IN SHELL YOUNGSTERS
By Dr Colin Walker BSc, BVSc, MRCVS, MACVSc
With breeding now in full swing I receive
regular calls from fanciers that fertile eggs are failing to hatch
i.e. that the embryos these eggs contain are dying through the incubation
time. Many fanciers immediately think of Salmonella when they see
this, when in fact all infections together including Salmonella account
for less than 5% of all dead in-shell youngsters.
And so just why do these youngsters die? Most youngsters that
die in the egg usually die either in the first few days of incubation,
or alternatively the last few days of incubation. In the first
few days embryo death is usually due to either inadequate incubation
leading to too low a temperature to keep the chick alive, excessive
jarring of the egg that either fatally damages the chick or yolk,
or alternatively, a genetic problem affecting the chick which is
incompatible with life.
Towards the end of incubation, chicks usually die as a result
of problems associated with hatching. As incubation ends the chick
has to shift from getting its oxygen through the membranes that
surround it, to breathing air and also re-absorb its yolk sac (which
supplies it with both food and immunity). If the temperature or
humidity is incorrect at this time these processes fail to occur
correctly and the chick can die.
Between the beginning and end of incubation the chick is essentially
just growing and it is here that nutrition and infection become
more important. If the young chick is lacking a nutrient it needs
for growth or becomes infected it dies.
This year has been a particularly good breeding season for me
in that I have not failed to wean a single fertile egg i.e. every
egg that was fertile has hatched and been weaned. I have now weaned
50 youngsters. Although pleasing this situation is unusual despite
the best of care. I did however, have one fancier mention to me
last week that he had had 30% of all fertile eggs fail to hatch.
He did not seen overly concerned and appeared to think that nothing
could be done. This is far from the truth. An embryo fatality of
5% could be regarded as normal. Anything more than this should
arouse suspicions of a problem.
For those of you having a problem with dead-in-the-shell youngsters,
lets have a look at the potential problems that can arise with
each of these periods of incubation in more detail, so that hopefully
the problem can be solved.
Embryonic Death At The Start Of Incubation
Deaths early in incubation can be detected by opening the egg
and seeing that it is in fact fertile, but that the embryo is only
poorly developed. As mentioned earlier, the usual cause is poor
incubation leading to the egg becoming cold after development has
started. Possible causes include improper nesting material, over
interference by the fancier, inadequate control of nest bowl mites
or pigeon flies, failure to provide second nest bowl for next pair
of eggs, too many birds in a section, older arthritic birds, poor
nest box design, competition with other birds within the loft,
poor parenting, nest box too hot or too cold or poorly ventilated,
disturbance outside loft etc. Also as mentioned earlier, eggs are
very vulnerable to vibration type injuries early in incubation.
Shaking or jarring can kill the developing embryo either directly
or by rupturing the yolk. This is of particular relevance when
eggs are being transferred for fostering. The effect of thunderstorms
is a total myth. Embryos that are unlucky enough to have genetic
abnormalities usually also die early in incubation. Genetic problems
are more likely to occur with in-breeding.
Deaths From Day 4 To Day14 Of Incubation
This is the longest period through incubation and yet is the time
when least deaths occur. The embryo is simply growing. The growing
chick receives its nutrition from the yolk and deaths here can
reflect nutritional problems in the hen. Hens that are correctly
fed produce nutritious yolks that support healthy embryos. The
effect of stock bird nutrition is very underrated. By simply feeding
a blend of 2-3 grains and grit it is not possible to prepare the
stock hens well for breeding. Fanciers who believe they can do
this often accept an elevated embryo death rate or several weak
chicks in the nest, as normal.
Although embryos can die of infection at any time through incubation,
it is at this time of growth that they are most vulnerable. Certainly
there are some infections that can be carried by the hen such as
Chlamydia and Salmonella, that can infect the ovary. These can
be incorporated into the egg at the time of its formation, and
subsequently infect and kill the embryo as it grows. Infection
can also pass through the oviduct wall into the egg. These types
of infections, that enter the egg prior to laying, are in the minority
however. Most infections that embryos develop are caught after
hatching in the nest. Nests that are dirty, poorly ventilated or
excessively humid lead to egg- shell contamination and movement
of infectious agents into the egg. Egg quality is also important
here. Cracked, thin, mis-shapen, rough eggs allow easier entry
of infection and are more subject to trauma. Poor eggs can be due
to oviduct disease, but are more often associated with a nutritional
deficiency in particular calcium deficiency. Some fanciers will
have noticed eggs with translucent clear lines running around the
outside of the egg, showing the eggs rotations, as it was passing
down the oviduct. These thin areas can be an early sign of calcium
Embryonic Deaths At The End Of Incubation
Through incubation a membrane called the chorioallantois develops
around the chick. The chorioallantois acts a bit like a human placenta,
in that it delivers air to the embryo after it diffuses through
the shell. At the end of incubation the chick must swap from a
chorioallantoic respiration to breathing air. It does this in two
stages. First it internally pips. This involves cutting a small
hole into the air chamber at the end of the egg and starting to
breath the air it contains. At this stage vibrations can be felt
in the egg and chick will sometimes vocalize. After another 12-24
hours the chick then cracks the shell and breaths external air.
While this is happening the last of the yolk sac (the chicks nutrition
during incubation) is drawn into the navel (and eventually ends
up as a tiny sac in the wall of the small intestine called Merkels
diverticulum which lasts the whole life of the bird). Interestingly,
during this time, the chick also drinks the clear fluid around
it called the amniotic fluid. This amniotic fluid, and also the
yolk sac contain the antibodies that protect the chick from infection
in the first few weeks of life.
While all this complex physiology is going on the chick is vulnerable
to problems. Too high or low temperature or humidity during this
time will adversely affect the chick. The usual problem, is however,
too high a temperature, or too low a humidity. This combination
causes the shell and shell membrane to become hard and dry. This
can lead to even a healthy chick becoming exhausted. In addition
to this, the chick quickly becomes dehydrated. I am sure many of
you, myself included, have helped these chicks hatch only to find
them dead later. These chicks die because they are dehydrated.
Such chicks if given small drops of water will often suck them
down greedily and survive. These dehydrated chicks are called sticky
chicks because of the way they stick to the dry shell membranes.
They are often found dead after hatching ¼ to1/2 the way.
If removed from the shell they often have unabsorbed yolk sacs
and there is often dry, gluggy albumen still in the egg. For consistently
high hatch rates, it is vital the stock birds have access to either
rain or a bath around this time. If not possible the underside
of the hen and also the eggs can be lightly misted with water from
a spray bottle. Ideally the nest box should have a temperature
of 20-25 degrees celsius, and a humidity of 70%. If unsure, a thermometer
and hygrometer can be placed in the nest box.
In summary, in most lofts hatchability can be dramatically improved
by three simple steps:
Improving stock bird nutrition in the months prior to breeding.
A fresh nest bowl for every round, and ongoing nest box hygiene.
Access to rain or a bath around hatching.
If attending to these matters does not help, your avian veterinarian
will usually want to test the hen for infection, or alternatively
do an egg autopsy.