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Humidity levels in the nests of incubating canaries (Serinus canarius)
Camp. Biochem. Physiol. Vol. WA, No. 3, pp. 721-125, 1987
0300-9629/87$3.00+ 0.00 0 1987Pergamon Journals Ltd
Printed in Great Britain
HUMIDITY
LEVELS IN THE NESTS OF INCUBATING CANARIES (SERZNUS CANARZUS)
MICHAEL D. KERN Biology Department, The College of Wooster, Wooster, OH 44691, USA. Telephone: (216) 263-2000 (Received 6 October 1986)
Abstract-l. Vapor pressure in the nest cups (PN) of common canaries Serinus cunurius averaged 12 torr, but varied considerably both between and within nests during 4-6 consecutive days of incubation. 2. Much of this variation (73-95%) could be accounted for by concurrent variations in ambient humidity (P,) around the nest. 3. The estimated amount of water (12.6% of the egg’s initial mass) lost by canary eggs during 13 days of incubation under conditions of P, that existed in this experiment is not significantly different from the amount (18.8%) that would be lost if P, = P,. 4. These data indicate that canaries do not keep the P, constant by ventilating the nest cup while incubating.
MTRODUCHON
that a relatively constant fraction (ca 15%) of an egg’s mass is lost from virtually any egg during incubation and that freshly laid eggs and chicks at pipping have the same relative degree of hydration (Rahn et al., 1976; Rahn et al., 1977a) led Ar and Rahn (1980) to propose that birds precisely regulate water loss from eggs while incubating. Such regulation may involve passive factors such as the conductance of the eggshell to water (GHIO);and/or active ones such as the behavior of incubating birds on the nest (Ar and Rahn, 1980). The hypothesis of Ar and Rahn (1980) implies that birds actively ventilate the nest in order to keep the vapor pressure or humidity in it (PN) constant and to thereby maintain an environment around the eggs that is optimal for hatching. Certainly there are nesting situations in which it would be advantageous for a bird to regulate PN. In rain forests, underground burrows, or in floating nests partially immersed in water, eggs might be exposed to ambient vapor pressures (P,) high enough to impede water loss and reduce hatchability. In desiccating environments such as high mountains, deserts, or xeric islands, an egg might lose too much water and consequently fail to hatch. Variations in the GHZOof the eggshell do minimize water loss or gain by eggs in such extreme environments. The GH20of eggs of species that normally nest under highly humid ambient conditions is higher than would be expected on the basis of their mass (Lomholt, 1976; Birchard and Kilgore, 1980; Seymour and Ackerman, 1980; Vleck et al., 1983; Davis et al., 1984). The GHZoof eggs of species that nest under potentially desiccating conditions is often much lower than that of eggs of related species nesting under less severe conditions or lower than would be predicted for eggs of their size (Packard et al., 1977; Rahn et al., 1977b; Carey, 1980; Sotherland et al., 1980; Whittow, 1980; Grant, 1982; Carey et al., 1983; Walsberg, 1985).
The discovery
721
This raises a question about whether birds also regulate (or even need to regulate) PN actively (behaviorally) as Rahn and his colleagues (e.g. Rahn et al., 1976) have suggested. For example, the belly-soaking behavior of black-necked stilts (Himuntopus mexicunus) produces increases in PN (Grant, 1982), but such changes could be. an accidental result of the bird’s thermoregulatory activity. Experimentally, the water content of eggs can be. changed considerably either by removing allantoic fluid (Simkiss, 1980) or by altering the G,, of the eggshell (Carey, 1986), yet the chicks hatch normally. So perhaps the level of hydration is not as critical to the egg’s hatchability as it was formerly thought to be, or perhaps the chick has an unexpectedly high tolerance for dehydration while developing in the egg. In domestic fowl, nest and ambient humidity vary in parallel during incubation (Chattock, 1925), i.e. PN depends on PI and the former is not constant. When dry or watersaturated air was injected into nests of house finches (Curpoducus mexicunus) and phainopeplas (Phuinopeplu nitens) to see if it would affect the ventilating behavior of incubating birds, it did not (Walsberg, 1983). Vapor pressure in the nests of mourning doves (Zen&u mucrouru) changes two-fold from the early to the late part of the nesting season. All of these findings suggest that PN is not actively regulated and need not be (Walsberg, 1980). In the experiments described below, PN was measured in nests of common
canaries
(Serinus cunurius)
during each of several consecutive days of incubation to test the hypothesis that canaries actively maintain the nest’s vapor pressure within a narrow range of values. Measurements of this kind have been made in the nests of domestic fowl (Chattock, 1925), pheasants, and several species of waterfowl (Rahn et al., 1977a), but not in the nests of any passerine. MATERIALS AND METHODS During August-September 1984, the vapor pressure was measured in nests of five canaries and in the indoor aviary
MICHAEL
D.
