Humidity Levels in Pied Flycatcher Nests Measured Using Capsule Hygrometers

The Auk 112(3):564-570, 1995

HUMIDITY LEVELS IN PIED FLYCATCHER NESTS MEASURED USING CAPSULE HYGROMETERS

MICHAEL D. KERN• AND RICHARDJ. COWIE2 •BiologyDepartment, The College of Wooster,Wooster, Ohio 44691, USA; and 2Schoolof Pureand Applied Biology, University of Wales,College of Cardiff,P.O. Box 915, CardiffCF1 3TL, Wales,United Kingdom

ABSTP,ACT.--We usedplastic capsules that were perforatedat one end and filled with silica gel to measurethe water vapor pressure(P, in nestsof Pied Flycatchers(Ficedula hypoleuca) during 1992 And 1993.Nest humidity measureddaily for 7 to 16 consecutivedays of incubation in 31 nestsaveraged 2.1 kPa or 15.7tort (range 1.8-2.5 kPa). Humidity varied greatly from one nestbox to anotherand within individual nests,but did not affectthe rate of egg water lossor hatching success.Nest humidity often was significantly correlatedwith variations in ambient humidity. Large clutcheshad significantlylower averagePN than small ones. In general,nest humidity waslow at the beginningof incubation,rose during midincubation, and remainedelevated thereafter. Received 8 December1994, accepted 6 February1995.

Ec,c;s THAT LOSE ABNORMAL AMOUNTS of water may change the shell's water-vapor conduc- during incubation frequently do not hatch tance (Kern et al. 1990). (Snyder and Birchard 1982, Carey 1986). Since Facedwith the drawbacksof usingdesiccant- egg water lossdepends in part upon differences filled eggsto measurehumidity in the nestsof between the water-vapor pressurein the egg smallbirds, we developedanother hygrometer and in the air around it (Landauer 1967, Lorn- that is small enoughto use in their nests,that holt 1976,Rahn and Paganelli 1990),humidity is virtually unbreakable,and in which desiccant levels of nest air can affect hatching success, can be replacedat will. In principle, it is the particularly under harsh environmental con- sameas that usedby Rahn et al. (1977)--a po- ditions (e.g. Grant 1982).Even though the wa- rous shell containing desiccant.In this case, ter-vapor pressure(PN) commonly varies in the however, the shell is a plasticcapsule, one end nest (Howey et al. 1984, Kern 1987, Kern et al. of which has been perforated so that water va- ! 990)and is not apparentlyregulated (Walsberg por can penetrate and bind to the desiccantin- ! 980, 1983),it may be responsiblefor otherwise side.This hygrometeris like an egghygrometer unexplainable hatching failure and its mea- in the sensethat it provides one value of nest surement may be therefore of considerable humidity representingconditions that presum- practical importance. ably existthroughout a set period of time, usu- However, few direct measurementsof PNwere ally 24 h. If there are markedchanges in water- made before the late 1970s when Rahn et al. vapor levels in a nest during that period, as (1977) introduced the egg hygrometer. Since sometimeshappens (Howey et al. 1984, Kern that time, many such measurementshave been and Knapic 1991),neither hygrometerwill rec- made, particularly in nests of birds which lay ord the variations.Our capsulehygrometer is large eggs (Rahn and Paganelli 1990). Egg hy- unlike an egghygrometer, however, in the sense grometerscan be used to measurehumidity in that it measuresthe quantity of water in nest nests of such large speciesbecause it is rela- air, whereas an egg hygrometer does not be- tively easyto install a screwcap in large, thick- cause20 to 30% of its porous surfaceis often in shelled eggs in order to add or remove desic- contactwith the skin of the incubating adult cant. Such eggsare robustand readily accepted and such contactscan be an important source by an incubating bird as part of its clutch. It is, of water vapor (Handrich 1989). however, difficult to make hygrometersfrom Using these capsule hygrometers, we mea- eggs of small birds becauseof the egg's small suredhumidity levels in the nestsof Pied Fly- sizeand thin shell. It is possibleto sealdesiccant catchers(Ficedula hypoleuca) during 1992 and into a small egg,but it cannotbe replacedwhen 1993, not only becausefew such measurements it becomeswaterlogged, and removing water exist for cavity-nestingbirds, but alsoto seeif from it by gently heating the egg in an oven PNaffected egg water lossand hatchingsuccess.

