A Comparison of Brown-Headed Cowbirds and Red-Winged Blackbirds

The Auk 115(4):843-850, 1998

TEMPERATURE, EGG MASS, AND INCUBATION TIME: A COMPARISON OF BROWN-HEADED COWBIRDS AND RED-WINGED BLACKBIRDS

BILL g. STRAUSBERGER • Departmentof BiologicalSciences, University of Illinoisat Chicago, 845 WestTaylor Street, Chicago, Illinois 60607, USA

ABSTRACT.--Eggsof the parasiticBrown-headed Cowbird (Molothrus ater) often hatch be- forethose of its hosts.I artificiallyincubated the eggsof the cowbirdand the taxonomically relatedbut nonparasiticRed-winged Blackbird (Agelaius phoeniceus) to determine if cowbird eggshave unique adaptations. To determine if cowbirdshave a wider toleranceof acceptable incubationtemperatures, the eggsfrom both specieswere incubatedat 35, 38, and 40øC. Neitherspecies' eggs hatched at 35 or 40øC,whereas 24 out of 42 cowbirdeggs and 19 out of 36 redwing eggshatched at 38øC.When correctedfor differencesin mass,cowbird and redwingeggs hatched 10 and 15%, sooner than expected, respectively, suggesting that cowbirds do not have an acceleratedrate of embryonicdevelopment. Cowbirds may be adapted to lay smallereggs that generallyhatch sooner. Using values obtained from allometricequa- tionsthat relatebody mass and eggmass, cowbird and redwingeggs were 25 and4% smaller,respectively, than expected. For cowbirds, there was no correlationbetween egg mass and length of incubationperiod, whereasthere was a positive(but not significant)correlation forredwings. Additional cowbird eggs were incubated at 36and 39øC to determinethe effect on the length of the incubationperiod. The incubationperiods of individual cowbirdeggs rangedfrom 10.49 to 14.04days at 39 and36øC, respectively, whereas eggs incubated at 38øC had a meanincubation period of 11.61days (n = 24), suggestingthat cowbirdscan tolerate somevariation in incubationtemperature and indicatinga strongtemperature effect. I found evidencethat internalegg temperature varies with clutchcomposition. This may partially explainthe variation in cowbirdincubation periods and why cowbirdeggs often hatch before hosteggs. Received 2 July 1997, accepted 20 November1997.

THE BROWN-HEADED COWBIRD (Molothrus adaptation to parasitism (O'Connor 1984). By ater)is a well-studiedbrood parasite that pos- hatching before host eggs, parasiteswould sessesa variety of adaptationsfor parasitism likely be better competitorsfor food. Fried- includingparasitizing nests when hostsare mann(1927) first suggested that cowbirds have leastlikely to be present(Scott 1991), rapid lay- evolved an accelerated rate of embryonic ing of eggs (Sealyet at. 1995), and producing growthand an unusuallyshort incubation pe- eggs with unusuallythick shellsthat resist riod (10 days), whereasHoffman (1929) sug- damage (Blankespooret al. 1982, Spaw and gestedthat cowbirdslay partially developed Rohwer 1987).Brown-headed Cowbirds (here- eggs,as do somecuckoos (Perrins 1967, Vernon after "cowbird") parasitize many different 1970).Studying the ShinyCowbird (Molothrus species(Friedmann and Kiff 1985) that have bonariensis),Kattan (1995) found a relationship different incubation temperatures (Huggins betweenan egg'sincubation period and energy 1941,Rahn and Ar 1974, Webb 1987), a variable content. that strongly affectsegg hatchability(Drent Nice (1953), however, found that cowbird 1973,White and Kinney1974). As a result,it eggs(n = 62)took at least11 days to hatch,and hasbeen suggested that broodparasites such sheconcluded that thecowbird's incubation pe- ascowbirds and parasitic cuckoos have flexible riod was similarto that of somenonparasitic ic- incubationtemperature requirements (Groeb- terids.Other reportsof cowbirdincubation pe- bets and M6bert 1930, Graber 1955). riods range from 10 to 14 days,with means Relativelyshort incubation periods of some ranging from 11.6 to 11.9 days (Norris 1947, broodparasites have been suggested to be an Briskieand Sealy1990, Scott and Lemon1996). Becausehigher temperaturesgenerally in- E-mail: [email protected] creasethe rate of embryonicdevelopment (Ro-

