Page 81 - Hand rearing birds second

64 Hand-Rearing Birds

­ Egg­Weight-Loss­Management

Eggs must lose the appropriate amount of weight during incubation in order to hatch successfully.
Generally, this is about 15% of the initial egg weight at laying, or fresh weight, for most species.
Because this is not a metabolic function but occurs by the physical process of water evaporation
through the pores of the eggshell, it is linear and can be manipulated by increasing or decreasing
the humidity during incubation. Carefully monitoring and controlling egg weight loss is key to
hatching success.
Eggs should be weighed as soon after laying as possible. Using a starting weight taken after eggs
have started incubation will result in inaccurate weight loss calculations that may affect hatchabil-
ity. If a true fresh weight taken within 24 hours of laying cannot be obtained, it can be approxi-
mated by the following formula:

CW FW
CD est
1 0 15
.
TD
Where CW = current weight, CD = days incubated from setting to current, TD = total days of
incubation expected (incubation term), and FW est = estimated fresh weight. This equation
assumes that the egg has had a weight loss trend of 15% under the incubating parents, as repre -
sented by the term “0.15” in the equation. This number can be adjusted as indicated by individ-
ual circumstances.
Another, more accurate method of estimating egg fresh weight is to use a species‐specific con-
stant, K w (Hoyt 1979; Burnham 1983). This relies on the shape of eggs, more specifically the rela-
tionship between egg dimensions and volume, that is unique to each species and can only be
determined based on data from large number of eggs:

FW
K w
LB 2
Where K w = observed weight coefficient, FW = fresh weight, L = length of egg (mm), and
B = breadth of egg (mm). Coefficients for intensively managed species, including Peregrine Falcons
(Burnham 1983), California Condors (Kasielke personal communication), and Attwater’s Prairie
Chickens (Bailey personal communication) have been determined from existing data. Estimated
fresh weights for future eggs with known length and breadth can then be calculated as follows:

KLB 2 FW est
W
Once the fresh weight is known or estimated, the expected weight loss can be calculated and
the actual weight loss tracked graphically. This can be done using computer software designed
especially for hatchery management, such as Tracks Software Egg Log, a generic spreadsheet
application, or simply graph paper and pencil. Egg weight is plotted on the y‐axis, with a scale
range from fresh weight to fresh weight less 20% (approximately). Days of incubation are plotted
on the x‐axis, with the set date as day 0 and continuing a few days beyond the expected incubation
term. If the expected weight loss is 15%, lines that bracket this, usually 13% and 17%, should be
plotted on the graph (Figure 3.1). Then, as actual egg weights are plotted, it will be easy to see
whether the weight loss is staying within the expected range and whether the slope of the line
projects the appropriate trend.