Weight Loss Determinations
Nearly all weight loss is due to the diffusive transport of water vapour across the eggshell (Rahn and Ar, 1974; Ar and Rahn, 1980). The rate of water loss is determined by a species-specific water-vapour conductance, number of pores, pore structure and thickness of the shell (Ar et al.,1974; Rahn et al., 1976). Egg shell conductance of each species has evolved in close coordination with egg mass, incubation period, and microclimate of the nest (Carey, 1980).
The weight loss rate from lay to pip is linear if the eggs are exposed to the same temperature and humidity during this period (Olsen and Olsen, 1987) and can be controlled by differing the level of humidity in the incubator. If the egg is losing weight too quickly, raising the humidity will slow the rate of evaporation through the egg's pores and conversely lowering the humidity will increase the rate of weight loss. Convection or a draft created by the incubator fan has a minimal effect on egg water loss (Weinheimer and Spotila, 1978).
Some recommend observing the size of the air-cell as a means of checking for correct weight loss or the age of the egg. This can be inaccurate due to the different shapes of eggs and variation in the amount of membrane separation and protrusion of the embryo into the air cell. For example; a moluccan cockatoo egg of unknown age was pulled and candled showing what appeared to be a large air-cell occupying about a third of the egg and it looked clear. Based on this it was thought the egg was old and infertile and was not incubated. On breakout a small one week old embryo was found and the air cell was actually small in volume as it wrapped around the egg membranes causing the embryo to protrude into what appeared to be a large air-cell. Needless to say HARI no longer uses the air-cell size or rate of size increase as an indicator of egg age or rate of weight loss.
Several formulas can be used to determine the accepted and observed rate of weight loss or overall per cent weight loss and to then correct the humidity if the observed values are off. The zoo community is aware of these formulas (Hoffman, 1987; Johnson, 1984) however private aviculturists usually do not have accurate enough scales or the propensity to work out these calculations.
From the normal incubation period (p in days), accepted mean overall fractional weight loss (the percentage of the initial egg weight lost) (fn) for that species and fresh egg weight (wf in grams) the predicted daily weight loss (dp in grams/day) for that particular egg can be calculated as follows;
wf x f n / p = d p
wf - w s / t = d 0
If the fresh weight of the egg is not known it can be calculated from the egg size. The length (l) and breath (b) (maximum diameter) of the egg can be accurately determined with calipers. These measurements can be used in the formula worked out by Hoyt (1979) where kw is a species-specific weight coefficient. Saunders and Smith (1981) found no significant difference in kw between five species of cockatoo and determined its mean value: kw = 0.554 ± 0.015. Based on the 26 species that Hoyt (1979) examined kw has mean value of 0.548 ± 0.016 and this can be used as kw if the species-specific value is not known;
w f = kw x l x b²
d0 x p / wf(100) = f0
Most larger amazon and cockatoo species lay eggs weighing 15 to 30 grams with macaw eggs weighing up to about 37 grams when freshly laid. A 15% mean fractional water loss was observed in 32 species of altricial eggs (Ar and Rahn, 1980). Based on a 26 to 28 day incubation period and this fractional loss, the daily loss calculates out in the range 0.06 to 0.19 g/day for the 15 to 30 g eggs and up to 0.22 g/day for the larger macaw eggs. An electronic balance with an accuracy of ± 0.01 g would be needed to observe such a small daily egg weight loss however if the egg is weighed over a period of a week an accuracy of ± 0.1 g may be sufficient.
The weight loss rate after piping and up to hatching may be greater than that during the "setting" period of incubation. To determine the entire incubation weight loss the egg should be weighed moments before hatching which is difficult to time correctly. Burnham (1983) took this measurement as the embryo was turning in the egg and determined that peregrine eggs lost 2.2% of the fresh egg weight between piping and hatching in a forced air hatcher set at 37.0°C and 60% humidity. This weight loss could be used as an approximate estimate for other altricial species to add to lay to pip data to determine the total fractional weight loss over the entire incubation period.
The fn loss during incubation in the eggs of seven species of terns was 14% (Rahn et al., 1976). The similar water loss amoung the terns was found in eggs with a four-fold difference in egg size, an incubation range of 21-36 days and in species which incubate their eggs at mean ambient temperatures of about 10°-28°C (Rahn et al., 1976).
The lay to pip fractional egg weight loss for successfully hatched Barn Owl eggs was significantly different between artificially incubated (37.5°C and 48% rh with 93.5% hatchability), with a 11% loss, and naturally incubated eggs which lost 14% (Marshall, 1986). Bird and Lague's (1980) study of successfully hatched American kestrels eggs also found differences between artificial and natural incubation egg weight loss but only during about three days of the first week of incubation and with no differences in hatchability. The mean total weight loss for piped eggs was 12.1% for artificial and 16.0% for natural incubation (Bird and Lague, 1980). A low fractional egg weight loss from lay to pip of 10% has been reported for Australian kestrels (Olsen, 1987). Fentzloff (1984) reports the best hatching results in White-tailed sea eagles using 37.0°C and 64% r.h. during setting and 36.5°C and 85-90% r.h. in the hatcher, resulting in a 13-14% weight loss over the entire incubation period.