843 844 BILLM. STRAUSBERGER [Auk,Vol. 115 manoff 1960, O'Connor 1984), variation in in- TABLE1. Observedand expectedegg massand incubationperiods may result from different incubationperiod of Brown-headedCowbird and Red-wingedBlackbird eggs incubatedat 38.0øC. cubationtemperatures. Observedvalues are • _+SD, with n in parentheses. A strong positive allometric relationship usually existsbetween egg massand incuba- Egg mass Incubationperiod tion period (Rahn and Ar 1974) and between (g) (days) egg massand femalebody mass(Rahn et al. Brown-headed Cowbird 1975).Species that lay relativelysmall eggs, Observed 3.04 _+0.28 (65) 11.61 _+0.24 (24) such as someparasitic cuckoos (Lack 1968), Expected 4.05 _+0.09 a 12.88b may experiencea reduced incubationperiod. Red-winged Blackbird Similarly,within species,relatively small eggs Observed 4.09 _+0.42 (52) 12.09 _+0.40 (19) alsomay hatchsooner than larger ones(Lan- Expected 4.24 _+0.09 a 14.17b dauer 1967, Parsons 1972, Drent 1975; but see •From Rahn et ai. (1975)for passerines:E = 0.34Wø677, where E is MacCluskie et al. 1997). Species-specificincu- expectedegg massand W is female body mass. bFrom Vleck and Vieck (1987) for aitricial birds: log I - 0.97 + bation periodsoften are correlatedwith taxo- 0.29(iogE), where I is incubationperiod and E is eggmass. nomic group (Lack 1968). Icterids (i.e. "blackbirds") collectivelylay relatively small eggs (Weatherheadand Teather1994). Briskie and Sealy (1990), however,did not find a smaller eggs.All eggswere weighed(_+ 0.01 g) on a Mettler balance (Model AE 100). egg massfor parasitic(n = 4) comparedwith In 1995, I assessed the effects of incubation tem- nonparasitic(n = 17) icterids. peratureon hatchability.I placeda randomsample Using eggsof the relatednonparasitic Red- of eggsof both speciesinto TX7 incubators(Lyon winged Blackbird(Agelaius phoeniceus), I pre- ElectricCompany) with temperaturesof 35.0, 38.0, dictedthat artificiallyincubated cowbird eggs and 40.0øCand 64%relative humidity. All eggswere would toleratea wider range of temperatures turned automaticallyevery hour. Of these,only eggs than Red-winged Blackbird (hereafter"red- incubated at 38øC hatched. Therefore, in 1996 I in- wing") eggs.Next, I testedthe hypothesesthat cubatedeggs at 38øCand 64% relativehumidity, to cowbirdeggs hatch sooner than predicted from determinethe meanlength of eachspecies' incubation period at this temperature.To determinethe ef- egg massand/or lay smaller eggsthan pre- fectsthat slightlydifferent temperatures have on the dicted.I predictedthat artificiallyincubated lengthof the incubationperiod, I incubated5 and 11 cowbirdeggs would hatch sooner than thoseof cowbirdeggs at 36.0 and 39.0øC,respectively. After redwings,after correctingfor egg massdiffer- 10 daysof incubation,all eggswere checkedapprox- ences. To test whether cowbirds laid smaller imatelyevery 4 h for pipping.Once pipping was ob- eggsthan predictedfrom femalebody mass,I served,eggs were checked more often. Hatching was comparedthe expected egg masses of both spe- definedas the momenta chickforced the capoff the cieswith the actualmass of collectedeggs. Fi- shelland fully extendedits neck,and wasmeasured to the nearest15 minutes.I used linear regression nally,I suggestan explanationfor the variation analysisto determineif eggmass and incubationpe- in the lengthsof reportedcowbird incubation riod were correlated. periods and earlier hatchingcompared with For other analyses,I comparedobserved and ex- manyhost eggs. pectedvalues obtained from regressionsof eachpa- rameter.Expected incubation periods and associated STUDY AREA AND METHODS 95% confidenceintervals based on eggmass were obtained from Vleck and Vleck (1987): I collectedfresh redwing and cowbirdeggs at The log I = 0.97 + 0.29(logE), (1) Morton Arboretumin northeasternIllinois during the 1995to 1997nesting seasons. Cowbird eggs were where I is incubationperiod in days and E is egg collectedfrom a varietyof hostspecies' nests. I mon- massin g (Table1). The mean incubationperiod of itored nests daily, and collectedeggs before 1000 an egg was defined as the interval betweenplace- CST on the day they were laid and only from nests mentin an incubatorand hatching.The meanlength whereincubation had not yet started. To prevent nest of eachspecies' incubation period was determined at abandonmentafter egg removal, I replacedredwing 38øC.For each species, deviations of meanegg mass eggswith markedeggs removed from other redwing from an expectedvalue based on femalebody mass nests.Cowbird eggswere replacedwith plasterof and 68% confidence intervals were obtained from Paris eggs that were painted to resemblecowbird Rahn et al. (1975).The expectedegg mass(g) for a October1998] EmbryoDevelopment in Blackbirds 845