564 July 1995] Humidityin PiedFlycatcher Nests 565

We describehow the hygrometerswere made where Gwat,ris the water-vapor conductanceof the and presentinformation aboutwater-vapor lev- capsule(in mg day-• kPa-•),A•i ..... is the changein els in flycatcher nests during each day of in- the capsule'smass (the amount of water taken up) cubation. per day (in mg day •), Pois the water-vaporpressure of the environment outsidethe capsule(in kPa), and Pt is the water-vapor pressureinside the capsule.Be- causethe capsulewas filled with silica gel, Pt was 0 METHODS kPa and the equation reduced to Capsulehygrometers.--Hygrometers were madefrom O..... =/• ..... /Po. (2) replacementcapsules for model TM transmitters(Mini- Mitter Company, Sunriver, Oregon). These capsules Since the capsuleswere sealed in a small container are cylindrical (1 cm in diameter) and consist of a over water, we assumedthat the atmospherearound tubular capinto which a hollow plug fits. We cut the them was 100% saturatedwith water vapor. Under plug down to a length of about 0.5 cm in order to these conditions,Po is directly related to water tem- shorten the overall length of the hygrometer to 2.0 perature and can be obtained from Weast (1975). to 2.3 cm. Six evenly spacedholes were made in the We calibrated the capsulesbefore and after each top of the capsulewith a 25-gaugehypodermic nee- fieldseason. Their conductance values, even after they dle. For a desiccant,we usedsilica gel (Tel-tale brand, had beenused repeatedly, changed less than 10%:by mesh 6-16, Fisher Scientific, Pittsburgh, Pennsylva- an average of -1.8% (range -11.5 to 8.5%, n = 32 nia). Sincethe top of the capsulewas semitransparent, capsules)in 1992;and 6.3% (range 2.9 to 11.1%,n = it was possibleto seethe silica gel inside and to de- 40 capsules)in 1993. termine approximately how much water it had ad- The amountof water taken up by the silicagel in sorbed(anhydrous silica gel is deep purple; as water the capsule hygrometers depends on Po.In environ- combineswith the gel, its colorchanges to light blue; ments that were 100% saturated with water (Poca. 3.9 when it is waterlogged,the gel is colorless).The des- kPa),the mass of the silicagel increasedby an average iccantshould be changeddaily becauseof the amount of 5.2%per day (range 3.9 to 6.1%,n = 35 capsules). of water that it adsorbsin a 24-h period. Under typical ambient conditionsin the field (Poca. We filled about75% of the capsulewith silica gel 1.6 kPa), the increasewas only 2.0% per day (range (350-500 rag). The hollow plug was lined insideand 1.6 to 2.4%, n = 25 capsules). out with Clingfilm, so that a layer of it was between Valuesof water-vaporpressure obtained using these the two parts of the capsulewhen they were con- capsulehygrometers corresponded closely to those nected.We used the wooden handle of a dissecting obtained with a hygrothermograph(calibrated with needle to push Clingfilm into the hollow center of a sling psychrometer and a precision thermometer). the plug. The two partsof the capsulewere then fitted We averagedambient vapor pressuremeasured at 2-h together, the dessicant was shaken down into the intervalsover a 24-hperiod with a hygrothermograph plug, and excessClingfilm was cut off with a scalpel and comparedthat value with the vapor pressure blade. Clingfilm did not provide a completeseal be- measuredconcurrently by 25 capsulehygrometers. tween the cap and the plug, but made it easierto take The differencebetween the valuesprovided by the them apart. The junction between the two parts of hygrothermographand 13 capsuleswithout plugsav- the capsulewas sealedwith electrician'stape. In our eraged 0.04 kPa. The value from the hygrothermo- experience, Scotch Super 33+ vinyl electrical tape graph and the averagePo obtained from 12 capsules (3M ElectricalProducts, Austin, Texas)works partic- with plugs were identical. The coefficientsof varia- ularly well. tion were 4.8% for capsuleswithout plugs and 1.9% When necessary,the lengthof the capsulehygrom- for those with plugs. eter wasreduced even further (to 1.7cm) by not using Sealedcapsules were essentiallyimpervious to wa- the plug, but simply sealingdesiccant into the cap. ter vapor. Six unperforatedcapsules were filled with In this case,we filled the capsulecompletely with silica gel, sealed,and kept in a 100% humidified at- silicagel (550-600mg), coveredthe open end with a mosphereat 30øC(Po = 4.2 kPa);they gained lessthan small piece of Clingfilm cut to fit, and sealedthe end I mg over a three-day period. Two groups of perfo- with electrician'stape. rated capsules(n = 26 and 45) were sealedand kept We determined the water-vapor conductanceof at a Poof about1.3 kPa;they gainedon averageonly capsulehygrometers by putting them in a small,closed 1.7 and 0.6 mg, respectively,over periods of six to container over water at a constanttemperature. The seven days. capsuleswere weighed beforehandand then at ap- Field measurements.--During1992 and 1993, we proximately 12-h intervals for 1 to 1.5 days. Their measuredhumidity levels in nest boxesoccupied by conductancewas determinedusing the formula (Ar incubatingPied Flycatchers,as well asin unoccupied et al. 1974): nest boxes, in Llanwrthwl woods, 1.5 km north of Newbridge-on-Wye, Powys, south-central Wales C..... = t• ...../(Po - P,), (1) (52øN, 3øW). 566 KeRUAND COWlE [Auk,Vol. 112

