Proximate Mechanisms of Parasite Egg Rejection by Northern Mockingbirds

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Proximate Mechanisms of Parasite Egg Rejection by Northern MockingbirdsAuthor(s): John Quinn and Kim Marie TolsonSource: The Wilson Journal of Ornithology, 121(1):180-183. 2009.Published By: The Wilson Ornithological SocietyDOI: http://dx.doi.org/10.1676/08-015.1URL: http://www.bioone.org/doi/full/10.1676/08-015.1

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The Wilson Journal of Ornithology 121(1):171–173, 2009

Accidental Egg Removal by Incubating Piping Plovers

Corie L. White,1,3,4 R. Mark Brigham,1 and Stephen K. Davis1,2

ABSTRACT.—Clutch reduction (the disappearanceof 1 or more eggs) is often reported in studies exam-ining avian reproductive success and has typicallybeen attributed to nest predation. We recorded clutchreductions at 20 (11%) of 188 Piping Plover (Char-adrius melodus) nests at Chaplin Lake, Saskatchewanfrom 2002 to 2004. Partial clutch reductions were ini-tially assumed to be the result of predation. However,all egg disappearances at three nests we monitored us-ing video cameras were due to accidental removal byincubating parents. Our observations suggest that ac-cidental removal may occur more frequently than ex-pected in alkaline environments, and are likely mis-classified as losses due to predation. Received 10 De-cember 2007. Accepted 1 July 2008.

Estimating demographic rates and identi-fying factors that influence these rates are es-sential for understanding population ecology(Stearns 1992, Miller and Knopf 1993, Lukaset al. 2004), and effectively managing wildlifespecies (Sarno et al. 1999). A large proportionof applied ornithological research has focusedon estimating reproductive success, in partic-ular nest survival (Beauchamp et al. 1996,Knetter et al. 2002, Davis 2003). Critical tothis understanding is the correct classificationof nest fate. Unfortunately, logistical con-straints often influence nest visitation sched-ules and make accurate assessment of nest fatedifficult (Pietz and Granfors 2000). For ex-ample, reduction in clutch size between nestvisits is typically attributed to predation, butcould result from brood parasitism (Payne1977), accidental breakage by the nest ownersthemselves, or by abiotic factors such asstrong winds and heavy rains (Sealy 1994).

1 Department of Biology, University of Regina, Re-gina, SK S4S 0A2, Canada.

2 Canadian Wildlife Service, Environment CanadaPrairie and Northern Region, Regina, SK S4P 4K1,Canada.

3 Current address: Saskatchewan Watershed Author-ity, 420-2365 Albert Street, Regina, SK S4P 4K1, Can-ada.

4 Corresponding author; e-mail: [email protected]

Video monitoring has become a relativelycommon means of studying nesting behaviorand allows an accurate assessment of nest fate(Pietz and Granfors 2000, Sanders and Ma-loney 2002, Williams and Wood 2002). Wedocumented the accidental removal of eggs byincubating Piping Plovers (Charadrius melo-dus) breeding at Chaplin Lake, Saskatchewan(50� 26� N, 106� 40� W) during a study whichused video monitoring of nests (White 2005).

OBSERVATIONS

Chaplin Lake is a large saline lake covering11,777 ha. The basin is composed of 10 in-terconnected pools used for harvesting brineshrimp (Artemia franciscana) and extractionof sodium sulfate. This lake supports a largenesting population of Piping Plovers with upto 23% of the Saskatchewan population in agiven year. Nest searches and monitoring wereconducted from early May until mid Augustin 2002–2004 following Murphy et al. (1999).Nests were checked every 3–5 days to assesshatching success or failure. Four video sys-tems were used during each of the threebreeding seasons to monitor nests (n � 24 to-tal). Small (29 mm diameter, 74 mm long) re-mote color/infrared cameras (National Elec-tronics Bullet C/IR, B&E Electronics, Regina,SK, Canada) were hidden in artificial ‘‘rocks’’constructed from hollow wood and placed atrandomly selected nests around the lake. Thecameras provided color video images duringthe day and black and white footage at night.Video was continuously recorded (24 hrs) ona time-lapse videocassette recorder (VCR,Sanyo Real Time SRT 2400DC, or SanyoReal Time 4040AC). We reviewed all videotape records from nests at which egg losseswere recorded during nest visits.

