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Egg Rejection by Cowbird Hosts in Grasslands

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Egg Rejection by Cowbird Hosts in Grasslands The Auk 117(4):892-901, 2000 EGG REJECTIONBY COWBIRDHOSTS IN GRASSLANDS BRIAND. PEER,TM SCOTT K. ROBINSON,2 AND JAMESR. HERKERT3 1DepartmentofZoology, University ofManitoba, Winnipeg, Manitoba R3T 2N2, Canada; 2IllinoisNatural History Survey, 607 East Peabody Drive, Champaign, Illinois 61820, USA; and 3IllinoisEndangered Species Protection Board, 524 South Second Street, Springfield, Illinois 62701, USA ABSTRACT.--Wetested Field Sparrows(Spizella pusilia), Vesper Sparrows (Pooecetes gra- mineus),Lark Sparrows (Chondestes grammacus), Grasshopper Sparrows (Ammodramus savan- narum),Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna), and Western Meadowlarks(S. neglecta) to determine whether the low level (< 10%)of observed parasitism byBrown-headed Cowbirds (Molothrus ater) on these grassland hosts is a resultof eggre- jection.Western Meadowlarks rejected 78% of artificial and real cowbird eggs, Eastern Mead- owlarksrejected 36% of artificial cowbird eggs, and Dickcissels rejected 11% of artificial cow- birdeggs. None of the other hosts regularly rejected cowbird eggs. Thus, egg rejection may accountfor some,but not all, of thelow levelof observedparasitism on grasslandhosts in theMidwest. Meadowlarks were also tested with nonmimeticeggs, and the remaining hosts weretested with undersizedmimetic and nonmimetic eggs when possible. All hosts,with theexception ofthe Field Sparrow, demonstrated some level of rejection of thenonmimetic eggs.These results suggest that some grassland hosts, which apparently have been in contact withcowbirds the longest, have evolved some form of rejectionbehavior that might have selectedfor mimetic eggs in cowbirds.The intermediate levels of rejectionby both species of meadowlarksalso may indicate that rejection is increasingin thesepopulations. Received 5 April 1998,accepted 20 February2000. BROWN-HEADEDCOWBIRDS (Molothrus ater) Kofordet al. 2000).The samegrassland hosts areknown to haveparasitized at least227 host in the Midwestare parasitizedless frequently species(DeGeus and Best1991, Ortega 1998). than hosts in adjacenthabitats: parasitism is Despitetheir generalistnature, cowbirds ex- highest on forest species,intermediate on hibit considerablegeographic variation in par- shrublandspecies, and loweston grassland asitism. For example,parasitism frequencies species(Strausberger and Ashley 1997; Robin- for commonhosts such as Red-wingedBlack- son et al. 1999, 2000). birds (Agelaiusphoeniceus), Song Sparrows (Me- Grasslandhosts in the Midwest may experi- lospizamelodia), and Wood Thrushes (Hylocichla ence relatively low levelsof parasitismfor sev- mustelina)vary widely in differentregions of eral reasons.Cowbird densities are highestin North America (Freeman et al. 1990, Hoover the Great Plains (Priceet al. 1995),and the andBrittingham 1993, Smith and Smith 1998). Great Plains are dominated by grasslands, Not onlydo parasitismfrequencies vary for in- whereas shrublands and forests are more avail- dividualhosts, they also vary for entirehost as- able in the Midwest; host nestsmay be easier semblages.Most grassland hosts in the mid- for cowbirds to find in these other habitats western United States (hereafter, "the Mid- (Robinsonet al. 1999,2000). Grassland hosts in west"), for example,contain cowbird eggs in the Midwest also may have evolveddefenses lessthan 10% of their nests(Strausberger and againstcowbird parasitism, particularly egg Ashley 1997;Robinson et al. 1999, 2000; see rejection.Rejection could account for thelow also Hahn and Hatfield 1995), whereas grass- frequencyof cowbirdparasitism on grassland land speciesin the GreatPlains often are par- hosts in the Midwest (Rothstein 1975b, Scott asitizedat twicethis frequencyor higher(Her- 1977,Sealy and Neudorf 1995). genrader1962, Newman 1970, Hill 1976,Elliott In this study,we testedthe hypothesisthat 1978, Zimmerman 1983, Davis and Sealy 2000, the relativelylow level of cowbirdparasitism observedon grasslandhosts in the Midwestis 4Present address: Department of Biology,Law- a resultof eggrejection by hosts.Many grass- renceUniversity, Appletown, Wisconsin 54912, USA. landhosts have eggs that closelyresemble cow- E-mail:[email protected] bird eggs(Fig. 1), and recognitionof cowbird 892 October2000] EggRejection by Grassland Hosts 893 eggsby hostswhose own eggsresemble those larksresponded to cowbirdparasitism rather than to of cowbirdsapparently is difficult (Rothstein artificialeggs per se.We usedRed-winged Blackbird 1975b; Burhans and Freeman 1997; see also eggsin addition to real cowbirdeggs because Red- Peerand Sealy2000). Therefore, we alsotested winged Blackbird eggs clearly are nonmimetic, whether thesehosts reject nonmimetic eggs, whereascowbird