CHICK
RESCUE
By Dr Colin Walker BSc, BVSc, MRCVS, MACVSc
(Avian health)
Approximately one-third of all embryos that die during
incubation die during the 3 – 4 days prior to hatching. This is a physiologically
demanding time for the chick. During this time, the chick must essentially change
from being a non-breathing baby floating in a sphere of fluid to an independent
air-breathing bird. The fact that the vast majority of chicks do this without
incident seems remarkable.
During incubation, nutrients (such as calcium from the shell) and oxygen pass
to the chick through a membrane called the chorio-allantois. This is actually
a two-in-one membrane that forms when the chorion (which develops as an outpouching
from the chick’s body wall) fuses with the allantois (which develops
from the chick’s bowel). Once formed, the membrane pushes out as the
embryo grows, and ends up sitting just inside the internal shell membrane.
If the egg shell is well formed and clean, the chick is healthy and no infection
is present, and the egg is kept at the correct temperature (approximately 37.4
C for a parrot) and humidity (approximately 56%), normal development should
occur. During growth, the embryo gains its nutrition primarily from its yolk
sac. This amazing structure is attached directly to its bowel and becomes smaller
and smaller as the chick uses the nutrition it contains and so grows. The last
of the yolk sac is absorbed at the navel around the time of hatching. The nutrition
contained in the yolk is very dependant on the level of nutrition of the hen
prior to breeding. Well-fed hens produce eggs with good shells and nutritious
yolks. Towards the end of incubation, the chick starts to gulp fluid from another
sac called the amnion, which closely envelops it. This fluid helps to keep
the chick hydrated but also contains vital immunogolobulins that protect the
chick against many common germs and play a similar role to colostrum in mammals.
Hopefully, at the end of incubation, the chick is well fed, hydrated, immunocompetent
and therefore ready for the tiring job of hatching. As the chick grows, it
not only becomes too big for the egg but insufficient oxygen can diffuse through
the shell to meet the chick’s demands. As a result, the level of carbon
dioxide rises and the chick starts to twitch. In a normal hatch, the chick’s
head is pulled down under its right wing and comes to rest on the underside
of the egg’s air chamber. With further twitching of the chick’s
neck muscles, the beak pushes through into the air chamber, aided by the small
egg tooth at its tip, and the chick gulps its first breath of air. The chick
is now air breathing but the small amount of oxygen in the air chamber is quickly
depleted. Once again, with rising carbon dioxide levels, the chick twitches.
Twitching neck muscles lead to perforation of the shell, while twitching abdominal
muscles cause the chick to rotate (usually anticlockwise) in the egg. The result
is that the beak acts a bit like a can opener with the pointy end of the egg
eventually popping off, leaving the chick ready to wriggle free.
Particularly in valuable eggs, it can be useful for aviculturalists to know
what is involved in the normal hatch so that abnormalities can be identified.
Many chicks in trouble can be saved with appropriate intervention. The attached
series of photos follow the course of one such chick. The bird in question
is a German Beauty Homer, bred from a pair of recently imported birds. The
parents had only recently come out of quarantine, where nutrition, although
regarded as adequate, may fall short of that provided in a home loft. The egg
had been naturally incubated but during incubation it was noted that the hen
in particular often failed to cover both eggs adequately, almost certainly
leading to undesirable temperature fluctuation. Hygrometer readings in the
loft showed an average humidity of 45%, which is lower than the ideal for both
incubation (56%) and hatching (65%). Both parents are also related and the
youngster, by 14 days of age, showed evidence of a deformed mandible (lower
beak), both factors suggesting a possible genetic involvement.
These joint nutritional, incubation and possible genetic problems were thought
in combination to cause the abnormal hatch. Such a chick, without intervention,
would be expected to die. The degree of intervention, although high, did result
in a healthy weaned youngster for the fancier. The accompanying photos were
taken over a 48-hour period.
At the start of hatching, the initial external pip is in an abnormal position,
indicating chick malposition. Blood on the outside of the shell is associated
with rupture of membrane blood vessels, possibly due to the chick struggling.
The pip area appears very dry. Such changes should immediately alert the astute
aviculturalist to problems. The nest mate had died in the shell at the external
pip stage.
Some of the shell has been removed with tweezers to allow the chick to breathe,
plain water (fluids containing electrolytes are best avoided here) is placed
on the shell membranes in an attempt to increase hydration and soften them.
More of the shell has been removed. Removing too much shell too early can allow
insufficient time for yolk sac resorption and does not allow carbon dioxide
levels to pass the threshold for muscle twitching to occur.
The chick is starting to lift its head after more of the shell has been removed.
5, 6 The chick is wriggling free of its egg. Note its head in the correct position,
under the right wing.
Now 36 hours into the hatch, the chick is tired. The red skin and dry mouth
indicate early dehydration. A drink of lactated Ringer’s solution is
given by syringe.
Because of the chick’s weak swallow reflex, it was regarded as safer
to give further fluids by crop tube.
The chick , now pink and better perfused, is kicking free. Note that the yolk
sac has still not been fully resorbed and the dryness of the shell membranes.
The chick was then placed in a brooder (temperature 36.9 C, humidity 65%)
and crop-tubed with lactated Ringer’s solution and glucose for24 hours
while the last of the yolk sac was resorbed, before being placed under foster
parents. |