KERN
Table 1. Data concerning the microclimate of canarv nestsa
16
E (6)
22
E (5)
12
M (5)
46
M (5)
2
L (4)
22
L (5)
35.1 + 1.5
19.8 + 2.0
17.2 + 1.6
10.0 + 3.6
(33.&.6)
(17.7-22.6)
(l4.2~la.3)
(5.3-12.3)
35.7 + 1.3
28.4 + 1.8
(35.0-37.5)
(26.3-30.1)
(42.2-48.3)
(5.6-12.1)
35.1 + 2.1
21.6 + 1.0
42.5 + 5.0
11.0 + 3.8
(32.&36.7)
(20.5-22.4)
(37.3-46.3)
(7.4-14.2)
35.4 + 2.0
27.7 + 1.7
43.4 + 4.8
(33.c-37.5)
(26.2-29.6)
(37.7-48.3)
(4.6-13.2)
33.6 + 1.5
21.9 + 6.0
39.2 + 3.4
11.4 + 4.5
(32.7-34.9)
(17.1-26.1)
(37.1-42.0)
(8.9-15.4)
38.5 + 0.9
29.4 + 3.3
51.1 + 2.6
15.8 + 5.2
(37.7-39.3)
(26.6-33.4)
(48.9-53.3)
(11.;20.7)
35.6 + 1.7
24.8 + 4.4
43.8 + 4.2
43.9 + 3.1
a.4 + 3.1
a.3 + 3.8
5.8 + 2.0
25.3
32.5
7.2
35.4
38.0
2.6
31.5
34.4
3.0
35.1
36.5
1.4
27.8
31.8
4.0
35.3
41.6
6.3
35.8 + 3.9
4.1 + 2.4
(4.1-7.5) a.0 2 3.5 (5.5-12.0) 6.9 + 3.9 (4.1-12.3) 7.3 + 2.4 (5.G.O) 9.5 + 2.3 (7.0-12.0)
All nests
. . .
7.9 + 1.7
31.7 + 4.6
‘Values in
the table are means k 95% confidence limits (range). bE = early incubation (days I-6 of incubation); M = mid-incubation (days 4-10); L = late incubation this column are the number of days in which the nest’s microclimate was sampled.
in which they were housed at 24-hr intervals, for periods of 4-6 consecutive days, using egg hygrometers. The ambient conditions to which the incubating birds were exposed were exaggerated with a vaporizer to increase the moisture content of ambient air, or a dehumidifier to reduce it. The birds were exposed to repeated cycles of 3 days of normal room humidity followed by 34 days of altered humidity. Pt ranged from 4.1 to 12.3 torr (Table 1). The canaries were kept by pairs in standard doublebrooder cages (23 x 36 x 28 cm) and exposed to a long daily ohotoneriod (16 L:8 D. liehts on from 0600 to 2200 EDT) and to ambient temperatures of 18.9-29.8”C (average temperature = 23.8”C). As is commonly done by canary breeders, the birds were provided with lined plastic nest pans in which to lay their eggs and incubate. The nest liners were pieces of the padding material used under large rugs, cut to fit the nest pan. They do not impede effective incubation (chicks hatch readily in these nests), although they may absorb water vapor from the nest cavity. It was thought that they would be likely to do so to the same extent in all of the nests used in the experiment, so any observed differences in P, between birds would be due to their behavior while incubating, rather than to differences in the composition of their nests. Also, the clutch size in this experiment was kept at four eggs. Calibrated, natural eggs were used, rather than eggs filled with silica gel (as per Rahn et al., 1977a), as hygrometers to measure P, and P,. To my knowledge, Grant (1982) is the first person to have used hygrometers of this kind, although he used them to determine the GHZO of eggs in nests, rather than to measure P, per se. Natural eggs can only be used to measure PN if one can establish that the temperature of the egg (T.) is reasonably constant during incubation since a natural egg’s vapor pressure (PA) varies exponentially with its temperature. Consequently, one must measure 7’, during incubation. With this information, one can obtain PN or P, by rearranging factors in the equations (Ar et al., 1974) GuZO= M/(P, - PN) and G,,, = M/(P, - P,). respectively; 1
I
-
(days 8-14). Numbers
in parentheses
here P, is specified by T, (in this case the average daily T, for the day on which PN or P,was determined); units of GHlo are mg/day per torr, M mg/day, and P torr. T, was monitored in each nest with a 30-gauge copper-constantan thermocouple (calibrated against a high precision thermometer) implanted in the center of one egg in the clutch. It was assumed that the temperature of this egg is the same as that of the egg hygrometer in the same nest. Given that canaries turn their eggs frequently (Kern, unpublished observations), that all of the eggs in the nest cup were in contact with each other, and that clutch size was only four eggs, I believe this assumption is valid. Also, 30-gauge copper-constantan thermocouples were used to monitor room temperature (T,). In this case, the thermocouple was inserted in the center of an egg that was placed beside the hygrometers on a shelf outside the nest. Egg and room temperatures were recorded for periods of 2 min at approximately 30-min intervals throughout the experiment using strip chart mini-recorders (Cole-Parmer Model R-8377-15). Average daily T, in different nests was 32.7-40.8”C. Within individual nests, the difference between daily minimum and maximum T, averaged 2.2-3.5”C over many days of incubation (Fig. 1). However, on any given day, the 95% confidence limits for the daily average r, within any nest did not exceed k 0.32”C. The maximum error in a determination of P, using a natural egg hygrometer exposed to an average T, with 95% confidence limits of this size is 7.8%. The natural egg hygrometers that were used to measure