TABLE2. Hatchingsuccess of Brown-headedCow- TABLE3. Incubation period for Brown-headed bird and Red-wingedBlackbird eggs incubated at Cowbird eggsincubated at 36.0 and 39.0øC.Only differenttemperatures and 64%relative humidity. 2 of 5 eggsand 5 of 11 eggshatched at 36.0 and 39.0øC,respectively. Temperature(øC) 35.0 38.O 4O.O Egg Incubation number (days) za P Brown-headed Cowbird 36.0øC No. of eggsincubated 11 42 16 672A 14.04 10.39 <0.001 No. of eggshatched (%) 0 (0) 24 (57) 0 (0) 672B 13.96 10.04 <0.001

Red-winged Blackbird 39.0øC No. of eggsincubated 15 36 20 573A 11.04 -2.43 <0.05 No. of eggshatched (%) 0 (0) 19 (53) 0 (0) 573B 11.25 - 1.54 >0.05 574A 11.11 -2.18 <0.05 5111A 10.79 -3.50 <0.001 speciesof known averagebody mass(g) canbe es- 6111A 10.49 -4.79 <0.001 timated from: aAll z-valuesobtained by comparingan egg'sincubation period at a giventemperature with the meanvalue obtainedfrom eggsincu- egg mass= a(femalebody mass)b, (2) bated at 38.0øC(x - 11.61 days, n - 24). where a and b are constants for each taxonomic group (Rahn et al. 1975).Values for femalecowbird and redwing body masses,38.8 and 41.5 g, respec- thanZebra Finch eggs, and theytended to restat the tively,were obtainedfrom Dunning (1993).Values of bottom of the nest.Care was takento placethe egg the constants(a = 0.340,b = 0.677)were obtained with the inserted thermistor at the level in the nest from Rahn et al. (1975)for passerines.I usedthe z- at whichit would naturallyrest when not attached statisticto determineif individual cowbird eggsin- to a thermistor.Egg temperature was recorded con- cubatedat 36 or 39øChad incubationperiods similar tinuouslyby a LinearInstrument electronic data log- to the meanincubation period of eggsincubated at ger.The meanegg temperaturewas determinedby 38øC.For eachspecies, the deviationsof eggmasses averagingegg temperaturerecorded every 6 min at and incubationperiods from predicted valueswere a constantambient air temperatureof 22.0øC.A total calculatedas the proportionof the observedvalues of 331 and 427 observations(i.e. 6-min intervals)was to expectedvalues. recordedfrom: (1) a finchegg in a clutchof threeoth- To obtain the incubationtemperature of eggsin er fincheggs, and (2) a finchegg in a clutchof two unparasitizednests and experimentallyparasitized othersand a cowbirdegg, respectively. Because both nests,I replacedan incubatingZebra Finch (Taeniop- sexesof Zebra Fincheswill incubatesimultaneously, ygiaguttata) egg with a freshcowbird egg. Although the male was removed. the Zebra Finchis not a naturally occurringhost of thecowbird, its body mass (ca. 12 g) is similarto that RESULTS of frequently parasitized species,including Field Sparrows(Spizella pusilla) and Chipping Sparrows Temperaturetolerance.