Artificial Incubation Applied to Small Numbers of Altricial Bird Eggs : Part 2

Candling and Fertility

Candling is important to determine the approximate age of an egg and its fertility. A candling lamp should have a narrow cool beam and be portable so that eggs do not have to be handled. The translucency of white-shelled parrot eggs allows candling, smaller eggs being more translucent since they have thinner shells (Grau, 1987). Candling is best done in dim light or in the dark so that only the light passing through the shell is observed.

Fertility is observed at about 4-7 days when definite opacities will be seen, along with reddish blood-vessels lining the membrane. If no development is observed after 2 weeks of incubation the egg should be removed and broken to see if any blood rings or dead embryos can be found. A blood ring is left when an embryo dies very early and the extra-embryonic blood vessels have continued to move down the yolk membrane.

As the embryo gets bigger it is nearly opaque and very difficult to observe. The air space will become larger as the embryo develops. When the embryo is about 3/4 through its incubation, the live embryo may be detected by shinning the light into the air space and observing for motion of the embryo against the air sac membrane. The embryo is not always moving but this activity will increase and be more observable closer to the piping date which is about 2-3 days before the hatching date.


The rate of embryonic development is dependent on temperature. Incorrect temperature may alter the timing of the hatch and may result in incomplete absorption of the yolk.

Poor quality temperature measurement and control has, in the past, been responsible for most failures in artificial incubation (Klea, 1983). Ether filled wafer thermostats are affected by barometric pressure, become inaccurate with use and even when new have a greater variability than other types of control (Klea, 1983). If the wafer bellows fails the eggs could be cooked or poisoned by the released gas. Reliable solid state thermostats are now the norm on most incubators. Add-on solid state kits are available from several manufacturers.

If an accurate thermometer, 1/2C ( 1/4F), does not come with the incubator, it should be purchased as an accessory. Kuhl Corp. have a Saybolt Thermometer available (item # 0167) with an accuracy of 0.2F and a range of 94 to 108F, perfect for dry bulb incubation temperatures.

Forced-air incubators have a continuously running circulation fan keeping the conditions within uniform. Still-air incubators have no such fan which usually results in a temperature gradient increasing to the top of the incubator and closer to the heating element. Still-air models need to be operated at slightly warmer temperatures so that the optimum average temperature of the eggs is achieved. Each model may have different gradient or zone characteristics and this variability makes still-air incubators difficult to monitor and control at the required steady temperature.

Researchers at the University of California, Davis found that incubating cockatiel eggs at a temperature of 37.5C (99.5F) and a relative humidity of 56% produced best results. Temperatures 1.4C higher or lower than 37.5C produced very poor hatchability and increased the incidence of abnormalities (Cutler and Abbott, 1986). HARI's experiences with this temperature have also been favorable, however, Stoodley (1983, 1984), a large and successful amazon and pionus parrot breeder, incubates at the lower temperature of 36.9C (98.5F) with favorable results. Remote monitoring of egg temperatures during natural incubation has found lower temperatures than those commonly used for artificial incubation (Klea, 1987; Schwartz et al. 1977).

Some recommend that artificially incubated eggs be cooled once a day to recreate the natural cooling which occurs when the brooding parent leaves the nest to eat (Reininger, 1983). It was found with American Kestrel eggs that this cooling has no effect on the hatch, detrimental or helpful (Snelling, 1972). Therefore opening the incubator regularly to check fertility or for piped eggs is probably not harmful and will also bring in fresh air.

In reptiles, incubation temperature has been shown to affect the sex of the hatchlings. It appears that these eggs have the genetic potential to develop into either sex. This environmental sex determination works by producing more males at warmer incubation temperatures and more females at cooler temperatures.

This does not appear to occur in birds.


A thermometer can be used to determine the humidity by making it into a "wet-bulb thermometer". This is made by fitting the thermometer with a wet wick and noting its lower temperature due to evaporative cooling. The rate of evaporation is dependant on the humidity of the incubator. These wicks should be kept free of mineral buildup which can be avoided by using distilled water. The wet-bulb temperature can be converted to per cent relative humidity using standard tables for the particular dry-bulb temperature used (Table 1).

Incubators and hatchers have pans filled with water to provide humidity. Some equipment, such as the Marsh Farms Roll-X, contain quadrants in the lower tray that are flooded to provide the humidity. The total surface area of water determines the humidity and should be adjusted until the correct humidity is achieved.

Some incubators have heated water trays which can significantly increase the humidity. With these systems the correct water tray temperature needs to be set or too high a humidity may occur.

The humidity can also be controlled by the size of the intake-exhaust air vents. However proper ventilation is needed to provide O2 and remove CO2 from the incubator and restriction of this flow to increase incubator humidity is not recommended.

Low (1987) advises towards lower humidity levels and provide little or no water in the incubator. This may be true for very humid areas such as Florida but in all cases the correct humidity must be established by a combination of wet bulb readings and proper egg weight loss.

Table 1. Percent relative humidity at different wet bulb readings and incubation temperatures.

                    Relative Humidity (%)
	Wet Bulb        Incubation Temperatures
 	Temperatures 36.9C   37.5C    Optimum
	(C)	(F) (98.5F) (99.5F)    levels		
	32.8	91	75	73	]-Hatching 
	32.2	90	72	69	] levels
	31.7	89	68	66	  
	31.1	88	65	64	
	30.6	87	62	61	]-Setting
	30.0	86	59	58	] levels 
	29.4	85	56	55	] 
	28.9	84	53	52	  
	28.3	83	51	49
	27.8	82	48	47
	27.2	81	46      44  
           (Reinhart, 1982)

The pans of water provide growing media for pathogenic bacteria, especially in the warm environment of an incubator, and should be disinfected periodically. The use of boiled water, that has been cooled, in incubator trays will reduce the chance of unwanted organisms being introduced from the tap water.

At HARI, wet bulb readings are in the range 30.0-30.5C (86-87F) which represents a higher 58-61% relative humidity than those previously reported. With these levels (and those indicated below for the hatcher) we have experienced excellent hatchablility amoung cockatoo eggs (approx. 90%) but poorer hatchability among our macaw and amazon eggs (approx. 70%). Random bacteriology on some of the dead-in-shell eggs found them to be free of pathogens. Malpositions seemed to be more common.

Stoodley (1984) reports good results with pionus and amazon eggs maintaining his main incubator at 53% r.h. but has several others at higher and lower r.h. levels to correct excess or insufficient loses. The Macdonald Raptor Research Centre of McGill University incubates more than 500 kestrel eggs per year with modified Marsh Farms Roll-X incubators maintained at 37.5C and the humidity (approx. 55%) is provided by flooding one quadrant of the incubator floor (Bird, 1987).

For the next breeding season (1990) HARI will set up a second incubator to separately set amazon and macaw eggs at a lower relative humidity, probably around 55%.

Part 3
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