eggs closely resemble meadowlark eggs(Fig. 1). and in the cases of the smaller hosts, we used Hostsalso were parasitizedwith artificialnonmi- both nonmimetic and mimetic undersized metic eggs(Table 1). Easternand Westernmeadow- eggs. We predicted that nonmimeticeggs larks were parasitizedwith the samewooden eggs shouldbe rejectedat higherfrequencies than describedabove, but they were painted indigo blue. mimetic eggs.We used undersizedeggs to fa- Field Sparrows (Spizellapusilla), Vesper Sparrows cilitateejection by smallerhosts, because many (Pooecetesgramineus), Lark Sparrows (Chondestes small hostsseem to be incapableof grasping grammacus),and GrasshopperSparrows (Ammodra- cowbird eggs in their bills (Rothstein1975b, mussavannarum) were parasitizedwith plasticeggs Sealy 1996). that were filled with white glue and painted indigo blue.Dickcissels (Spiza americana), which lay immac- ulate eggs(Fig. 1), were parasitizedwith the same METHODS plastic eggs but with white or beige backgrounds with spots.It is possiblethat the smallerhosts rec- Studyareas.---We experimentally parasitized nests ognized cowbird eggsbut were unable to remove during the 1997 and 1998breeding seasonst Experi- them. Therefore,when possible,hosts that rejected mentswere conductedat the SavannaArmy Depot the smallernonmimetic eggs also were experimen- (42ø01'N, 90ø02'W), Ayers Sand Prairie Nature Pre- tally parasitizedwith smaller plastic eggs that re- serve (42ø03'N, 90ø06'W), and Thompson-Fulton sembledtheir own eggs.This allowedus to ascertain Sand Prairie Research Natural Area (41ø57'N, whetheracceptance was due to the large sizeof cow- 90ø06'W) in northwestern Illinois, and at the Mide- bird eggs or becausehosts could not distinguish win National TallgrassPrairie in northeasternIlli- cowbirdeggs from their own eggs. nois (41ø03'N,88ø00'W). EasternMeadowlarks (Stur- Eggswere added to nestsduring the host'slaying nella magna)and Western Meadowlarks (S. neglecta) or incubationperiod. The time of parasitismusually nestedat the SavannaArmy Depot;for this reason, hasno effecton host response (Rothstein 1975b; Sea- specieswere distinguishedby call or song.To in- ly 1996;but seeRothstein 1976) if parasitismoccurs creaseour sample sizes, we monitored nests that after at least one host egg is present(Peer and Bol- were naturally parasitizedby cowbirdsat the afore- linger 1997).No hosteggs were removed in conjunc- mentionedstudy sites and at Thompson-FultonSand tion with experimentalparasitism, and nestswere Prairie Nature Preserve(41ø56'N, 90ø06'W) in north- parasitized from 0500 to 1600 CST. Nests were western Illinois, Nachusa Grasslands (41ø08'N, checkedfor evidenceof rejectionevery one to three 89ø04'W) and Green River Conservation Area daysuntil the cowbirdegg was rejected or the host's (41ø06'N, 89ø05'W) in north-central Illinois, and Des eggshatched. Eggs were consideredrejected if they Plaines Conservation Area (41ø03'N, 88ø01'W) and disappearedfrom an active nest (i.e. ejected)or if GooseLake Prairie (41ø03'N,88ø02'W) in northeast- they were damagedby the host(i.e. pecked).Artifi- ern Illinois. Nestswere checked approximately every cial eggswere checkedclosely for peckmarks. The two to four days.We also report parasitismfrequen- plastic eggswere soft and could be compressed,so cies that we recorded for hosts at the above sites from presumablyhosts could inflict noticeable damage; in 1995 to 1998,although not all siteswere monitored all likelihood,these eggs were small enoughto be every year. ejected.Similarly, the woodeneggs were softenough Experimentalparasitism.---Nests were parasitized for Mourning Doves(Zenaida macroura) to make peck with one of severalegg types,with one exception marks(Peer and Bollinger 1998), despite the delicate (seebelow). All hostswere parasitized with artificial bills of this species(Mirarchi and Baskett1994). cowbirdeggs that were made of wood and painted Therefore, we assumed that smaller hosts, with to mimicreal cowbirdeggs. The artificialeggs effec- stronger bills, could inflict noticeabledamage to tively mimicked cowbird eggs;hosts responded in theseeggs as well. At the very least, thesehosts the samemanner to theseeggs as they did to real shouldhave been able to chipthe paint off the eggs. cowbird eggs,and they acceptedartificial wooden We also noted whetherhost eggswere missingor eggsthat mimickedtheir own (Peer1998; Peer and damaged.Eggs that remainedin nestsfor at leastfive Bollinger 1997, 1998). Western Meadowlarks also days were consideredto be acceptedbecause most were experimentallyparasitized with real cowbird rejectersremove cowbird eggs within 24 h and near- eggs that we collectedfrom other host nests,and ly alwayswithin five days (Rothstein1975b, Sealy with Red-wingedBlackbird eggs. These eggs were 1996, Peer 1998). added as controls to ensure that
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