PN and
P, were S- to lCday-old
canary eggs that had not
been incubated and had Gus0 values (measured by the method of Rahn et al., 1977a) averaging 98.7% (N = 14 eggs) of the GH20Predicted for them on the basis of their mass when freshly laid (Ar and Rahn, 1978). Weighed hygrometers were placed in the nests (for determination of PN)or on a shelf near the nests (for determination of P,)and weighed at 24-hr intervals thereafter, to obtain daily values of P, and P,. I did not use data collected
in
Humidity in canary nests
723
20 -
_ 16f = 14-
T,
36
+.,
(‘C)
36
w 5 2
12-
: a
IO-
8 6-
64I
I
I
I
I
I
123456 DAY
OF
1
I
1
I
I
IO
II
I2
I3
I4
INCUBATION
Fig. 1. Humidity in the nests (PN) of incubating canaries fluctuates considerably from day to day, often in parallel with changes in P,, as illustrated here for nest 22. T, = average daily egg temperature (“C); the vertical lines around each r, are 95% confidence limits. from any nest on any day on which the 95% confidence limits for T, exceeded f 0.32”C or on any day during which a bird left her nest for more than a few minutes at a time. Removing an egg so that it could be weighed and then returning it to the nest did not disturb the incubating female canaries. In many cases , they had to be lifted from their clutches in order to handle their eggs. They returned to the nest as soon as the eggs were finished with. These natural egg hygrometers were used to measure PN for only 4-6 consecutive days of incubation because the GHIoof canary eggs declines significantly 9 days after they are exposed to incubation T,s (Kern, 1986). This G,, change does not occur in eggs that are exposed to T, (Kern, 1986) and hence, natural egg hygrometers were used to measure P, for up to 20 days. Unless otherwise specified, the data that appear in this paper are means f 95% confidence limits.
RESULTS AND DISCUSSION
The vapor pressure in the five nests averaged 12 torr (Table l), a volume within, but near the lower end, of the 6- to 26-torr range reported for other species of birds (Walsberg, 1980). That the vapor pressure was near the low end of the range may mean that the nest liner absorbed moisture from air in the nest cup during incubation. Nest humidity varied as much as 9.9 torr from one nest to another on any given day of incubation. It also varied considerably from day to day within a single nest (Fig. 1). The average difference between maximal and minimal daily values of PN in individual nests during the 4-6 days of sampling was 7.0 torr; the greatest difference was 9.2 torr (Table 1). Such intra- and internest variation suggests that there is no one level of P., that is optimal for hatching of canary eggs and hence no specific narrow range of P, that is maintained by incubating canaries. Average PN exceeded average PI by 1A-7.2 torr, or by an average of 4.1 torr (Table 1). PA averaged
44 torr (range = 37-53 torr; Table l), which is consis-
tent with published values (42-51 torr; see Walsberg, 1980). It was 25-35 torr higher than PN. Rahn et al. (1976) reported differences between PA and PN of 21-35 torr. Hence, there was always a steep vapor pressure difference (averaging 32 torr) favoring the movement of water vapor from the egg into the nest, but a much smaller one (averaging 4 torr) favoring the movement of water vapor from the nest cup to the surrounding air. Daily variations in PN and PI were directly and significantly correlated in three of the five nests (Table 2). In other words, changes in the moisture content of the air around the nest were accompanied by similar changes in the moisture content of the nest cup. The two variables fluctuated independently in the other two nests and in a second run on one nest. In nests where PN and P, were significantly related, Table 2. Relationships between pN and P, for canary nests Periodof Nest
P vs. P -N -1
b
sHnplinga
2 f
f
16
E (6)
0.67
0.44
22
E (5)
0.85
0.72
12
M (5)
0.94 $G>
0.87 sb
46
M (5)
0.97 -‘eu<
0. 95 ,wti<
2
L (4)
0.90
0.81
22
L (5)
0.88 +<
0.77 +
‘See Table 1 for definitions of terms in this column. Vomlation coefficients (I) and coefficients of determination (?) followed by asterisks are statistically significant at the 0.05 (*), 0.02 (**), and 0.005 (***) levels.
724
MICHAEL D. KERN Table 3. Differences between the actual water loss from eggs in canary nests and the water loss that would occur if P, = P,
6
16
4.8
10.4
16.5 20.9 14.3
22 (E)
6
7.2
15.6
12
5
4.0
10.4
46
5
5.9
15.4
20.0
2
4
1.6
5.3
13.2
5
7.0
18.3
27.7
...
