Heat flow through eggs in natural incubation is evidently different from that in eggs incubated in conventional incubators, particularly incubators with fans in which the air temperature is uniform. Sophisticated mathematical modeling has illustrated clear differences between the two incubation regimes, both in the temperature difference supported across eggs and in the change in embryonic temperature as incubation progresses (Turner, 1991).

The influence of blood circulation within the egg becomes dominant late in incubation, exceeding the effect of increased metabolism. In naturally (contact) incubated eggs, this usually has the effect of increasing heat loss from the egg and reducing embryonic temperature. Conversely, in conventional incubators, eggs have no cool zone to liberate excess heat and in consequence metabolic heat usually causes a rise in embryo temperature.

Whilst it is accepted that eggs of fowl and other domesticated birds fair well in machines, probably because of adaptation or inadvertent selection over many generations, eggs of altricial and undomesticated birds do less well. Hence the present attempt to engineer an incubator which provides an environment much more closely matching the bird/nest combination found in nature.

The present project has existed in concept for some twenty five years and indeed Brinsea’s first attempt to build a machine using an artificial brood patch was in 1979. The breakthrough came with the use of an air inflated ‘skin’ to provide the necessary contact pressure to ensure reasonably uniform heat transfer to each egg. The skin can be made of any thin, impermeable but flexible material such as polyethylene, latex rubber or polyurethane elastomer. The latter is our preferred choice because of strength and transparency, though all work well so long as the material is thin enough to conform to the profile of the top of a group of eggs and accept some egg size variation. Inflation of the skin is achieved by sealing it to the underside of the incubator lid and pressurizing the space above with a small computer type fan blowing air in from outside. This positive pressure is sufficient to press the skin against the egg tops. As a bonus, the system can easily be enhanced by reversing the fan to extract the air, lift the skin from the eggs and thus mimic normal periodic disturbance or departure of an incubating parent.

After the first trial machine was built in 1998, it was decided to produce eleven prototype machines based on Brinsea’s Polyhatch cabinet. They have programmable control of egg turning to coincide with the skin lifting. A series of tests were conducted by Brinsea in house with simultaneous field trials in a variety of locations with essentially wild species.

Of the out-place units, all but one reported better hatching success than with conventional artificial incubation. In some cases the improvement was substantial, including hatchling survival. The Wildfowl & Wetlands Trust at Slimbridge raised some 30% more Chilean Flamingos with their contact prototype than with parallel conventional incubation. The one disappointing placement was a psittacine breeder in Loxahatchee, Florida. This case was investigated carefully and there appear to have been mitigating factors associated with management and hygiene. However, it did reveal shortcomings with the egg turning system, which have been addressed in the production product, recently introduced.

It is my hope that this new machine be a useful tool for conservation, and that it will facilitate investigation into various aspects of incubation in wild birds.

Turner, J. S. (1991). The thermal energetics of incubated bird eggs.

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