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 eggrejection.Western Meadowlarks rejected 78% of artificial and real cowbird eggs, Eastern Mead- owlarksrejected 36% of artificial cowbird eggs, and Dickcissels rejected 11% of artificial cowbirdeggs. 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 hostsin 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 immaculate 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 Western Meadow- Desertionof parasitizednests, including burial of 894 PEER,ROBINSON, AND HERKERT [Auk, Vol. 117

FIG.1. Clutchesof hoststested for egg-rejectionin this study.Host eggsare to the left, and the last egg in eachrow is a cowbirdegg. Clutchesare as followsfrom top to bottom:Grasshopper Sparrow, Vesper Sparrow,Lark Sparrow,Field Sparrow,Dickcissel, Eastern Meadowlark, and WesternMeadowlark.

cowbird eggs, sometimesis consideredrejection be- bird eggs away from their nests(Rothstein 1975b, havior.However, birds desert their nestsfor a variety Peer1998). It is likely that sucheggs were knocked of reasons(Rothstein 1975b, Hill and Sealy 1994), out of nests when females flush from the shallow and egg burial is usually a continuation of nest ground nests typical of grasslandspecies. Indeed, building (Rothstein1975b, Hobson and Sealy 1987; duringthe courseof thisstudy we observedeggs be- but seeSealy 1995). Thus, we did not considerthese ing knockedout of two meadowlarknests when the behaviorsas rejectionsunless they occurredconsis- femalesflushed (see also Peer and Bollinger1998). tently in responseto parasitism. Eggs that were Statistics.--Weused chi-square tests to analyzeegg found just outsidenests also were not consideredre- rejectionand Fisher exact tests when expected values jectionsbecause rejecter species usually carry cow- were lessthan five.Tests were two-tailedexcept for October2000] EggRejection by Grassland Hosts 895

TABLE1. Observed parasitism frequencies(n = RESULTS numberof nests)on grasslandhost speciesin Il- linois, 1995 to 1998, and sizes of real (Baicichand Four of the seven hosts we monitored were Harrison1997) and artificiala eggs involved in this study. parasitizedin lessthan 10%of their nests,and the remainingthree hosts were parasitizedin % 13 to 21% of their nests(Table 1). Parasitism Mean egg Western Meadowlark.--The Western Meadow- Species/eggtype (n) size (mm) lark was the only host that regularly rejected Field Sparrow 13.1 (336) 18 x 13 mimetic and nonmimeticeggs (Table 2). Exper- VesperSparrow 7.7 (52) 21 x 15 imentally added real cowbirdeggs and artifi- Lark Sparrow 20.9 (43) 20 X 16 cial cowbird eggswere rejectedat similar fre- GrasshopperSparrow 4.0 (318) 19 x 14 Dickcissel 13.1 (222) 21 x 16 quencies(Fisher exact test, P > 0.99;Table 2). Eastern Meadowlark 1.8 (221) 28 X 20 Nonmimeticeggs were rejected at a higherfre- Western Meadowlark 2.2 (46) 28 X 21 quencythan mimetic cowbirdeggs (Table2), Unknown Meadowlark spp. 0.0 (15) -- Brown-headed Cowbird -- 21 x 16 but the differencewas not significant(Fisher Artificial cowbird -- 22.4 x 16.5 b exact test, one-tailed, P = 0.17). All rejections Artificial undersized -- 15.4 x 11.8 c for which the time required to reject was Includesboth mimetic and nonmimetic egg types. known (n = 19) occurredwithin 24 h. One host bMean mass = 2.3 -+ 0.04 g (n = 18). egg was missingfollowing an ejectionof an ar- cMean mass = 1.1 _+0.03 g (n = 15). tificial cowbirdegg. Parasitism was acceptedat the one known naturally parasitizednest (Ta- ble 2). A nestat whichan artificialcowbird egg was caseswhere we predicteddifferences in rejectionbehavior, in which case we used one-tailed tests. We ejectedwithin 24 h was subsequentlyexperi- predicteda priorithat nonmimeticeggs would be re- mentallyparasitized with a real cowbirdegg jectedmore frequently than mimeticeggs when egg that was not ejected,and with a Red-winged size was held constant. However, when size differed Blackbird egg that was ejected within 24 h. betweenegg types (e.g. for GrasshopperSparrows, These data are not included in the above anal- Lark Sparrowsand VesperSparrows) we did not pre- yses becausethis nest was parasitizedwith dict a difference,because despite the smaller size morethan oneegg type. that may facilitaterejection, smaller eggs also were Eastern Meadowlark.--Eastern Meadowlarks similar to host egg size,which shouldmake recog- rejected mimetic and nonmimetic cowbird- nition more difficult. Egg measurementsare givenas sized eggs at an intermediatelevel (Table2), •+SE. but the frequencyof rejectiondid not differ be-

TABLE2. Rejectionrates of hostsin responseto naturaland experimental nest parasitism in Illinois.Mimetic eggswere painted to resemblecowbird eggs and nonmimeticeggs were painted solid indigo blue or with white or beigebackgrounds and darkerspots (for use only in Dickcisselnests). Values are percentages, with numberof nestsin parentheses.