--No eggs hatched at 35 (S. passerina;Dunning 1993). Finch eggs are much or 40øC,whereas 24 out of 42 (57%) and 19 out smallerthan cowbirdeggs, weighing approximately of 36 (53%) cowbirdand redwing eggs,respec- 1.1 g (Williams 1996), whereas cowbird eggs are nearly three times heavier (Table 1; see Lowther tively, hatchedat 38øC(Table 2). Subsequent 1993). To measureegg temperatures,a thermistor candling revealed that most embryosbegan (YSI model 42SC telethermometer with 400 series growthat 35øCbut soondied, whereas those at probe) was placed 1 mm beneaththe upper surface 40øCsurvived approximately seven days. of a finchegg, i.e. approximatelyat thelocation of the Temperatureand cowbirdincubation period.- blastoderm,as describedby Haftorn (1978).No em- Two out of five cowbirdeggs hatched at 36øC, bryonicdevelopment took placein eggsthat had a whereas5 out of 11 hatchedat 39øC.Both eggs thermistorinserted, whereas others developed nat- hatchedat 36øChad significantlylonger incu- urally. The temperaturemeasurements were unlike- bation periodscompared with the mean of 24 ly to be influencedby embryonicheat production, eggsat 38øC,whereas each of five eggsat 39øC whichgenerally is too low to be significantin small passerines(Kendeigh 1963:898). The movementof had shorterincubation periods (Table 3). the eggwith the insertedthermistor was restricted Egg mass.--Totalobserved mean egg masses due to an attachedwire probe.The remainingeggs for both cowbirds and redwings were lower werefree to moveduring egg turning by the attend- than predicted from female body mass (Table ing adult.Cowbird eggs are muchlarger and heavier 1). Cowbird and redwing egg massesranged 846 BILLM. STRAUSBERGER [Auk, Vol. 115

12.5 135

y = 11.593 + O.006x y = 10.676 + 0.336x 0 12.0 13.0 r = 0.317 r = 0.007 0 0 0 0000 0 0 0 0 0 O oO•OO O 115 12.5 0 0 0 8 0 0 o o O 11.0 12.0 o o oo goOoo o

10.5 • • • 11.5 I i 2.0 2,5 3,0 3.5 4.0 35 4.0 4.5 50 5.5

Egg mass (g) Egg mass (g)

FIG. 1. Relationship between Brown-headed FiG.2. Relationshipbetween Red-winged Black- Cowbird egg massand incubationperiod (P = 0.974, bird egg massand incubationperiod (P = 0.185,n = n = 24) at 38.0øCand 64% relativehumidity. 19) at 38.0øCand 64%relative humidity.