Clutch size reduction occurred at 10–17%of monitored nests each year (n � 188 nests,Table 1). Review of the video (1,946 hrs) atthe 3 nests (1 in each year), where video cam-eras were used and which experienced clutch

172 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 121, No. 1, March 2009

TABLE 1. Partial clutch reductions recorded at Piping Plover nests at Chaplin Lake, Saskatchewan, 2002–2004.

2002 2003 2004 Totals

Nests located 36 41 75 188Video-monitored nests 11 8 5 24Nests with partial clutch reduction (%) 5 (14) 7 (17) 8 (10) 20 (11)Eggs monitored 136 145 277 558Missing eggs 8 11 15 34Eggs lost to known partial predation 0 0 0 0Eggs lost to known accidental egg removal (%) 1 (12) 1 (9) 1 (7) 3 (8)

reduction, revealed in each case that an eggwas accidentally removed by an incubatingparent. None of the missing eggs was the re-sult of predation (Table 1). In each instance,the egg adhered to the abdominal feathers ofthe incubating adult and was carried by theadult as it left the nest.

Accidental egg removal accounted for theonly partial clutch reductions documented onvideo. These removals, without the video rec-ord, would have been attributed to predators.In two of the three events, the adult searchedfor the egg within the scope of the video cam-era and either incubated the egg where it de-tached from the feathers or attempted to rollthe egg back into the nest bowl. The egg thatwas incubated where it fell hatched. The otheregg was rolled into the nest by one or bothtending adults over the course of 26 hrs. Thislatter egg was damaged and failed to hatch. Inthe third case, the adults did not search for theegg within the field of view of the camera andthe fate of the egg is unknown. Additionally,at the same study site in 2001, one egg fromeach of three plover nests was found outsidebut near nests (�3.0 m), consistent with ac-cidental removal.

DISCUSSION

Reports of accidental egg removal havealso been reported for Black Stilts (Himanto-pus novaezelandiae), Black-fronted Terns(Sterna albostriata), and Double-banded Dot-terels (Charadrius bicinctus) (Sanders andMaloney 2002), all of which nest in marinehabitats. One explanation for eggs sticking tothe plumage is that high salinity leads to theegg becoming encrusted in salt, which adheresto the feathers. Chaplin Lake is highly salineand a number of plover eggs have been en-crusted in salt, particularly after heavy rains

(CLW, pers. obs.). An additional reason foregg adherence to feathers may be due topipped eggs that catch on the feathers of theincubating adult, although only one of thethree eggs removed by parents in our studywas close to hatching.

Removal of a single egg from a nest doesnot represent reproductive failure for a breed-ing pair, but an egg failing to hatch representsan energetic cost to the bird that laid it as wellas to the individuals that participated in in-cubation (Koenig 1982, Clutton-Brock 1991).Partial clutch reduction was recorded at 10–17% of nests in any given year at ChaplinLake. Although it is unknown whether acci-dental removal by incubating parents was thecause at all nests, our results suggest that re-searchers should exercise caution when attri-buting partial clutch loss to predators, partic-ularly on highly alkaline lakes. Misidentifi-cation of sources of reproductive loss may re-sult in misdirected management efforts forthis endangered species. Our observationsprovide additional examples of the uncertaintyassociated with inferring the fate of eggs whendirect evidence of nest fate is unavailable.