T^M,E1. 2•/•,,,,,PN, and hatchingsuccess in PiedFlycatcher nests as functionof clutchsize. Values in parenthesesare ranges and sample sizes.

Days of Clutch incubation size(no. 2•/•,,,, PN duringwhich Hatchingsuccess eggs) (mg day-1) (kPa) P• measured (percentof eggs) 5 22.2 + 1.7 (22.0-25.6, 5) 2.12 + 0.20 (1.93-2.48, 7) 9-14 83 (60-100, 7) 6 -- 2.32 + 0.13 (2.23-2.52, 4) 7-10 88 (67-100, 4) 7 23.1 + 3.1 (19.4-27.2, 7) 2.07 -+ 0.11 (1.87-2.19, 9) 11-14 73 (43-86, 8) 8 -- 2.08 + 0.05 (2.00-2.15,5) 10-14 80 (50-100, 5) 9 24.0 + 5.6 (17.3-26.7,6) 1.95 + 0.13 (1.84-2.20,6) 11-16 57 (33-89, 6)

Capsulehygrometers were made and calibrated (as sure PNon each of 7 to !6 days of incubationin 3! describedabove) at the LlysdinamField Centre (of nests (6 in !992 and 25 in !993). All of the birds used the University of Wales Collegeof Cardiff) in New- in our study exceptone (which had a sterile clutch) bridge-on-Wye,weighed to the nearest0.! mg on a hatchedtheir eggsand reared broodssuccessfully. Mettier microbalance, and buried in a sealed contain- In 1993,we also measuredambient vapor pressure er of silicagel until they were used.They were em- (Pt)in the woodswith hygrometercapsules placed in beddedin the walls of flycatchernests with the per- empty nest boxesat the study site. These capsules foratedends exposed to nestair underneaththe clutch. were treated in the sameway as those placed in oc- After approximately24 h in the nest,they were re- cupied nest boxes. moved, sealedin Clingfilm, and returned to the field Nest and ambient water-vapor pressures(in kPa) stationwhere they were reweighed. were determinedfrom the mass(in mg day-•) gained By exchangingcapsules daily, we were ableto mea- by the capsuleswhen they were in a nest box and their water-vaporconductance (in mg day-• kPa-1), using the general formula