DISCUSSION from 2.27 to 3.66 g (• = 3.04 g) and 3.40 to 5.00 g (• = 4.09 g), respectively.The expectedegg To supportthe hypothesisthat an attributeis massfor a passerinethe massof a femalecow- a specializationfor parasitism,one must dem- bird or redwing is 4.05 g (68% confidencein- onstratethat the attributeis unique to para- terval 3.38 to 4.94 g; Rahn et al. 1975)and 4.24 sites.My evidencesuggests that cowbirdsand g (68% confidenceinterval 3.47 to 5.16 g; Rahn redwingshave similar incubation temperature et al. 1975),respectively. The ratio of observed requirements,and that cowbirdsdo not havea to predicted egg mass was 0.75 for cowbirds uniquely acceleratedrate of embryonicdevel- and 0.96 for redwings. opment.However, cowbird eggswere smaller The mean incubationperiod was shorter than expected,which is consistentwith the hy- than predictedfrom eggmass for hatchedcow- pothesis that cowbirds lay small eggs that bird and redwingeggs incubated at 38øC(Table hatch sooner. 1). The incubationperiods for cowbirdand red- Temperaturetolerance.--Given the effects of wing eggsincubated at 38øCranged from 11.08 temperatureon physiologicaland developmen- to 11.96 days (œ= 11.61 __+SD of 0.24 days,n = tal processes,species may be constrainedin 24) and 11.71 to 13.10 days (• = 12.09 + 0.40 their ability to hatchunder different incubation days, n = 19), respectively.The mean incuba- temperatures (Dawson 1984:Table 1, Webb tion period was below the lower limit of the 1987), even if suchan attribute is favorable.In 95% prediction interval derived from Vleck and Vleck's(1987) equation for cowbirds(12.05 TABLE4. Egg temperature(øC; œ ñ SD) in Zebra to 13.77 days; predicted incubationperiod = Finch nest with and without a Brown-headed 12.88days) and redwings (12.90to 15.28days; Cowbird egg. Ambient air temperature was con- predicted incubationperiod - 14.17 days; Ta- stant at 22øC. ble 1). For cowbirdeggs, there was no correlation betweenegg massand lengthof the incu- Egg re- Egg Nest contents corded temperaturea nb bationperiod (Fig. 1). For redwings,there was a positive,but not significant,correlation be- 4 fincheggs Finch 37.3 _+0.74 331 3 fincheggs, tween egg massand length of the incubation 1 cowbirdegg Finch 36.8 _+0.78 427 period (Fig. 2). The additionof a cowbirdegg Differencein temperaturessignificant (Mann-Whitney U-test, P < to the nest significantlyreduced the internal 0.001). temperatureof the Zebra Finchegg (Table4). Observationrecorded every 6 min. October1998] EmbryoDevelopment in Blackbirds 847 my experiment,failure to hatchsuggests that birds to effectively parasitize small species. neither speciesis tolerant of continuousincu- Cowbirds usually parasitize species smaller bationat 35 or 40øC.Cowbird eggs hatched at than themselves (Robinson et al. 1995, Scott 36, 38, and 39øC,confirming they are able to and Lemon 1996, but see Peer and Bollinger withstand some variation in incubation tem- 1998),and in captivity they havebeen shown perature.For both species,38øC may not be the to preferentiallyparasitize nests with eggs optimalincubation temperature. Hatching suc- smallerthan their own (King 1973,1979). Al- cess that I measured did not exceed 57%, ternatively,smaller eggs may enablecowbirds whereasthat of naturally incubatedeggs of to lay moreeggs or eggsthat hatchsooner A both species,and of avianeggs in general(not variety of other factors,including degree of lost to total nestfailure), hasbeen found to be sexual size dimorphism, are also correlated ->88%and 90%, respectively(Southern 1958, with a species'mean egg mass(Weatherhead Koenig1982, Beletsky 1996:212). Other factors and Teather1994), making it difficult to draw (e.g.lack of temperaturevariation) alsomay be generalconclusions from two species.In an ex- responsiblefor the relativelylow hatchingsuc- tensive study, Briskie and Sealy (1990) found cess that I measured. that egg volumesfor parasiticicterids (includ- Incubationperiod.