12.6
18.8
22 (0 All wts
sE = early incubation; L = late incubation. bWater loss (in %) over 13 days of incubation = [(loss during sampling period)/(sampling period)] x 13. ‘Predicted water loss = (lOO)(I)(G,)(P, - P,)/W, where I = incubation period (13 days), G, = water vapor conductance of the egg (in g/day per ton), and W = initial weight of the egg (in g). See Walsberg (1980).
changes in P, explain 73-95% of the observed variations in P,. The average amount of water (12.6%) that would be lost from eggs incubated for 13 days (= length of the canary’s incubation period) under the conditions of vapor pressure that were measured in the nest cup was not significantly different (P > 0.05, Student’s t-test) from the amount that would be lost if PN = P, (Table 3). The same is true for nine other avian species (Walsberg, 1980). This finding, as well as the variation in PN between and within nests (Table I), the small difference between PN and P, (Table l), and the significant correlation between PN and P, in three of the five nests (Table 2), argue against the hypothesis that common canaries keep P, constant by actively ventilating their nests while incubating. Acknowledgements-This project was underwritten Work Study funds of The College of Wooster.
with
Carey C., Garher S. D., Thompson E. L. and James F. C. (1983) Avian reproduction over an altitudinal gradient. II. Physical characteristics and water loss of eggs. Physiol. Zool. 56, 340-352.
Chattock A. P. (1925) On the physics of incubation. Phil. Trans. R. Sot. Lond. B 213, 397-450.
Davis T. A., Platter-Reiger M. F. and Ackerman R. A. (1984) Incubation water loss by Pied-billed Grebe eggs: adaptation to a hot, wet nest. Physiol. Zool. 57, 384-391. Grant G. S. (1982) Avian incubation: egg temperature, nest humidity and behavioral thermoregulation in a hot environment. Omithol. Mono. no. 30. Am. Omithol. Union, Washington, D.C. Kern M. D. (1986) Changes in water-vapor conductance of Common Canary eggs during the incubation period. Condor 88, 390-393.
Lomholt J. P. (1976) Relationship of weight loss to ambient humidity of birds eggs during incubation. J. camp. Physiol. B 105, 189-196. Packard G. C., Sotherland P. R. and Packard M. J. (1977) Adaptive reduction in permeability of avian eggshells to water vapour at high altitudes. Nature, Lond. 266, 255-256.
REFERENCES
Ar A., Paganelli C. V., Reeves R. B., Greene D. G. and Rahn H. (1974) The avian eee: water vanor conductance. shell thickness; and functi&al pore area. Condor 76; 153-158. Ar A. and Rahn H. (1978) Interdependence of gas conductance, incubation length, and weight of the avian egg. In Respiratory Function in Birds, Adult and Embryonic (Edited by Piiper J.), pp. 227-236. Springer, New York. Ar A. and Rahn H. (1980) Water in the avian egg: overall budget of incubation. Am. Zool. 20, 373-384. Birchard G. F. and Kilgore D. L. (1980) Conductance of water vapor in eggs of burrowing and nonburrowing birds: implications for embryonic gas exchange. Physiol. Zool. 53, 284-292.
Carey C. (1980) Adaptation of the avian egg to high altitude. Am. Zool. 20, 449459. Carey C. (1986) Tolerance of variation in eggshell conductance, water loss, and water content by Red-winged Blackbird embryos. Physiol. Zool. 59, 109-122.
Rahn H., Ackerman R. A. and Paganelli C. V. (1977a) Humidity in the avian nest and egg water loss during incubation. Physiol. Zool. 50, 269-283. Rahn H., Carey C., Balmas C. K., Bhatia B. and Paganelli C. (1977b) Reduction of pore area of the avian eggshell as an adaptation to altitude. Proc. natn. Acad. Sci. U.S.A. 74, 3095-3098.
Rahn H., Paganelli C. V., Nisbet I. C. T. and Whittow G. C. (1976) Regulation of incubation water loss in eggs of seven species of terns. Physiol. Zool. 49, 245-258. Seymour R. S. and Ackerman R. A. (1980) Adaptations to underground nesting in birds and reptiles. Am. Zool. 20, 431441.
Simkiss K. (1980) Water and ionic fluxes inside the egg. Am. Zool. 20, 385-393.
Sotherland P. R., Packard G. C., Taigen T. L. and Boardman T. J. (1980) An altitudinal cline in conductance of Cliff Swallow (Petrochelidon pyrrhonota) eggs to water vapor. Auk 97, 177-185. Vleck C. M., Vleck D., Rahn H. and Paganelli C. V. (1983) Nest microclimate, water-vapor conductance, and water loss in heron and tern eggs. Auk 100, 7683.
Humidity in canary nests Walsberg G. E. (1980) The gaseous microclimate of the avian nest during incubation. Am. 2001. 20, 363312. Walsberg G. E. (1983) A test for regulation of nest humidity in two bird species. Physiol. Zool. 56, 231-235. Walsberg G. E. (1985) A test for regulation of egg dehy-
725
dration by control of shell conductance in Mourning Doves. Physiol. 2001. 58, 473477. Whittow G. C. (1980) Physiological and ecological correlates of prolonged incubation in sea birds. Am. Zool. 20, 427436.