2.6 i• = ,• .... /G...... (3) In 1993, we also weighed the eggs in 18 clutches two or three times during the incubation period to see if the rate at which they lost water (i.e. mass) •.8 dependedon the humidity in the nest. In eachcase, the eggswere brieflyremoved from the nestbox (we •.4 replacedthem with clay eggs)and transportedto the 2.4 field centre,where they were weighed to the nearest 0.! mg with a Mettier microbalance.Usually, the eggs were out of the nest box for less than 2 h. We have 2.0 ignored the small amount of massthat they lostwhile in transit.Weighing timeswere three to six daysapart. •.6 Datawere examinedstatistically with studentt-tests, regressionanalyses, and analysesof variance fol- 2.6 ///•'• ß lowed when necessaryby Student-Newman-Keuls / multiple-range tests(Zar 1974). 2.2 \ / RESULTS

1,4 During 7 to 16 days of incubation, water-vapor levels in nest boxesaveraged 2.1 + SD of 0.07 kPa (range 1.8 to 2.5 kPa; Table 1). Large

0.1 a t • t t t t clutcheshad significantlylower averagePN than 2 4 6 8 I0 12 14 smallones (clutch size versusaverage P•, r2 = 0.18, P < 0.02, n = 31 nests).Hatching success DAY OF INCUBATION did not depend on the averagehumidity level Fi;. [. Day-to-dayvariation o• P• i• nestso• five in the nest box (r 2 = 0.06, P > 0.1, n = 30 nests) incubatin; •ied F[ycatche;s. or on the maximum changethat occurredin the July 1995] Humidityin PiedFlycatcher Nests 567

led on average0.8 + 0.2 kPa and, in somecases, as much as 1.3 kPa. o o ß 2.2 o In general, PN WaSlOW at the onset of incu- o o O ß ß bation and rose as incubationcontinued (Fig. 2). In 1992, it rose sharply during the first few o ß o o days of incubation and remained reasonably z 2.0 constant thereafter. In 1993, however, it remained low through day 5 of incubation, and 1.9 o then rosesteadily through day 14. Averagedai- o ly PN (in kPa) and day of incubation were tin- early related: 4 6 8 I0 12 14 PN = 1.937 + 0.020 Day (4) DAY OF INCUBATION (r 2 = 0.47, P < 0.001, n = 26). Fig. 2. Daily averagePN in representativenests of Pied Flycatchers. DISCUSSION

As Table 2 illustrates,humidity levels in the PNof the nest during incubation(r 2 = 0.005, P nests of small birds increaseas the nest cup > 0.5, n = 30 nests).Day-to-day variationsin becomes more enclosed. Open-bowl nests, PNwere differentfrom onenest to another(Fig. which are presumably well ventilated, com- 1). In somecases, PN rose more or lesssteadily monly have low values of PN, whereas domed during the incubationperiod, but in others,it and cavity nestswith more stationaryair mass- declinedduring midincubation,or includedone es,have higher values.Underground burrows, or more pronounced,but brief changes.Even which are poorly ventilated and in which the with suchdifferences, PN and P• were signifi- atmosphereis nearly saturatedwith water va- cantly correlated in 16 of 25 nestsin 1993 (r 2 = por, have still higher values. Levels of water 0.35 to 0.92).Humidity levelswere on average vapor in the nest boxesof Pied Flycatchersare 0.5 + 0.1 kPa higher in occupiednest boxes consistentwith this general picture (Table 2). than in empty ones. However, the humidity in flycatchernests is no There was no significant relationship be- higher than that in the open-bowlnests of some tween the average PN in individual nests and passetinesthat breed in xeric environments, the rate at which eggsin the clutch lost water specificallyHouse Finches(Carpodacus mexican- (r• = 0.09, P > 0.1, n = 18 nests),even though us) and Phainopeplas(Phainopepla nitens; Wats- daily values of PN within individual nests vat- berg 1983). Such seeminglyanomalous condi-

T^BLE2. Relationshipbetween PN and nesttype amongsmall birds.