Experimental Cowbird-sizedeggs Undersizedeggs Species Natural Mimetic Nonmimetic Mimetic Nonmimetic Field Sparrow 0 (28) 0.0 (3) -- -- 0.0 (4) VesperSparrow 0 (4) 0.0 (4) -- 100 (1) 75.0 (4) Lark Sparrow 0 (5) 0.0 (2)a -- 0.0 (2) 66.7 (3) GrasshopperSparrow 0 (11) 0.0 (10) -- 25.0 (8) 42.9 (14) Dickcissel 0 (15) 11.1 (9) -- -- 100 (3) Eastern Meadowlark 0 (2) 35.7 (14) 40 (10) -- -- Western Meadowlark 0 (1) 77.8 (18)b 100 (9) c -- -- Unknown Meadowlark -- 100 (2) ------Experimentalcowbird parasitism was alsoaccepted at two additionalLark Sparrownests for four daysbefore the nestswere depredated. Includes11 of 14 rejectedartificial cowbird eggs and threeof four rejectedreal cowbirdeggs. Includeseight artificialnonmimetic eggs and one real Red-wingedBlackbird egg. 896 PEER,ROBINSON, AND HERKERT [Auk, Vol. 117 tweenthe two eggtypes (Fisher exact test, one- metic eggs (Table2). A cowbird egg was par- tailed, P > 0.50; Table 2). More nonmimetic tially buried in a naturally parasitized nest. eggs (4 of 4) than mimetic eggs(2 of 5) were Three more parasitizednests had beendesert- rejectedwithin 24 h (Fisherexact test, one- ed when found, and a fourth was desertedby tailed, P = 0.11).Two host eggswere missing the secondvisit with only the cowbirdegg re- following the ejectionof an artificialblue egg mainingin the nest.We did not considerthese from one nest. Cowbird eggswere acceptedat rejections. the two known naturally parasitizednests (Ta- ble 2). DISCUSSION Dickcissel.--Dickcisselsrejected 11.1% of artificialcowbird eggs (Table 2). It is possiblethat Low frequencyof observedparasitism.--Low this ejectionwas a caseof partial predationbe- frequenciesof cowbird parasitism are some- causea host egg was also missingfrom this timesexplained by rejectionof cowbirdeggs by nest. All undersized nonmimetic eggs also hosts. Several rejecter speciesoften remove wererejected (Table 2), and they were rejected cowbirdeggs immediately after they have been more frequentlythan were normal-sizedcow- parasitized,which leads to an underestimateof bird eggs (Fisher exact test, one-tailed,P = actual parasitism(Rothstein 1977, Scott 1977, 0.02). A naturally parasitized nest was found Sealy and Neudorf 1995). Egg rejection by with an emptyhost egg with a holein it, plus Western Meadowlarks, and to a lesser extent two intact host eggs and a real cowbird egg. EasternMeadowlarks, may contributeto the The damagedegg was gone when the nestwas low levels of observedparasitism on these revisited three days later. Cowbird eggs plus hosts in the Midwest. Western Meadowlarks re- hosteggs disappeared from two othernatural- jectedmost cowbird eggs, and all rejectionsoc- ly parasitizednests, but theseappeared to be curredwithin a day.Eastern Meadowlarks also depredated. rejectedcowbird eggs, but lessfrequently, and GrasshopperSparrom--Grasshopper Sparrows only 40% of rejectionsoccurred within a day. acceptedall artificial cowbirdeggs and natural Nests must be inspectedsoon after cowbirds cowbird parasitism(Table 2). Undersizedeggs lay their eggs around sunrise to determine of both types were rejected. All undersized whether eggs are being rejected(Scott 1977). eggscombined were rejectedmore frequently Dickcisselsalso may have rejected cowbird than the normal-sizedcowbird eggs (8/22 vs. eggs,though this may have been partial pre- 0/10, respectively;Fisher exact test, P = 0.04), dation. Even if it was rejection,it is unlikely and all nonmimeticeggs combinedtended to that this low level of rejectionsignificantly af- be rejectedmore frequentlythan all mimetic fectedthe observedfrequency of parasitism. eggs combined (6/14 vs. 2/18, respectively; The parasitismfrequencies we observedap- Fisher exact test, P = 0.10). pear to be very closeto actualparasitism fre- VesperSparrow.