68 
ALAN LILL 
17-25°C and 6-14°C respectively. Eggs were col¬ 
lected under permit from four of the sites for 
experimental work. Nesting habitat comprised 
artificial and natural ponds, dams, lakes and 
marshes, often with emergent stands of Typha 
sp. and Eleocharis sp. 
The initial mass of eggs (± 0.01 g) was deter¬ 
mined: (a) by weighing newly-laid eggs in the 
field on a wind-shielded Shinko Denshi portable 
balance, and (b) for a further sample of partly 
developed eggs, by estimation from their linear 
dimensions (± 0.05 mm), using the equation 
W = K w x LB max 2 (Hoyt 1979), 
where W is fresh mass (g), L is length, B ma? is 
maximum breadth (mm) and K vv is a species- 
typical constant which was empirically deter¬ 
mined for both species. 
The mean daily mass loss of incubated eggs at 
various stages of development was determined 
by marking them with indelible ink and weigh¬ 
ing them (as above) at intervals of 5-22 days. 
Since nearly all mass loss can be attributed to 
water vapour loss (Drent 1970, Rahn & Ar 
1974), this value equals the mean daily rate of 
water loss (Mh 2 o> * n mg.day -1 )- 
Nest humidify was determined by the method 
of Rahn et al. (1977a). The gain in mass of 
hygrometer eggs (consisting of eggshells fitted 
with metal screw caps and filled with dry silica 
gel) when placed in active nests for periods of 2- 
4 days was divided by their water vapour con¬ 
ductance. The latter quantity was determined by 
measuring their mean daily mass gain at 20°C 
and 76% RH in the laboratory over a 4 day 
period. 
G h ,o °f intact eggs was determined by the 
method of Ar et al. (1974). The daily weight loss 
(± 1 mg) of eggs placed over dry silica gel in a 
desiccator at 20°C was measured over 8-9 days. 
Values obtained were corrected to a barometric 
pressure of 101.33 kPa and also to a temperature 
of 25°C to facilitate interspecific comparisons. 
Determinations were mostly done on fresh eggs, 
but a few older eggs were included to give an 
indication of whether shell conductance 
increased markedly during incubation, as re¬ 
ported for a few bird species (Carey 1979). 
Shell parameters influencing the egg's G H ,o 
were measured on dried shells which were cut 
with a dental drill with a cutting bit into equat¬ 
orial, pointed and blunt pole sections. True shell 
thickness (L) and pore density were measured on 
12 equatorial and 6 polar (3 from each pole) frag¬ 
ments from each egg. The fragments were first 
briefly boiled in 5% NaOH to remove the shell 
membranes. L (± 1 pm) was measured with a 
Tesa dial micrometer fitted with an hemispheri¬ 
cal anvil to accommodate the curvature of the 
shell. Pore density was determined for the same 
fragments by painting the inner surface with 
acid fuchsin, which penetrated the pores so that 
they could be detected as discrete spots on the 
outer surface (Ar & Rahn 1985). The fragments 
were mounted on a Nikon V20A Profile Projec¬ 
tor and the number of pores in a 25 mm 2 area 
counted from the magnified (x 20) image. For 
each species, the number of pores in an eee of 
average size (N) was calculated as the product of 
mean pore density and surface area, which was 
estimated from equation (12) of Hoyt (1976). 
Pore shape and form were examined by viewing 
platinum-coated, radial sections of shell under 
an Hitachi S570 SEM. 
The relative magnitude of egg water vapour 
flux parameters in swamphens and moorhens 
was assessed by comparing observed values with 
those predicted from egg mass/incubation 
period (M/I) (relative embryonic growth rate) 
using established regression equations based on 
data for many species (sources in Table 1). 
RESULTS 
Egg size measurements and mean values for egg 
parameters which influence its water vapour 
flux are summarised for both species in Table 
1 . 
In the purple swamphen, P n averaged 3.7 1 
0.4 kPa (range 2.9-4.1 kPa; n = ^determin¬ 
ations for 10 nests); only three values were less 
than 3.5 kPa. 36.3 g, the average of the two esti¬ 
mates derived respectively by weighing and 
from dimensions, was taken as the best estimate 
of mean initial egg mass. At the mean M H ,o 
observed, an egg of this mass would have an esti¬ 
mated fractional mass loss of c, 12% over the 24 
day incubation period. G H ,o corrected to 25 : C 
ranged from 60.23 to 102.63 mg.(day.kPa) l :the 
mean values for the three older eggs and the nine 
fresh eggs, which had equivalent mean initial 
masses, were not significantly different (75.65 + 
12.61 versus 70.60 ± 11.24 mg (day.kPa)“ J ;t„ 0 ) 
= 0.613, p >0.05). The mean AP H2 q in the nest 
was 2.5 kPa. The mean saturated vapour pres¬ 
sure of the egg contents was therefore 6.1 kPa. 
which translates into a mean incubation 
temperature of 36-37°C. Pore density averaged 
28 ± 10,25 ± 8 and 35 ± 8 pores per 25 mm 2 al 
the pointed pole, equator and blunt pole respect¬ 
ively. > 
In the dusky moorhen, P n averaged3.6 ±0.4 
kPa (range 2.9-4.2 kPa; n = 11 determinations 