Species Type of nest P• (kPa) Source Barn Swallow (Hirundorustica) Open bowl 1.2 Bitchard and Kilgore (1980) CommonCanary (Serinuscanarius) Open bowl 1.6 Kern (1987) 1.3-2.8 Kern and Knapic (1991) House Finch (Carpodacusmexicanus) Open bowl 1.7 Walsberg(1983) Phainopepla(Phainopepla nitens) Open bowl 2.1 Walsberg(1983) Song Sparrow (Melospizamelodia) Open bowl 2.1 Kern et al. (1990) Red Bishop(Euplectes orix) Domed woven 2.1 Woodall and Parry (1982) (grass)nest Great Tit (Parusmajor) Nest box 2.4 Lornholt (1976) (2.0-2.9) ProthonotaryWarbler (Protonotariacitrea ) Nestbox 2.1a Blem and Blem (1994) Pied Flycatcher(Ficedula hypoleuca ) Nestbox 2.1 This study (1.8-2.5) Bank Swallow (Ripariariparia) Tunnel 2.7 Bitchardand Kilgore (1980) EuropeanBee-eater (Merops apiaster) Tunnel 2.9-4.0 White et al. (1978) ßCalculated from average values of nesttemperature and relative humidity. 568 KERNAND COWIœ [Auk,Vol. 112 tions have been observed before. Walsberg egg's G,•at,rseems to be much more important (1980), for example, in reviewing what was and in an evolutionarysense is generallyadapt- known aboutPN up to 1980,noted that the av- ed to the environmental conditions under which eragePN of desertspecies was higher than that a speciesnests. It is high in wet nestsand mesic of nondesert species(2.8 vs. 2.5 kPa). Grant habitats (e.g. Lomhott 1976, Birchard and Kil- (1982)discovered that the PNof severalspecies gore 1980, Davis et at. 1984), but low in xeric of Charadriiformesbreeding under extremely habitats(Grant 1982)and at all but the highest hot, dry xericconditions was an unexpectedly altitudes(Leon-Vetarde et at. 1984,Carey et at. high 2.7 to 3.3 kPa. 1987, 1989). Furthermore, it often increases dur- Watsberg (1983, 1985) showed, both by ing incubation concurrentwith the embryo's changingP• experimentallyand by observing increasing demands for oxygen (Kern et al. natural changesin the nest's microclimate dur- 1992). ing the breeding season,that PN in wild birds The fact that daily values of PN within and affectsegg water loss,a relationshiplong since among flycatchernests were so varied (Fig. 1) known for domesticatedbirds (Murray 1925, suggeststhat: (1) P• is not closelyregulated by Romanoff1929); the higher the humidity, the the birds; (2) no particular level is optimal for lower the/9/,•a•r. all nests, as has now been demonstrated for It hasbeen arguedthat this is preciselywhy many other species(Watsberg 1980, 1983, How- P• is so important (Rahn et at. 1977, Morgan et ey et at. 1984, Andersen and Steen 1986, Kern at. 1978, Rahn and Paganetli 1990). Uncharac- 1987,Kern et at. 1990, Kern and Knapic 1991); teristic degreesof egg water loss causeabnor- and (3) normal variations in P• do not have mal embryonicdevelopment and reducehatch- seriouseffects on egg water loss,as Watsberg's ing success.High water lossmay interfere with (1980) model predicts.We were not, for exam- formation of the chorioatlantoic membrane ple, able to demonstratea relationshipbetween (Snyderand Birchard1982), reduce embryonic P• and egg water lossfor Pied Flycatchers,per- growth rates (Sirekiss 1980a, b), or block late haps becausenatural variations in PN were not stagesof embryogenesis(Carey 1986).Low wa- large enoughto producesubstantial changes in ter loss preventsfully developed chicksfrom /9/...... Extremedifferences in P• fromday to pipping successfully(Watsberg and Schmidt day of incubation averagedonly 0.8 kPa and 1992).In domesticfowl (Gallus;which may not did not exceed1.3 kPa in any nest.By contrast, be