--Vesper Sparrows accepted quenciesfor Field Sparrows,Vesper Sparrows, all artificial cowbird eggsand natural cowbird Lark Sparrows,and GrasshopperSparrows be- parasitism(Table 2). All undersizedeggs com- cause all accepted natural and experimental bined were rejectedmore frequentlythan the cowbirdparasitism. Our resultsare supported normal-sized cowbird eggs (4/5 vs. 0/4, re- by findings that VesperSparrows accept arti- spectively;Fisher exact test, P = 0.05), and all ficial cowbird eggs (n = 3; Rothstein1975b) nonmimetic eggs tended to be rejected more and that Field Sparrowsaccept real and artifi- frequentlythan mimeticeggs (3/4 vs. 1/5, re- cial cowbirdeggs (n = 27; Burhans1996). In- spectively;Fisher exact test, P = 0.21). steadof rejectingcowbird eggs, Field Sparrows Lark Sparrow.--LarkSparrows accepted all apparently desertnests in responseto parasit- artificial cowbirdeggs (Table 2) and seemingly ism, especiallywhen they observea female all natural cowbird parasitism.Two of three cowbirdat their nest (Burhans2000). This may undersizednonmimetic eggs were rejected,but explain the naturally parasitized nests we neither of two undersizedmimetic eggs were found that were deserted.Thus, rejectionmay rejected(Table 2). accountfor someof the relativelylow levelsof Field Sparrom--FieldSparrows acceptedall parasitismon grasslandhosts in the Midwest. artificial cowbird eggsand undersizednonmi- In addition,it appearsthat grasslandhabitats October2000] EggRejection by Grassland Hosts 897 are simplyavoided by cowbirds.Similar to the hosts, whereas cowbirds do not (Davies and findings in the Midwest (Strausbergerand Brooke 1998, Fleischer 1985; but see Alderson Ashley1997; Robinson et al. 1999,2000), hosts et al. 1999). Nevertheless,several other host nestingin old-fieldhabitats in New York are speciesthat do not nestin grasslandsostensi- alsoparasitized less frequently than those nest- bly have been in contactwith cowbirdsfor a ing in adjacentforests (Hahn and Hatfield relativelyshort time, but they too have eggs 1995). The reasonsfor the avoidanceof these that resemblethose of cowbirds(e.g. Rose- grasslandhabitats remain unclear, but theveg- breasted Grosbeak [Pheucticusludovicianus], etative structure and secretive behavior of Northern Cardinal [Cardinaliscardinalis], East- hostsin grasslandsmay makeit moredifficult ern Towhee [Pipilo erythropthalmus],Yellow- for cowbirdsto locate these nests compared breastedChat [Icteriavirens]). Chats also reject with thosein shrublandsand forests(Robinson nonmimeticeggs more frequently than mimet- et al. 1999, 2000). ic eggs(Burhans and Freeman 1997). Therefore, Rejectionof nonmimeticand undersizedeggs.- it is possiblethat cowbirdssimply have a gen- WesternMeadowlarks, Vesper Sparrows, Lark eralized egg morph that fortuitouslymatches Sparrows,and GrasshopperSparrows tended eggsof many of its hosts,making recognition to rejectnonmimetic eggs more frequently than of cowbird eggsby thesehosts difficult. We mimetic eggs (see Fraga 1985, Burhansand cannotrule out this possibility.However, egg Freeman 1997). Apparently, nonmimeticeggs colorationhas gonethrough considerable evo- differedenough for thesehosts to recognizethe lution in different speciesof cowbirds(Roth- differencebetween their eggsand the foreign steinand Robinson1998), suggesting that the eggs,as was evidentat the WesternMeadow- Brown-headed Cowbird has evolved a mimetic lark nest that was parasitizedwith three egg egg. types. This meadowlark rejected the Red- We had no controlsfor undersizedeggs. winged Blackbirdegg and an artificialcowbird Consequently,Vesper Sparrows, Lark Spar- egg,but acceptedthe real cowbirdegg. Despite rows, GrasshopperSparrows, and Dickcissels the similarrate of rejectionof real andartificial may have removedundersized eggs because cowbirdeggs (Table 2), the spotson real cow- the eggswere artificial, or becausethey viewed bird eggstended to be somewhatlarger than them as inviable eggsor as