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

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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 cow- birds 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% small- er,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 the cowbird'sincubation 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 in- cubationperiods may result from different in- cubationperiod of Brown-headedCowbird and Red-winged Blackbirdeggs 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. "black- birds") 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 random sample 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' incuba- tion 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 egg massand 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 ob- tained 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
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