representativeof birds in general), the em- much greater elevations in PN of 1.5 to 1.6 kPa bryo is particularlysensitive to egg water loss reducedegg water loss in Walsberg's(1983,1985) during early development (Snyder and Bir- studies.Furthermore, P• was only about0.5 kPa chard 1982) and percent hatch is highest when higher than Pt at our study site, and the frac- eggslose only 6 to 12%of their mass(i.e. water) tional massloss of the flycatcheregg during during incubation(Landauer 1967, Lundy 1969). incubation would have increasedonly 2% had Wild speciesappear to show much more tol- Pt and P• been the same.In addition, hatching erance to variations in egg water loss and P• successwas not significantlyaffected by the av- during incubationthan chickens(Carey 1986); erageP• in the nest during incubation,or by natural fluctuations in both, between and with- the degreeof changein PNduring incubation. in species,can be quite large (Hoyt 1979,Wats- Basedon valuesof/9/ ..... in ourstudy (Table berg 1980, Howey et at. 1984,Kern 1987,Kern 1), the egg would loseapproximately 23% of its et at. 1990, Kern and Knapic 1991). Some evi- massduring a typical 13-dayincubation period. dence suggeststhat even domestic fowl will This fractional massloss is somewhathigher toleratemarked variations in 3)/...... provided than we have found in our previouswork with theyoccur during late incubation (Sirekiss 1980a, Pied Flycatchereggs (14.2%; Kern et at. 1992), b, Snyder and Birchard1982). but still within the 10 to 23% range reported Several factorsbesides PN can influence egg for other species(Rahn and Ar 1974, Ar and water loss,including the G..... of the egg shell Rahn 1980, Rahn and Paganetli 1990). and the nest-related behaviors of incubating Nest-humiditylevels were low when incu- birds. Walsberg(1980) developeda model that bationbegan, increased during midincubation, examinesthe relative importance of such fac- andremained high until the chickshatched (Fig. tors. The model indicatesthat PN is not very 2). It is also during mid- and late incubation important.;changes would need to be extreme that the G..... and M ..... of Pied Flycatchereggs to push M,•at,rto lethal limits. By contrast,the are elevated and the egg's massloss is most July1995] Humidityin PiedFlycatcher Nests 569 pronounced (Kern et al. 1992). This could be a AR,A. 1990. Rolesof water in avian eggs.Pages 229- cause-and-effectsituation in which the eggsbe- 243 in Egg incubation:Its effectson embryonic comemore water permeableand lose more wa- developmentin birds and reptiles (D.C. Deem- ter, which remains in the confines of the nest ing and M. W. J.Ferguson, Eds.). Cambridge Univ. cup elevating PN, or it could indicate changes Press,Cambridge. AR, A., C. V. PAGA•qELLI,R. B. REEVES,D. G. GREE•qE, in the nest-relatedbehavior of the incubating ANDH. RAH•q. 1974. The avian egg: Water vapor bird that add water to the nest or prevent water conductance,shell thicknessand functional pore loss.The fact that PNwas significantlylower in area. Condor 76:153-158. large clutchesthan in small ones, in distinct AR, A., ANDH. RAIN. 1980. Water in the avian egg: contrast to the situation in bantam hens (Gallus Overall budget of incubation.Am. Zool. 20:373- domesticus;Andersen and Steen 1986), argues 384. against the first possibility. The fact that egg BIRCHARt),G. F., AND D. L. KILC,ORE, JR. 1980. Con- water loss did not depend on PN iS evidence ductanceof water vapor in eggs of burrowing against the second possibility. The birds do, and nonburrowing