empty eggshells thoseon the artificialeggs, and the larger spots owing to their low masses(see Kattan 1998). were very similar to those on meadowlark This seems unlikely. Rothstein (1975b) also eggs.As a result, real cowbird eggs appeared tested four accepterspecies using undersized moresimilar to meadowlarkeggs than to arti- cowbirdeggs and foundno differencesin host ficialeggs, which may explain why thereal egg responsesto theseeggs and normal-sized cow- was not rejected. bird eggs.Accepter species apparently do not The similarity of Brown-headedCowbird rejectrunt eggs,which are inviableand weigh eggsto thoseof mostof the grasslandhosts we lessthan normal eggs (Rothstein 1973), where- tested, in addition to those of many other asat leastone rejecter species rejects these eggs grasslandhosts, is striking (Fig. 1; seeBaicich (Peer 1998). and Harrison 1997).Combining this with our Most birds remove empty eggshellsfrom findingsthat somegrassland hosts tended to their nests (Kemal and Rothstein 1988);how- reject the nonmimeticeggs more frequently ever,relatively few speciesreject cowbird eggs thanmimetic eggs raises the possibility that the (Peer1998). Also, Field Sparrowsdid not reject Brown-headedCowbird evolved a mimeticegg the undersizedeggs (Table 2), and the sameis (see Elliott 1977). Grasslandhosts, including true for ChippingSparrows (Spizella passerina; thosein Illinois, apparentlyhave been in con- B. Peerunpubl. data). Yet, both of thesespecies tact with cowbirdsfor the longestperiod of removeeggshells from their nests(Carey et al. time (Mayfield 1965);hence, they have had the 1994, Middleton 1998).If the birds we tested longest time to evolve antiparasite defenses simplyresponded to the eggsas if they were suchas egg rejection.Subsequently, cowbirds empty eggshells,then we would expectField may haveevolved an eggthat mimicsthose of Sparrowsand ChippingSparrows to havere- its hosts,similar to CommonCuckoos (Cuculus movedthe eggs.They did not, whichis further canorus),although cuckoos parasitize specific evidencethat Vesper Sparrows, Lark Sparrows, 898 PEER,ROBINSON, AND HERKERT [Auk, Vol. 117

GrasshopperSparrows, and Dickcisselsdem- those in the Midwest. This is unusual because onstraterejection behavior. the only cowbirdhosts known to vary in rejec- Why did Dickcissels,Vesper Sparrows, Lark tion behavior are American Robins (Turdus Sparrows, and GrasshopperSparrows rarely migratorius;Briskie et al. 1992) and Brown reject normal-sized artificial cowbird eggs? Thrashers(Toxostoma rufum; Haas and Haas Dickcisselsshould have no difficulty distin- 1998). Eastern and western Warbling Vireos guishingcowbird eggs from their immaculate alsodiffer in rejectionfrequency, but thesetaxa blue eggs;hence, they do not risk makingrec- may be distinct species(Sealy 1996).Meadow- ognition errors. In contrast, eggs of Vesper larks in the northern Great Plains may not re- Sparrows, Lark Sparrows, and Grasshopper jectbecause they mistakenly treat cowbird eggs sparrows resemble cowbird eggs (Fig. 1), so laid early in first nestsas their own, as may be these speciesrisk rejectingtheir own eggs. the case in Brown Thrashers(Haas and Haas GrasshopperSparrow eggs typically are small- 1998).If so,then these examples would not con- er than cowbirdeggs (Table 1), althoughthey stitutegeographic variation in rejection. overlapin sizewith them (Lowther1993, Vick- The historic range of Brown-headedCow- ery 1996). Vesper Sparrow and Lark Sparrow birds includedthe prairiesof Illinoiswhere our eggs are similar in size to cowbird eggs, but studies were conducted as well as the Great their eggsdiffer enoughin maculationpatterns Plains (Mayfield 1965), so we would expect to be distinguishedfrom cowbirdeggs by most similarrejection frequencies by speciesin these humans (Table1, Fig. 1) and presumablyby two areas.Moreover, we would expectEastern these