WATER VAPOUR FLUX IN THE EGGS OF TWO SPECIES 
OF RAIL (RALLIDAE) DURING INCUBATION 
Alan Lill 
Department of Ecology & Evolutionary Biology and Department of Psychology, Monash University, 
Clayton, Victoria 3168 
Lill. A., 1990:11:30. Water vapour flux in the eggs of two species of rail (Rallidae) during 
mcubation. Proceedings of the Royal Society of Victoria 102(2): 67-70. ISSN 0035- 
Swamp-dwelling purple swamphens and dusky moorhens lay their eggs in shallow, open 
nests, typically built of reeds, placed a few centimetres above water level and consequently 
sometimes noticeably damp. Nest humidity and egg water vapour flux parameters were 
measured to determine the extent to which the eggs of these species have a water balance 
problem during incubation. Mean water vapour pressures in the nest were comparatively 
high (3.6-3.7 kPa). Calculated incubation temperatures averaged 36-37°C and conse¬ 
quently the water vapour pressure gradient across the eggshell was fairly shallow (2.5-2.6 
kPa); however, these values were not sufficient to cause a significant water flux problem in 
the egg. The egg’s mean rate of water loss (185-188 mg.day 1 ), fractional mass loss during 
incubation (12-13%), water vapour conductance (71.43-74.36 mg (day.kPa)-') and total 
functional pore area (1.04-1.15 mm 2 ) all conformed with allometric predictions based on 
the ratio of egg mass:incubation period. Despite their nest placement and nesting habits, the 
two species do not exhibit the wet-nesting syndrome typical of some freshwater birds such as 
grebes and loons. 
MOST bird eggs lose 12-18% of their initial 
mass as water vapour during the incubation 
period, mainly by diffusion through micro¬ 
scopic pores in the shell. This is crucial in pro¬ 
moting optimal hatching success. The rate of 
water loss is determined by the water vapour 
pressure difference across the shell (AP H ^ 0 ) and 
its water vapour conductance (G H ,o> in nig (day. 
kPa) -1 ). Gh 2 o is governed by the^ number, size 
and length‘of the pores (Ar & Rahn 1980). 
Although this gas flux system is now well under¬ 
stood, ecologically-induced, atypical patterns of 
egg water content regulation remain of great 
interest. 
Water vapour pressure in the nest (P n ) of birds 
breeding in a wide variety of conditions averages 
about 2.7 kPa (Walsberg 1980), but the nest 
microclimate of some grebes (Podicipedidae), 
loons (Gaviidae) and megapodes (Megapo- 
diidae) is significantly more humid (Seymour & 
Rahn 1978, Sotherland et al. 1984, Ar & Rahn 
1985). Nonetheless, the eggs of these so-called 
wet-nesters” maintain an optimal state of 
hydration during incubation through having 
such features as a plastron-like outer shell struc¬ 
ture resistant to liquid water influx (Board 1982, 
Sotherland et al. 1984), or a greatly elevated 
Gh,o (Ar & Rahn 1985). However, the extent 
and causes of the wet-nesting syndrome, and 
variations in this phenomenon, are still poorly 
documented. 
In eastern Australia, the dusky moorhen, Gal- 
linula tenebrosa , and purple swamphen, Por- 
phyrio porphyrio , commonly nest in dense reed 
beds in the same wetlands. Their nests are 
shallow bowls, made of reeds and rushes 
(although moorhens sometimes use twigs and 
bark) and often placed only a few centimetres 
above water level (Beruldsen 1980). Some nests 
are noticeably damp at times due to short-term 
water level fluctuations, condensation “run-off’ 
from the dense surrounding vegetation and from 
moisture transported on the incubating adults’ 
body. Both species are therefore potentially wet- 
nesters. This investigation examined whether 
the microclimate of moorhen and swamphen 
nests is unusually humid and, if so, whether this 
generates any water balance problem in the egg. 
METHODS 
Field data were obtained from September- 
December in several years from 7 locations 
within 150 km of Melbourne, including Heales- 
ville, Coldstream, Werribee and Heidelberg. 
During these months. Melbourne has an average 
relative humidity of 60-68% and average daily 
maximal and minimal temperatures of about 
67 
