriptive and detailed.
Initially, the yolk sac will appear clearly as a roughly circular red “spider” shape with the embryo
in the center. As this membrane surrounds the yolk and becomes more complex, it will appear less
distinct, an effect that is soon compounded by the vessels of the CAM as they line the inside of the
shell. This “streaky” stage is often incorrectly interpreted to be the breakdown of vessels. Eggs at
this stage have even been removed from incubation by those who were convinced they were dead
or dying. The vasculature of the CAM eventually becomes more distinct and heavier. It should
completely line the inside of the shell soon after the middle of incubation, usually forming a visible
“seam” along the back side of the egg. Insufficient turning radius and setting eggs with the air cells
uppermost will prevent the CAM from completing. This is easily checked by candling the egg from
the small end.
The first indication that the hatching process has begun is the apparent sudden enlargement of
the air cell, termed “draw down.” This is caused by the slight separation of the inner and outer shell
membranes rather than a true increase in air cell size. The inner shell membrane that has been taut
above the embryo during incubation is now loosely draped like a blanket, allowing the embryo
more flexibility of movement. Prior to internal pip, the shadow of the embryo’s beak may be seen
pushing against the membrane. The hatching contractions will appear irregular and jerky. Once
the embryo has internally pipped, or pierced into the air cell, regular, rhythmic respiration is
observed. After external pip, candling can verify whether the outer shell membrane has been
pierced, ensuring the embryo’s access to outside air. As the hatching process progresses, candling
at the small end and sides can be used to monitor the regression and shutdown of the CAM vessels,
at least in light‐shelled eggs. This is especially useful in deciding if and when hatching assistance
can be safely initiated.