Page 87 - Hand rearing birds second

70 Hand-Rearing Birds

­ Egg­Necropsy­and Analysis­of Hatching­Failures

Thorough analysis of hatching failures is crucial to future hatching success. This chapter will pro-
vide a simple outline for this process. The reader is strongly encouraged to review more detailed
references on the subject (Kuehler 1983; Langenberg 1989; Joyner and Abbott 1991; Ernst et al.
2004b).
The two most useful statistics are fertility and hatchability, and they are expressed as percent-
ages. Fertility is the ratio of the number of fertile eggs to the total number of eggs received. For
commercial poultry, this is usually 90% or more. Similar fertility should be seen in aviculture as
well with mature, healthy, compatible birds. Hatchability is the ratio of the number of hatched
eggs to the number of fertile eggs set (not the total number of eggs – infertile eggs are incapable of
hatching). This is typically 87–93% in commercial poultry hatcheries but tends to be a bit lower in
aviculture, even under good conditions, for a variety of reasons.
This expected or normal mortality falls into specific developmental periods. One third, or
about 2–5% of all expected mortality occurs during the first few days of incubation and is pri-
marily due to chromosomal anomalies when the gametes form. The remaining two thirds,
about 5–9%, occur during the last few days of incubation and are usually associated with the
hatching process, including malpositions. There should be little or no mortality during the
middle of incubation.
Break out and examine all unhatched eggs, including those that are “clear” on candling. If all
clear eggs are assumed to be infertile, efforts will likely be made to solve a fertility problem when,
in fact, there may be a hatchability problem with a vastly different cause.
The first step in a thorough egg necropsy is a review of the parental history and the incubation
record. The egg should be candled to locate the embryo, air cell, and any unusual characteristics. It
may be desirable to take microbial culture samples from the shell surface and/or egg contents,
particularly the yolk contents, but the embryo will likely have been dead for more than a day
resulting in a high number of decay organisms and masking infectious ones. It is sometimes very
difficult to determine whether an embryo is still viable and usually wise to risk leaving the egg
under incubation a few days longer. If egg infections are suspected, the interior of the incubator
may be swabbed for culture, particularly water sources.
Clear eggs and those with younger embryos are best opened in water. The shell can be cracked
(much like for cooking), and the egg held submerged in a bowl of water while the shell halves are
eased apart. Broken yolk is likely to obscure everything else, so it may be necessary to carefully
strain and rinse the egg contents to find membrane tissue or a tiny embryo. Eggs with older embryos
can be opened with forceps, beginning with a hole over the air cell and carefully noting the condi-
tion of the extraembryonic membranes and fluids, and the position of the embryo as the shell and
shell membranes are removed. Even badly decomposed eggs may yield useful information. Using
a standard system of characterizing embryos by developmental stage (Hamburger and Hamilton
1951), breakout results should be characterized as follows:

“Clear” on Candling:
I = infertile – blastodisc centrally dense white, not “donut‐shaped” as in fertile egg;
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PD = positive development – no discrete embryo but white membranous tissue on the surface of
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a watery yolk;
FND = fertile, no development – rare case of development ceasing at lay due to genetic defect or
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thermostabilized blastoderm.