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- field season.Their conductancevalues, 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 massof 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)

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