birds: Implications for em- however, sit very tightly near the end of the bryonic gasexchange. Physiol. Zool. 53:284-292. incubationperiod, which couldexplain why PN BLEM,C. R., ANDL. B. BLEM. 1994. Compositionand iS elevated then. microclimateof ProthonotaryWarbler nests.Auk 111:197-200. Ar (1990) suggestedrecently that PN iS not regulated directly, but nonethelessstays rela- CAREY,C. 1986. Tolerance of variation in eggshell conductance,water loss, and water content by tively constantbecause an incubatingbird reg- Red-winged Blackbird embryos. Physiol. Zool. ulates egg temperature(T•) closely.That does 59:109-122. not seem to be true for Pied Flycatchers(Figs. CAREY,C., F. LEON-VELARDE,G. CASTRO,AND C. MONGE. 1 and 2). If PNdepended on T, in Pied Flycatch- 1987. Shell conductance,daily water loss, and ers, Tewould be higher during the secondhalf water contentof Andean Gull and PunaIbis eggs. of the incubationperiod than it is during early J. Exp. Zool. (SuppL) 1:247-252. incubation.In a previousstudy (Kern et al. 1992), CAREY, C., F. LEON-VELARDE,O. DUNIN-BoRKOWSKI, T. however, we found no significantchanges in L. BUCHER,G. DE LA TORRE,D. ESPINOZA,AND C. Teduring the incubation period. MONGE. 1989. Variation in eggshell character- In both years of our study, PN was, as ex- isticsand gasexchange of montane and lowland pected, highest when hatching occurred. cooteggs. J. Comp. Physiol. B Biochem.Syst. En- Hatching should contribute water vapor to the viron. Physiol. 159:389-400. nest, since 20 to 40% of the egg's total water DAVIS. T. A., M. F. PLATTER-REIGER,AND R. A. ACK- loss occursduring external pipping (Ar and ERMAN. 1984. Incubation water loss by Pied- billed Grebe eggs:Adaptation to a hot, wet nest. Rahn 1980, Whittow 1982, Sotherland and Rahn Physiol. Zool. 57:384-391. 1987). GRANT,G. S. 1982. Avian incubation: Egg temperature, nest humidity and behavioral thermoreg- ACKNOWLEDGMENTS ulation in a hot environment. Ornithol. Monogr. 30. We thank: Fred Slater, warden of the Llysdinam HANDRICH,Y. 1989. Incubation water loss in King Field Centre, who is a constantsource of help to us Penguin egg. II. Does the brood patch interfere during our field seasons;the LlysdinamTrust for pro- with eggshellconductance? Physiol. Zool. 62:119- viding facilities at the centre;Pat Tantrum on whose 1'32. property we work; and C. R. Blem and two anony- HowEY, P., R. G. BOARD,D. H. DAVIS, AND J. KEAR. mousreviewers for their thoughtful commentson an 1984. The microclimate of the nests of water- earlier draft of our paper. This research was con- fowl. Ibis 126:16-32. ducted under license SB:7:93 issued to R.J.C. and HoY'r,D.F. 1979. Osmoregulationby avian embry- M.D.K. by the CountrysideCouncil for Wales in Ban- os: The allantois functions like a toad's bladder. gor. M.D.K. was supportedby a grant from the How- Physiol. Zool. 52:354-362. ard Hughes Foundation and faculty development KERN,M.D. 1987. Humidity levels in the nestsof funds from the College of Wooster. incubatingcanaries (Serinus canarius). Comp. Bio- chem.Physiol. A Comp. Physiol.87:721-725. LITERATURE CITED KERN,M.D., R. J. Cow, E,AND M. Y•AGER. 1992. Water loss,conductance, and structureof eggsof Pied ANDERSEN,•D.,AND J. B. STEEN.1986. Water economy Flycatchersduring egg laying and incubation. in bird nests.J. Comp. Physiol. B Biochem.Syst. Physiol. Zool. 65:1162-1187. Environ. Physiol. 156:823-828. KERN, M.D., AND M. KNAPIC. 1991. Short-term vari- 570 KERNAND COWlE [Auk,Vol. 112

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