0300-9629/87$3.00+ 0.00 0 1987Pergamon Journals Ltd
Printed in Great Britain
HUMIDITY
LEVELS IN THE NESTS OF INCUBATING CANARIES (SERZNUS CANARZUS)
MICHAEL D. KERN Biology Department, The College of Wooster, Wooster, OH 44691, USA. Telephone: (216) 263-2000 (Received 6 October 1986)
Abstract-l. Vapor pressure in the nest cups (PN) of common canaries Serinus cunurius averaged 12 torr, but varied considerably both between and within nests during 4-6 consecutive days of incubation. 2. Much of this variation (73-95%) could be accounted for by concurrent variations in ambient humidity (P,) around the nest. 3. The estimated amount of water (12.6% of the egg’s initial mass) lost by canary eggs during 13 days of incubation under conditions of P, that existed in this experiment is not significantly different from the amount (18.8%) that would be lost if P, = P,. 4. These data indicate that canaries do not keep the P, constant by ventilating the nest cup while incubating.
MTRODUCHON
that a relatively constant fraction (ca 15%) of an egg’s mass is lost from virtually any egg during incubation and that freshly laid eggs and chicks at pipping have the same relative degree of hydration (Rahn et al., 1976; Rahn et al., 1977a) led Ar and Rahn (1980) to propose that birds precisely regulate water loss from eggs while incubating. Such regulation may involve passive factors such as the conductance of the eggshell to water (GHIO);and/or active ones such as the behavior of incubating birds on the nest (Ar and Rahn, 1980). The hypothesis of Ar and Rahn (1980) implies that birds actively ventilate the nest in order to keep the vapor pressure or humidity in it (PN) constant and to thereby maintain an environment around the eggs that is optimal for hatching. Certainly there are nesting situations in which it would be advantageous for a bird to regulate PN. In rain forests, underground burrows, or in floating nests partially immersed in water, eggs might be exposed to ambient vapor pressures (P,) high enough to impede water loss and reduce hatchability. In desiccating environments such as high mountains, deserts, or xeric islands, an egg might lose too much water and consequently fail to hatch. Variations in the GHZOof the eggshell do minimize water loss or gain by eggs in such extreme environments. The GH20of eggs of species that normally nest under highly humid ambient conditions is higher than would be expected on the basis of their mass (Lomholt, 1976; Birchard and Kilgore, 1980; Seymour and Ackerman, 1980; Vleck et al., 1983; Davis et al., 1984). The GHZoof eggs of species that nest under potentially desiccating conditions is often much lower than that of eggs of related species nesting under less severe conditions or lower than would be predicted for eggs of their size (Packard et al., 1977; Rahn et al., 1977b; Carey, 1980; Sotherland et al., 1980; Whittow, 1980; Grant, 1982; Carey et al., 1983; Walsberg, 1985).
The discovery
721
This raises a question about whether birds also regulate (or even need to regulate) PN actively (behaviorally) as Rahn and his colleagues (e.g. Rahn et al., 1976) have suggested. For example, the belly-soaking behavior of black-necked stilts (Himuntopus mexicunus) produces increases in PN (Grant, 1982), but such changes could be. an accidental result of the bird’s thermoregulatory activity. Experimentally, the water content of eggs can be. changed considerably either by removing allantoic fluid (Simkiss, 1980) or by altering the G,, of the eggshell (Carey, 1986), yet the chicks hatch normally. So perhaps the level of hydration is not as critical to the egg’s hatchability as it was formerly thought to be, or perhaps the chick has an unexpectedly high tolerance for dehydration while developing in the egg. In domestic fowl, nest and ambient humidity vary in parallel during incubation (Chattock, 1925), i.e. PN depends on PI and the former is not constant. When dry or watersaturated air was injected into nests of house finches (Curpoducus mexicunus) and phainopeplas (Phuinopeplu nitens) to see if it would affect the ventilating behavior of incubating birds, it did not (Walsberg, 1983). Vapor pressure in the nests of mourning doves (Zen&u mucrouru) changes two-fold from the early to the late part of the nesting season. All of these findings suggest that PN is not actively regulated and need not be (Walsberg, 1980). In the experiments described below, PN was measured in nests of common
canaries
(Serinus cunurius)
during each of several consecutive days of incubation to test the hypothesis that canaries actively maintain the nest’s vapor pressure within a narrow range of values. Measurements of this kind have been made in the nests of domestic fowl (Chattock, 1925), pheasants, and several species of waterfowl (Rahn et al., 1977a), but not in the nests of any passerine. MATERIALS AND METHODS During August-September 1984, the vapor pressure was measured in nests of five canaries and in the indoor aviary
MICHAEL
D.