--The mean incubationpe- ing the Brown-headedCowbird) were not riod of cowbirds was 1.27 days (10%) shorter smallerthan those of nonparasiticicterids, sug- than expectedbased on egg mass.It is inter- gestingthat parasiticicterids do not lay smaller estingto speculatethat the cowbird'sshort in- eggsthan many nonparasiticicterids. Brown- cubationperiod is an adaptationfor parasitism. headedCowbirds, however, may lay unusually However, the nonparasiticredwing had an small eggsfor a parasiticicterid, or redwings evenshorter incubation period than predicted may lay unusuallylarge eggs for a nonparasitic by eggmass, hatching 2.08 days(15%) sooner icterid. Theseresults differ from expected values based Although egg mass generally varies allo- on Briskieand Sealy(1990), who foundthat the metricallywith body mass,substantial intra- mean incubation periods of host-generalist specificvariation exists within this generalre- parasiticand nonparasiticicterids averaged 0.7 lationship(Gill 1990).In my study,egg mass of daysshorter and 0.2 dayslonger, respectively, individual cowbird eggs was not correlated thanexpected from eggmass. It is unlikelythat with lengthof theincubation period, indicating the shorterincubation periods that I observed thatsmall cowbird eggs do nothatch sooner. In resultedfrom earlierhatching of artificiallyin- redwings,egg mass was positively (but not sig- cubatedeggs. The incubationperiod for artifi- nificantly) correlatedwith length of the incu- ciallyincubated redwing eggs was the sameas bation period. that usedby Briskieand Sealy(1990) and sim- Incubationtemperature.--The reported varia- ilar to that foundby othersfor naturallyincu- tion in the lengthof cowbirdincubation peri- bated eggs(Case and Hewitt 1963,Payne 1969, odsmay result from differentmean incubation Strehland White 1986).Why redwingshave an temperatures.Briskie and Sealy(1990) consid- apparently acceleratedrate of embryonicde- ered the possibilitythat the 10-dayincubation velopmentis unclear,but it could be due to in- periodsthey observedmay haveresulted from tensesibling competion favoring earlier hatch- higher incubationtemperatures from an un- ing. usuallyattentive host. In my experiment,cow- Egg mass.--Themean cowbird egg massin bird incubationperiods ranged from 10.49 to my study was much smaller (25%) than ex- 14.04 days at 39 and 36øC,respectively. Cow- pected, whereas redwing eggs were only bird eggsincubated at 36øCtook up to 34%lon- slightlysmaller (4%). Huxley (1927)found that ger to hatch than eggs incubatedat 39øC,in- the eggsof 436 oscinespecies with a mean dicatinga strongtemperature effect. body massof 37.92g (i.e. similar to that of cow- The ostensibleparadox of cowbird eggs birds and redwings) averaged 9% of adult hatchingin lesstime thanhost eggs, even when body mass.Cowbird and redwing egg masses the cowbirdeggs are larger (Hamiltonand Or- were 7.84 and 9.86%of femalebody masses,re- ians 1965,Payne 1977, Briskie and Sealy 1990, spectively,suggesting that cowbirdslay rela- McMasterand Sealy1997), may be explainedif tively small eggs.Small eggsmay enablecow- theyreceive sufficient heat for incubationwhile 848 BILLM. STRAUSBERGER [Auk, Vol. 115 interferingwith incubationof host eggs.Egg LITERATURE CITED temperaturesvary widely within a nest de- pending,in part,on the amountof contactwith BALDWIN,S. P., ANDS.C. KENDEIGH.1932. Physiol- ogy of temperatureof birds. ScientificPublica- the attendingbird's incubation patch (Baldwin tions of the Cleveland Museum of Natural His- and Kendeigh1932, Drent 1973).The addition tory Vol. 2. of anegg to a clutchmay decrease the efficiency BELETSKY,L. 1996.The Red-wingedBlackbird. Aca- of incubationbecause the energycosts of in- demic Press, New York. cubationincrease by 6 to 7%for eachadditional BLANKESPOOR,G. W., J. OOLMAN, AND C. UTHE. 1982. egg (Haftorn and Reinertsen1985, McMaster Eggshell strength and cowbird parasitism of and Sealy1997). Similarly, the