Abstract
Introduction
Just as parrots form a unique order of birds - psittaciformes - with only the most conjectural relationship to other avian groups, so have their eggs proved exceptional. The most striking difference is in the resistance the shell has against injury.
Other birds' eggs are more brittle.
A strong tap will chip them and the sharp edges of the pieces, as they are depressed with the blow, too readily cut through the egg membranes. When the albumen leaks out, or if air gets to lie under the inner-shell membrane, the embryo dies. The unique "spongy" structure of the proportionately thinner parrot egg shell gives it a greater pliability. A similar slight blow merely caves in a mild dent at whose edges the shell membranes hold firm. In practical terms this means that the psittaculturist can give first-aid repairs applying, say, a wipe of nail varnish, to a dented parrot egg with some likely success of it hatching.
General
Most information available on the incubation of eggs has come from investigations on poultry. Obviously much of what has been found will apply to other eggs. This will be particularly so for game birds. On the other hand, when it comes to parrots, some findings may mislead. For example, the fresh-hatched poultry chick obtains most of its body calcium by absorbing away the inner layer of the shell during the last third of the egg's incubation. But as parrot chicks have no need for a strong, supportive skeleton until much later in their life, their egg shell remains quite undigested.
Neither do parrot eggs have a cuticle: that water repelling ' 'varnish" to the outermost layer of the shell. However, the parrot egg similarly cannot absorb water by direct capillary action. If you moisten a parrot egg by submerging it for hours in water at its temperature, not a particle of moisture will enter. The high-gloss and submicroscopic openings to the shell pores keep it out. Or, put another way, spraying eggs (something commonly advocated in the cage-bird press) merely serves to lower their temperature. It chills them by evaporative cooling. Wetting eggs does not, unless the nest is practically inundated, make any difference to the rate at which they lose water.
As anyone who has regularly monitored eggs throughout natural or artificial incubation gets to know, they are exceptionally resistant to infection. The unincubated egg, unless it is cracked, does not go bad for weeks and weeks. Much of this "natural" protection comes from the excellence of the three-layered '' packaging" preventing bacteria from getting in. The smooth, waterproof shell overlies the two bacterially near-impermeable shell membranes. Should the shell be cracked, this inevitably will let bacteria in. Despite which, if the shell membranes remain intact, the contents can still remain sterile, for even the innershell membrane alone can prove to be remarkably efficient. If the broader end of the shell, above the air-sac, is perforated to increase the transpiratory rate, infection does not necessarily happen. O have yet to experience this in more than 50 examples.)
Once incubation begins, the inner· most shell membrane becomes closly underlined by the '' placental" tissues of the growing embryo. At this stage, however, even though the shell remains intact, when the embryo dies the egg will now get infected. This conversion of a good to a bad egg can be amazingly prompt in the heat of an incubator. Postmortem changes cause the contents of the egg to break down. This releases substances that damage shell membranes. The net result is that the shell membranes and, not infrequently, adjacent portions of the inside of the shell will become soft with moisture. Through this soggy lining, bacteria wander in. Therefore, non-hatching eggs, irrespective of why the embroyo has died, when sent to an investigatory laboratory, are found to be infected. Sadly, an inevitable, rapid infection and subsequent post-mortem decay is wellknown in all corpses that are not preserved. A pathologist may not be aware that this equally applies to incubated eggs. When this is the case, the egg autopsy report is irrelevant if it assumes that the infection has spread into the egg from the incubator or nest box and killed the chick.
This is a cart-before-the-horse example. Our wish is to know why the egg has died. Not what, inevitably, happens after death. True, a pipped egg, when the still incarcerated embryo is moist and with an unabsorbed yolk-sac, may get infected. This is uncommon.
To put this innate resistance to infection in another light, the owl's nest and eggs always stink because of the build-up of rotting carrion. Yet, amidst these fetid surroundings, they hatch. Likewise so do parrot eggs soiled with feces as they are.
This is not to say that a primary infection of eggs never happens. We know how notorious, politically, Salmonellosis became. However, even in this instance, it is still true to say that this bacterium, in most instances, is on the shell, not inside with the contents. Salmonellosis involves us when the egg is broken and the yolk and albumen are seeded with bacteria from the shell. It is then kept for a while at temperatures at which the organism can grow. The mayonnaise, or whatever foodstuff, that gives Salmonella to a person generally grew from this egg-shell seeding.
Nor can we deny that "vertical" transmission, that is, infection of the chick through the egg itself, cannot happen. It can occur when Salmonella affects the peritoneal cavity, ovaries, or oviduct as might, equally infrequently, take place with Ornithosis (Psittacosis). The contents of an egg have some ability to defy infection, for the albumen is rich in lysozymes (general anti-bacterial agents). It also contains specific antibodies against those infections to which the hen has developed resistance.