KERN
Table 1. Data concerning the microclimate of canarv nestsa
16
E (6)
22
E (5)
12
M (5)
46
M (5)
2
L (4)
22
L (5)
35.1 + 1.5
19.8 + 2.0
17.2 + 1.6
10.0 + 3.6
(33.&.6)
(17.7-22.6)
(l4.2~la.3)
(5.3-12.3)
35.7 + 1.3
28.4 + 1.8
(35.0-37.5)
(26.3-30.1)
(42.2-48.3)
(5.6-12.1)
35.1 + 2.1
21.6 + 1.0
42.5 + 5.0
11.0 + 3.8
(32.&36.7)
(20.5-22.4)
(37.3-46.3)
(7.4-14.2)
35.4 + 2.0
27.7 + 1.7
43.4 + 4.8
(33.c-37.5)
(26.2-29.6)
(37.7-48.3)
(4.6-13.2)
33.6 + 1.5
21.9 + 6.0
39.2 + 3.4
11.4 + 4.5
(32.7-34.9)
(17.1-26.1)
(37.1-42.0)
(8.9-15.4)
38.5 + 0.9
29.4 + 3.3
51.1 + 2.6
15.8 + 5.2
(37.7-39.3)
(26.6-33.4)
(48.9-53.3)
(11.;20.7)
35.6 + 1.7
24.8 + 4.4
43.8 + 4.2
43.9 + 3.1
a.4 + 3.1
a.3 + 3.8
5.8 + 2.0
25.3
32.5
7.2
35.4
38.0
2.6
31.5
34.4
3.0
35.1
36.5
1.4
27.8
31.8
4.0
35.3
41.6
6.3
35.8 + 3.9
4.1 + 2.4
(4.1-7.5) a.0 2 3.5 (5.5-12.0) 6.9 + 3.9 (4.1-12.3) 7.3 + 2.4 (5.G.O) 9.5 + 2.3 (7.0-12.0)
All nests
. . .
7.9 + 1.7
31.7 + 4.6
‘Values in
the table are means k 95% confidence limits (range). bE = early incubation (days I-6 of incubation); M = mid-incubation (days 4-10); L = late incubation this column are the number of days in which the nest’s microclimate was sampled.
in which they were housed at 24-hr intervals, for periods of 4-6 consecutive days, using egg hygrometers. The ambient conditions to which the incubating birds were exposed were exaggerated with a vaporizer to increase the moisture content of ambient air, or a dehumidifier to reduce it. The birds were exposed to repeated cycles of 3 days of normal room humidity followed by 34 days of altered humidity. Pt ranged from 4.1 to 12.3 torr (Table 1). The canaries were kept by pairs in standard doublebrooder cages (23 x 36 x 28 cm) and exposed to a long daily ohotoneriod (16 L:8 D. liehts on from 0600 to 2200 EDT) and to ambient temperatures of 18.9-29.8”C (average temperature = 23.8”C). As is commonly done by canary breeders, the birds were provided with lined plastic nest pans in which to lay their eggs and incubate. The nest liners were pieces of the padding material used under large rugs, cut to fit the nest pan. They do not impede effective incubation (chicks hatch readily in these nests), although they may absorb water vapor from the nest cavity. It was thought that they would be likely to do so to the same extent in all of the nests used in the experiment, so any observed differences in P, between birds would be due to their behavior while incubating, rather than to differences in the composition of their nests. Also, the clutch size in this experiment was kept at four eggs. Calibrated, natural eggs were used, rather than eggs filled with silica gel (as per Rahn et al., 1977a), as hygrometers to measure P, and P,. To my knowledge, Grant (1982) is the first person to have used hygrometers of this kind, although he used them to determine the GHZO of eggs in nests, rather than to measure P, per se. Natural eggs can only be used to measure PN if one can establish that the temperature of the egg (T.) is reasonably constant during incubation since a natural egg’s vapor pressure (PA) varies exponentially with its temperature. Consequently, one must measure 7’, during incubation. With this information, one can obtain PN or P, by rearranging factors in the equations (Ar et al., 1974) GuZO= M/(P, - PN) and G,,, = M/(P, - P,). respectively; 1
I
-
(days 8-14). Numbers
in parentheses
here P, is specified by T, (in this case the average daily T, for the day on which PN or P,was determined); units of GHlo are mg/day per torr, M mg/day, and P torr. T, was monitored in each nest with a 30-gauge copper-constantan thermocouple (calibrated against a high precision thermometer) implanted in the center of one egg in the clutch. It was assumed that the temperature of this egg is the same as that of the egg hygrometer in the same nest. Given that canaries turn their eggs frequently (Kern, unpublished observations), that all of the eggs in the nest cup were in contact with each other, and that clutch size was only four eggs, I believe this assumption is valid. Also, 30-gauge copper-constantan thermocouples were used to monitor room temperature (T,). In this case, the thermocouple was inserted in the center of an egg that was placed beside the hygrometers on a shelf outside the nest. Egg and room temperatures were recorded for periods of 2 min at approximately 30-min intervals throughout the experiment using strip chart mini-recorders (Cole-Parmer Model R-8377-15). Average daily T, in different nests was 32.7-40.8”C. Within individual nests, the difference between daily minimum and maximum T, averaged 2.2-3.5”C over many days of incubation (Fig. 1). However, on any given day, the 95% confidence limits for the daily average r, within any nest did not exceed k 0.32”C. The maximum error in a determination of P, using a natural egg hygrometer exposed to an average T, with 95% confidence limits of this size is 7.8%. The natural egg hygrometers that were used to measure