replacementof a Red-wingedBlackbirds. Auk 99:363-365. host'segg with a relativelylarger egg,such as BRISKIE,J. V., AND S. G. SEALY.1990. Evolution of a cowbird's,may decreasethe incubationeffi- short incubationperiods in the parasiticcow- ciencyof thehost's eggs (Friedmann 1929, Hug- birds, Molothrusspp. Auk 107:789-794. gins 1941, Hofslund 1957, Zimmerman 1983, CASE,N. A., ANDO. H. HEWITT.1963. Nesting and productivityof the Red-wingedBlackbird in re- Wiley 1985,Petit 1991,Sealy 1992, McMaster lationto habitat.Living Bird 2:7-20. and Sealy 1997, Peer and Bollinger 1998) and DAWSON,W. R. 1984.Metabolic responses of embry- result in extendedincubation periods and re- onic seabirdsto temperature.Pages 139-157 in duced hatchability.Sayler (1996) found that Seabirdenergetics (G. C. Whittow and H. Rahn, Canvasback(Aythya valisineria) eggs in nests Eds.). Plenum Press,New York. parasitizedby Redheads(A. americana)often DRENT,g. H. 1973.The natural history of incubation. had longer incubationperiods and hatched Pages262-311 in Breedingbiology of birds (D. asynchronouslyas a resultof lesseffective in- S. Farrier, Ed.). National Academy of Sciences, cubationdue to the addition of parasiticeggs Washington,D.C. (but seeMcMaster and Sealy1997). Although a DRENT,R. H. 1975.Incubation. Pages 333420 in Avi- larger samplesize is neededto eliminatealter- an biology (D. S. Farrier,J. R. King, and K. C. Parkes, Eds.). Academic Press,New York. nativeexplanations that may have been respon- DUNNING,J. B. 1993. CRC handbookof avian body sible for the observedtemperature reduction masses. CRC Press, Boca Raton, Florida. (e.g.the useof a thermistorthat inhibitedegg FRIEDMANN,H. 1927.A caseof apparentlyadaptive movement),evidence from my experimentis accelerationof embryonicgrowth-rate in birds. consistentwith the hypothesisthat replace- BiologicalBulletin (Woods Hole) 53:343-345. mentof a hostegg with a largercowbird egg FRIEDMANN,H. 1929.The cowbirds,a study in the reducesthe incubationtemperature of host biologyof socialparasitism. Charles C. Thomas, eggs.This, in turn, may resultin a longerin- Springfield,Illinois. cubationperiod and lowerhatchability of host FRIEDMANN,H., AND L. E KIFF. 1985. The parasitic eggs.Conversely, cowbird eggs that are rela- cowbirdsand their hosts.Proceedings of the tively smallcompared with hosteggs may re- WesternFoundation of VertebrateZoology 2: 225-304. ceiveless heat and experiencelonger incuba- GILL,E B. 1990.Ornithology. Freeman, New York. tion periods. As a result, host-eggsize and GRABBER,g. g. 1955. Artificial incubation of some clutchcomposition may affectthe lengthof a non-galliformeggs. Wilson Bulletin 67:100-109. cowbirdegg's incubation period, which may GROEBBELS,E, AND E MOBERT.1930. Ueber die le- help explainvariation in reportedcowbird in- bensdauervon vogelembryonenund die lebens- cubationperiods under natural conditions. dauerdes kuckucks in ei. OrnithologischeMon- atsberichte 38:89-90. ACKNOWLEDGMENTS HAFTORN,S. 1978.Egg-laying and regulationof egg temperatureduring incubation in the Goldcrest I am gratefulto Mary V. Ashley,Peter E. Lowther, Regulusregulus. Ornis Scandinavica9:2-21. D. Glen McMaster, William Moskoff, and Scott K. HAFTORN,S., AND g. E. REINERTSEN.1985. The effects Robinsonfor their commentsand suggestionsin of temperatureand clutchsize on the energetic manuscriptpreparation. Additionally, I thank The costsof incubationin a free-livingBlue Tit (Par- Morton Arboretum for allowing me accessto their us caeruleus).Auk 102:470-478. site,and David E. Springer,whose assistance helped HAMILTON,W. J., AND G. H. ORIANS.1965. Evolution makethis studypossible. This work was supported of broodparasitism in altricialbirds. Condor 67: in part by a grant from the National ScienceFoun- 361-382. dation (IBN-9601201). HOFFMAN,E. C. 1929. Cowbirds--decoys--incuba- October1998] EmbryoDevelopment in Blackbirds 849

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