The lay to pip weight loss of eggs incubated at HARI was determined with eggs that were pulled within a few days of being laid. Eggs that were pulled after more than a week of parental incubation were not used in this weight loss data. Eggs were weighed as soon as they were pulled and again when they pip and are transferred to the hatcher.
We found differences in the average fractional weight loss of eggs from lay to piping with different species. Double Yellow-Headed Amazons had a loss of 13.4% (n=8), Blue & Gold Macaws 12.9% (n=6), Moluccan Cockatoos 9.4% (n=4) and Medium Sulphur-Crested Cockatoos 9.7% (n=18) (Appendix 1). Remember these are losses to piping and further weight reduction from piping to hatching must be added to get the weight loss for the entire incubation period. All of these eggs hatched 36 to 60 hours after piping for a total incubation period of about 28 days (Table 2).
Work with cockatiels demonstrated an egg weight loss of 9.6 % of initial egg weight at lay over the first 15 days of incubation with these eggs piping at 16 days and hatching at 18 days (Cutler and Abbott, 1986). Stoodley (1983, 1984) reports optimum hatching in amazons and pionus with a 16% weight loss.
It therefore appears that a range of fractional weight losses resulting in successful hatching occurs within species and the average value for each species falls within a range of about 12-16%.
Table 2. The incubation periods of psittacines bred at The Hagen Avicultural Research Institute
=============================================================== Species Days _______________________________________________________________ Blue & Gold Macaw 28 Severe Macaw 27 Citron Crested Cockatoo 28 Medium Sulphur-crested Cockatoo 28 Moluccan Cockatoo 29 Umbrella Cockatoo 28 Double Yellow-headed Amazon 28 Yellow-fronted Amazon 28 ______________________________________________________________
Piping and Hatching
Approximately two-three days before piping the air cell gradually expands down one side of the egg. The act of shell piping usually coincides with the first breath, that is, the filling of lung and air sacs and the initial breathing movements. The air space or cell accommodates the increased volume of the embryo as lung inflation takes place. In the chicken this projected movement into the air cell continues for about nine hours during which the O2 tension in the air cell falls and the CO2 tension rises precipitously and appears to initiate the piping of the shell (Visschedijk, 1968).
Once eggs pip they should be placed into a hatcher that is maintained at a slightly lower temperature, about one °C less (about two °F), and much higher humidity than the incubator. Carpenter et al. (1987) hatch eagle eggs in hatching units kept at 36.9°C dry bulb and over 32.2°C wet bulb, 71% r.h. ("as humid as possible"). We have similarly experienced maximum hatching when the hatcher has a relative humidity of greater than 80% (wet bulb about 32-33¼°C, dry bulb 36.5°C). Some incubator manufacturers make separate hatchers but these are relatively expensive units, difficult to clean and may not reach the high humidities needed to hatch some types of eggs with maximum success.
HARI has put together a still-air hatcher/brooder that is composed of an aquarium sitting in a water bath, heated by a submersible aquarium heater, all surrounded in a clear plastic pen. Using this set up (see Appendix 2 for product item numbers), we are able to achieve a high humidity, temperature controlled hatcher and also use it as a subsequent brooder.
To set up the hatcher; lay the heater horizontally into the "pen" and pass the cord between the two sections, place the two aquariums into the "pen", fill the "pen" with warm water to about 6-7 cm, add a cup of salt to the water (this significantly reduces pathogenic micro-organism growth in the bath water), place a small hand towel over the top opening of the pen and a few paper towels in the aquariums. After allowing the heater to equalize with the water temperature, plug it in and turn it up until the pilot light comes on. Using an accurate thermometer keep turning up the heater in small increments until the bath water is 36.0°-37.5°C (97°-100°F) so that the inside of the aquariums are 35.0°-36.5°C (96°C-98.0°F). This should be set up several days before an egg is to pip. Evaporated water needs to be replaced or the heater will be exposed to the air and its glass case may explode. Clean the whole bath as needed, about every 3-4 weeks. Horizontally lay the egg, pip mark up, onto the dry paper towel inside the aquarium. Allow about 2-3 days for most eggs to hatch from the first pip. After 3 days the egg should be candled to see if the chick is stuck to the membranes and requires assistance.
HARI is now achieving better than 95% hatch from piped eggs using this setup. Previously when HARI used the hatching tray in the incubator hatching rates were only about 50% hatch from piped eggs and losses included six amazon and cockatoo eggs.
The setup can also be used as a brooder for the first 10-14 days of a chick's life. In this case, remove the towel from the top and lower the temperature a few degrees. The aquariums can be removed and cleaned every few days while the paper towels can be changed at each feeding. At HARI, wood shavings are used at 2 weeks of age, about the same time babies are removed from the brooder and placed in a warm room.
Researchers at the University of California, Davis place hatching eggs in a unit maintained at a hatching temperature of 36.9°C with 67% relative humidity (Cutler and Abbott, 1986). A hatchability of 81.0% of fertile cockatiel eggs was achieved by Cutler and Abbott (1986) using these levels and the previously mentioned incubation temperatures and humidity levels.
Larue and Hoffman (1981) position crane eggs in the hatcher so they are held firm, to allow the chick to rotate in the shell without rolling the egg.
A freshly hatched chick may still be attached to the allantoic sac by thin vessels which should no longer contain blood and will quickly dry and fall off.
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