PN and
P, were S- to lCday-old
canary eggs that had not
been incubated and had Gus0 values (measured by the method of Rahn et al., 1977a) averaging 98.7% (N = 14 eggs) of the GH20Predicted for them on the basis of their mass when freshly laid (Ar and Rahn, 1978). Weighed hygrometers were placed in the nests (for determination of PN)or on a shelf near the nests (for determination of P,)and weighed at 24-hr intervals thereafter, to obtain daily values of P, and P,. I did not use data collected
in
Humidity in canary nests
723
20 -
_ 16f = 14-
T,
36
+.,
(‘C)
36
w 5 2
12-
: a
IO-
8 6-
64I
I
I
I
I
I
123456 DAY
OF
1
I
1
I
I
IO
II
I2
I3
I4
INCUBATION
Fig. 1. Humidity in the nests (PN) of incubating canaries fluctuates considerably from day to day, often in parallel with changes in P,, as illustrated here for nest 22. T, = average daily egg temperature (“C); the vertical lines around each r, are 95% confidence limits. from any nest on any day on which the 95% confidence limits for T, exceeded f 0.32”C or on any day during which a bird left her nest for more than a few minutes at a time. Removing an egg so that it could be weighed and then returning it to the nest did not disturb the incubating female canaries. In many cases , they had to be lifted from their clutches in order to handle their eggs. They returned to the nest as soon as the eggs were finished with. These natural egg hygrometers were used to measure PN for only 4-6 consecutive days of incubation because the GHIoof canary eggs declines significantly 9 days after they are exposed to incubation T,s (Kern, 1986). This G,, change does not occur in eggs that are exposed to T, (Kern, 1986) and hence, natural egg hygrometers were used to measure P, for up to 20 days. Unless otherwise specified, the data that appear in this paper are means f 95% confidence limits.
RESULTS AND DISCUSSION
The vapor pressure in the five nests averaged 12 torr (Table l), a volume within, but near the lower end, of the 6- to 26-torr range reported for other species of birds (Walsberg, 1980). That the vapor pressure was near the low end of the range may mean that the nest liner absorbed moisture from air in the nest cup during incubation. Nest humidity varied as much as 9.9 torr from one nest to another on any given day of incubation. It also varied considerably from day to day within a single nest (Fig. 1). The average difference between maximal and minimal daily values of PN in individual nests during the 4-6 days of sampling was 7.0 torr; the greatest difference was 9.2 torr (Table 1). Such intra- and internest variation suggests that there is no one level of P., that is optimal for hatching of canary eggs and hence no specific narrow range of P, that is maintained by incubating canaries. Average PN exceeded average PI by 1A-7.2 torr, or by an average of 4.1 torr (Table 1). PA averaged
44 torr (range = 37-53 torr; Table l), which is consis-
tent with published values (42-51 torr; see Walsberg, 1980). It was 25-35 torr higher than PN. Rahn et al. (1976) reported differences between PA and PN of 21-35 torr. Hence, there was always a steep vapor pressure difference (averaging 32 torr) favoring the movement of water vapor from the egg into the nest, but a much smaller one (averaging 4 torr) favoring the movement of water vapor from the nest cup to the surrounding air. Daily variations in PN and PI were directly and significantly correlated in three of the five nests (Table 2). In other words, changes in the moisture content of the air around the nest were accompanied by similar changes in the moisture content of the nest cup. The two variables fluctuated independently in the other two nests and in a second run on one nest. In nests where PN and P, were significantly related, Table 2. Relationships between pN and P, for canary nests Periodof Nest
P vs. P -N -1
b
sHnplinga
2 f
f
16
E (6)
0.67
0.44
22
E (5)
0.85
0.72
12
M (5)
0.94 $G>
0.87 sb
46
M (5)
0.97 -‘eu<
0. 95 ,wti<
2
L (4)
0.90
0.81
22
L (5)
0.88 +<
0.77 +
‘See Table 1 for definitions of terms in this column. Vomlation coefficients (I) and coefficients of determination (?) followed by asterisks are statistically significant at the 0.05 (*), 0.02 (**), and 0.005 (***) levels.
724
MICHAEL D. KERN Table 3. Differences between the actual water loss from eggs in canary nests and the water loss that would occur if P, = P,
6
16
4.8
10.4
16.5 20.9 14.3
22 (E)
6
7.2
15.6
12
5
4.0
10.4
46
5
5.9
15.4
20.0
2
4
1.6
5.3
13.2
5
7.0
18.3
27.7
...
