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 36.9C, 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.