hosts. Meadowlarks and Western Meadowlarks to ex- Anotherfactor may be that larger eggsare hibit similarrejection frequencies, particularly moredifficult to rejectfor theserelatively small at the Savannasite where populationsostensi- hosts. Small hosts often must use puncture bly havebeen in contactwith cowbirdsfor sim- ejectionto removecowbird eggs because their ilar lengthsof time.Possibly, the necessarymu- bills are too small to grasp the eggs.Bills of tationsand recombinants for rejectionbehavior smaller hosts sometimes ricochet off thick- arose earlier in Western Meadowlarks than in shelledcowbird eggs during attempts at punc- EasternMeadowlarks (see Rothstein 1975a, b); ture ejection,which may damagethe hosteggs hence,the higherrejection rate demonstrated (Rohwer and Spaw 1988, Picman 1989).War- by the formerspecies. Testing the rejectionbe- bling Vireos(Vireo gilvus) are the smallesthosts havior of other populationsof EasternMead- known to eject cowbird eggs frequently,and owlarks and Western Meadowlarks is warrant- they do so without incurring significantcosts ed. (Sealy 1996). Dickcissels,Vesper Sparrows, The low-to-intermediatelevels of true eggre- Lark Sparrows, and GrasshopperSparrows jection demonstratedby the meadowlarkspe- haveshorter bills than that of theWarbling Vir- ciesmay representexamples of microevolution. eo (Peer1998), which may be a constraintwhen Very few North Americanhosts exhibit inter- the former speciesattempt to rejectthe larger mediate levels of true rejection (Rothstein cowbirdeggs (Rohwer and Spaw1988). 1975b),perhaps because rejection behavior has Geographicvariation and microevolutionof egg- a high selectivevalue and increasesrapidly un- rejectionbehavior.--The relatively high parasit- til it reachesfixation, thereby transforming a ism recorded for meadowlarks in the northern speciesfrom an accepterto a rejecter(Roth- Great Plains versusthe Midwest implies that stein1975a). Meadowlarks may be undergoing egg rejectionvaries geographically, especially such a transformation,making them unique for WesternMeadowlarks. Parasitism frequen- among cowbirdhosts. Both speciesare para- ciesfor this speciesrange from 43 to 45% in sitized throughoutmost of their ranges and Manitoba, Saskatchewan, and North Dakota raise fewer young in parasitizedversus unpar- (Davis and Sealy2000, Koford et al. 2000,S. Da- asitized nests (Elliott 1978, Davis and Sealy vis pers. comm.) versus 2 to 22% in Illinois, 2000). As a consequence,rejection may be in- Kansas, Minnesota, and Wisconsin (Lanyon creasingand approachingfixation in thesespe- 1957,Hill 1976,Johnson and Temple1990, this cies (Rothstein 1975a), which would be evi- study), suggestingthat northernGreat Plains denceof microevolutionarychange in response birds exhibitlower levels of egg rejectionthan to parasitism(see Soler et al. 1994). Alterna- October 2000] EggRejection by GrasslandHosts 899

., tively, meadowlarksmay be in an evolutionary CAREY, M., D. E. BURHANS, AND D. A. NELSON. 1994. equilibriumwith cowbirds.The similaritybe- Field Sparrow (Spizellapusilla). In The birds of tween meadowlarkand cowbird eggsmay in- North America, no. 103 (A. Poole and E Gill, creasethe likelihood of recognitionerrors that Eds.). Academy of Natural Sciences,Philadel- maymake acceptance of cowbirdeggs the bet- phia, and American Ornithologists' Union, Washington,D.C. ter strategyin theselarger hosts, yet thisseems DAVIES, N. B., AND M. DE L. BROOKE. 