12.6
18.8
22 (0 All wts
sE = early incubation; L = late incubation. bWater loss (in %) over 13 days of incubation = [(loss during sampling period)/(sampling period)] x 13. ‘Predicted water loss = (lOO)(I)(G,)(P, - P,)/W, where I = incubation period (13 days), G, = water vapor conductance of the egg (in g/day per ton), and W = initial weight of the egg (in g). See Walsberg (1980).
changes in P, explain 73-95% of the observed variations in P,. The average amount of water (12.6%) that would be lost from eggs incubated for 13 days (= length of the canary’s incubation period) under the conditions of vapor pressure that were measured in the nest cup was not significantly different (P > 0.05, Student’s t-test) from the amount that would be lost if PN = P, (Table 3). The same is true for nine other avian species (Walsberg, 1980). This finding, as well as the variation in PN between and within nests (Table I), the small difference between PN and P, (Table l), and the significant correlation between PN and P, in three of the five nests (Table 2), argue against the hypothesis that common canaries keep P, constant by actively ventilating their nests while incubating. Acknowledgements-This project was underwritten Work Study funds of The College of Wooster.
with
Carey C., Garher S. D., Thompson E. L. and James F. C. (1983) Avian reproduction over an altitudinal gradient. II. Physical characteristics and water loss of eggs. Physiol. Zool. 56, 340-352.
Chattock A. P. (1925) On the physics of incubation. Phil. Trans. R. Sot. Lond. B 213, 397-450.
Davis T. A., Platter-Reiger M. F. and Ackerman R. A. (1984) Incubation water loss by Pied-billed Grebe eggs: adaptation to a hot, wet nest. Physiol. Zool. 57, 384-391. Grant G. S. (1982) Avian incubation: egg temperature, nest humidity and behavioral thermoregulation in a hot environment. Omithol. Mono. no. 30. Am. Omithol. Union, Washington, D.C. Kern M. D. (1986) Changes in water-vapor conductance of Common Canary eggs during the incubation period. Condor 88, 390-393.
Lomholt J. P. (1976) Relationship of weight loss to ambient humidity of birds eggs during incubation. J. camp. Physiol. B 105, 189-196. Packard G. C., Sotherland P. R. and Packard M. J. (1977) Adaptive reduction in permeability of avian eggshells to water vapour at high altitudes. Nature, Lond. 266, 255-256.
REFERENCES
Ar A., Paganelli C. V., Reeves R. B., Greene D. G. and Rahn H. (1974) The avian eee: water vanor conductance. shell thickness; and functi&al pore area. Condor 76; 153-158. Ar A. and Rahn H. (1978) Interdependence of gas conductance, incubation length, and weight of the avian egg. In Respiratory Function in Birds, Adult and Embryonic (Edited by Piiper J.), pp. 227-236. Springer, New York. Ar A. and Rahn H. (1980) Water in the avian egg: overall budget of incubation. Am. Zool. 20, 373-384. Birchard G. F. and Kilgore D. L. (1980) Conductance of water vapor in eggs of burrowing and nonburrowing birds: implications for embryonic gas exchange. Physiol. Zool. 53, 284-292.
Carey C. (1980) Adaptation of the avian egg to high altitude. Am. Zool. 20, 449459. Carey C. (1986) Tolerance of variation in eggshell conductance, water loss, and water content by Red-winged Blackbird embryos. Physiol. Zool. 59, 109-122.
Rahn H., Ackerman R. A. and Paganelli C. V. (1977a) Humidity in the avian nest and egg water loss during incubation. Physiol. Zool. 50, 269-283. Rahn H., Carey C., Balmas C. K., Bhatia B. and Paganelli C. (1977b) Reduction of pore area of the avian eggshell as an adaptation to altitude. Proc. natn. Acad. Sci. U.S.A. 74, 3095-3098.
Rahn H., Paganelli C. V., Nisbet I. C. T. and Whittow G. C. (1976) Regulation of incubation water loss in eggs of seven species of terns. Physiol. Zool. 49, 245-258. Seymour R. S. and Ackerman R. A. (1980) Adaptations to underground nesting in birds and reptiles. Am. Zool. 20, 431441.
Simkiss K. (1980) Water and ionic fluxes inside the egg. Am. Zool. 20, 385-393.
Sotherland P. R., Packard G. C., Taigen T. L. and Boardman T. J. (1980) An altitudinal cline in conductance of Cliff Swallow (Petrochelidon pyrrhonota) eggs to water vapor. Auk 97, 177-185. Vleck C. M., Vleck D., Rahn H. and Paganelli C. V. (1983) Nest microclimate, water-vapor conductance, and water loss in heron and tern eggs. Auk 100, 7683.
Humidity in canary nests Walsberg G. E. (1980) The gaseous microclimate of the avian nest during incubation. Am. 2001. 20, 363312. Walsberg G. E. (1983) A test for regulation of nest humidity in two bird species. Physiol. Zool. 56, 231-235. Walsberg G. E. (1985) A test for regulation of egg dehy-
725
dration by control of shell conductance in Mourning Doves. Physiol. 2001. 58, 473477. Whittow G. C. (1980) Physiological and ecological correlates of prolonged incubation in sea birds. Am. Zool. 20, 427436.