1998. Cuckoos unlikely becausesuch errors rarely occurredin versushosts: Experimental evidence for coevo- our study.Further investigation is requiredto lution. Pages59-79 in Parasiticbirds and their resolve these issues. hosts:Studies in coevolution(S. I. Rothsteinand S. K. Robinson,Eds.). Oxford University Press, ACKNOWLEDGMENTS New York. DAVIS,S. K., AND S. G. SEALY.2000. Cowbird para- We thank B. Kleiman and S. Kleiman of Nachusa sitism and nest predation in fragmented grass- Grasslands,the Departmentof Defense,and the U.S. lands of southwesternManitoba. Pages 220-228 Army for permissionto work at the SavannaArmy in Ecology and management of cowbirds and Depot. We also thank B. Speakerfor logisticalsup- their hosts(J. N.M. Smith,T. L. Cook,S. I. Roth- port and S. Davisfor allowingus to citeunpublished stein,S. K. Robinson,and S. G. Sealy,Eds.). Uni- data. K. McKay, P. Enstrom,M. Rickefts,C. Saffer,S. versity of TexasPress,•Austin. Saffer,and D. Shepardprovided assistancein the DEGEUS,D. W., AND L. B. BEST.1991. Brown-headed field. Commentsby D. R. Curson,C. B. Goguen,S. I. Cowbirds parasitizeLoggerhead Shrikes: First Rothstein,S. G. Sealy,and an anonymousreviewer recordsfor family Laniidae. Wilson Bulletin 103: improvedthe manuscript.D. Willard of The Field 504-506. Museum of Natural History and S. G. Sealy of the ELLIOTT,P. E 1977.Adaptive significanceof cowbird University of Manitoba allowed us to photograph egg distribution.Auk 94:590-593. eggs.Funding was providedby the AudubonCoun- ELLIOTT,P. F. 1978.Cowbird parasitismin the Kansas cil of Illinois, Illinois Department of Natural Re- tallgrassprairie. Auk 95:161-167. sources, National Fish and Wildlife Foundation, the FLEISCHER,R. C. 1985. A new techniqueto identify Nature Conservancy,U.S. Fish and Wildlife Service, and the Wildlife Preservation Fund. and assessthe dispersionof eggsof individual brood parasites.Behavioral Ecology and Socio- biology 17:91-99. LITERATURE CITED FRAGA,R. g. 1985. Host parasite interactions be- ALDERSON, G. W., H. L. GIBBS,AND S. G. SEALY. 1999. tween Chalk-browedMockingbirds and Shiny Cowbirds. Pages 829-844 in Neotropical orni- Determiningthe reproductivebehaviour of in- thology (P.A. Buckley,M. S. Foster,E. S. Morton, dividual Brown-headedCowbirds using micro- satellite DNA markers. Animal Behaviour 58: R. S. Ridgely, and E G. Buckley,Eds.). Ornitho- 895-905. logical MonographsNo. 36. FREEMAN, S., D. E GORI, AND S. ROHWER. 1990. Red- BAICICH,P. J., AND C. J. O. HARRISON.1997. A guide to the nests,eggs, and nestlingsof North Amer- winged Blackbirds and Brown-headed Cow- ican birds, 2nd ed. Academic Press, New York. birds: Someaspects of a host-parasiterelation- BRISKIE,J. V., S. G. SEALY, AND K. A. HOBSON. 1992. ship. Condor 92:336-340. Behavioraldefenses against avian brood para- HAAS, C. A., AND K. H. HAAS. 1998. Brood parasit- sitism in sympatric and allopatric populations. ism by Brown-headed Cowbirds on Brown Evolution 46:334-340. Thrashers:Frequency and rates of rejection. BURHANS,D. E. 1996.Anti-brood parasitestrategies Condor 100:535-540. and nest-siteselection by forest-edgesongbirds HAHN, D.C., AND J. S. HATFIELD. 1995. Parasitism at in Central Missouri. Ph.D. dissertation, Univer- the landscapescale: Cowbirds prefer forests. sity of Missouri, Columbia. ConservationBiology 9:1415-1424. BURHANS,D. E. 2000. Morning nest arrivals in cow- HERGENRADER,G. L. 1962.The incidenceof nestpar- bird hosts:Their role in aggression,cowbird rec- asitismby the Brown-headedCowbird (Moloth- ognitionand hostresponse to parasitism.Pages rus ater) on roadsidenesting birds in Nebraska. 161-168 in Ecology and managementof cow- Auk 79:85-88. birds and their hosts(J. N.M. Smith, T. L. Cook, HILL, D. P., AND S. G. SEALY. 1994. Desertion of nests S. I. Rothstein,S. K. Robinson,and S. G. Sealy, parasitized by cowbirds: Have Clay-coloured Eds.). University of TexasPress, Austin. Sparrowsevolved an anti-parasitedefence? An- BURHANS, D. E., AND P. C. FREEMAN. 1997. Partial re- imal Behaviour 48:1063-1070. jection of immaculateforeign eggsby Yellow- HILL, R. A. 1976. Host-parasiterelationships of the breasted Chats. Auk 114:503-506. Brown-headedCowbird in a prairie habitat of 900 PEER,ROBINSON, AND HERKERT [Auk,Vol. 117

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