ACKNOWLEDGMENTS

Funding for our research was provided by the Sas-katchewan Watershed Authority (formerly Saskatche-wan Wetland Conservation Corporation), CanadianWildlife Service, Saskatchewan Environment, Sas-katchewan Minerals, Regina Natural History Society,and the University of Regina. We thank Robert Mur-phy and an anonymous reviewer for valuable com-ments on this manuscript. We thank Tony Lau, LowellStraus, Ken Kessler, Katherine Brewster, Susan Rever,Nick Simon, and Trent Linford for their work in thefield. We also thank Gladys and Bob Weppler, NolanMathies, Clem Millar, the Chaplin Nature Centre staff,and the communities of Chaplin and Morse for theirsupport and hospitality.


LITERATURE CITED

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CLUTTON-BROCK, T. H. 1991. The evolution of parentalcare. Princeton University Press, Princeton, NewJersey, USA.

DAVIS, S. K. 2003. Nesting ecology of mixed-grassprairie songbirds in southern Saskatchewan. Wil-son Bulletin 115:119–130.

KNETTER, J. M., R. S. LUTZ, J. R. CARY, AND R. K.MURPHY. 2002. A multi-scale investigation of Pip-ing Plover productivity on Great Plains alkalilakes, 1994–2000. Wildlife Society Bulletin 30:683–694.

KOENIG, W. D. 1982. Ecological and social factors af-fecting hatchability of eggs. Auk 99:526–236.

LUKAS, P. M., V. J. DREITZ, F. L. KNOPF, AND K. P.BURNHAM. 2004. Estimating survival probabilitiesof unmarked dependant young when detection isimperfect. Condor 106:926–931.

MILLER, B. J. AND F. L. KNOPF. 1993. Growth and sur-vival of Mountain Plovers. Journal of Field Or-nithology 64:500–506.

MURPHY, R. K., B. G. ROOT, P. M. MAYER, J. P. GOOS-SEN, AND K. A. SMITH. 1999. A draft protocol forassessing Piping Plover reproductive success onGreat Plains alkali lakes. Pages 90–107 in Pro-ceedings, Piping Plover and Least Terns of theGreat Plains and nearby (K. F. Higgins, M. R.Brashier, and C. D. Kruse, Editors). South DakotaState University, Brookings, USA.

PAYNE, R. B. 1977. The ecology of brood parasitismin birds. Annual Review of Ecology and System-atics 8:1–28.

PIETZ, P. J. AND D. A. GRANFORS. 2000. Identifyingpredators and fates of grassland passerine nestsusing miniature video cameras. Journal of WildlifeManagement 64:71–87.

SANDERS, M. D. AND R. F. MALONEY. 2002. Causes ofmortality at nests of ground nesting birds in theUpper Waiteaki Basin, South Island, New Zea-land: a 5-year video study. Biological Conserva-tion 106:225–236.

SARNO, R. J., W. R. CLARK, M. S. BANK, W. S. PREXL,M. J. BEHL, W. E. JOHNSON, AND W. L. FRANKLIN.1999. Juvenile guanaco survival: management andconservation implications. Journal of AppliedEcology 36:937–945.

SEALY, S. G. 1994. Observed acts of egg destruction,egg removal and predation on nests of passerinebirds at Delta Marsh, Manitoba. Canadian FieldNaturalist 108:41–51.

STEARNS, S. C. 1992. The evolution of life histories.Oxford University Press, Oxford, United King-dom.

WHITE, C. L. 2005. Reproductive ecology and parentalcare strategies of Piping Plover (Charadrius mel-odus) breeding at Chaplin Lake, Saskatchewan.Thesis. University of Regina, Regina, Saskatche-wan, Canada.

WILLIAMS, G. E. AND P. B. WOOD. 2002. Are traditionalmethods of determining nest predators and nestfates reliable? An experiment with Wood Thrush-es (Hylocichla mustelina) using miniature videocameras. Auk 119:1126–1132.


Gray Jays Accept Brown-headed Cowbird Eggs

Spencer G. Sealy,1,4 Brian D. Peer,2 and Dan Strickland3

ABSTRACT.—Results of simulated brood parasit-ism on five Gray Jay (Perisoreus canadensis) nestssuggest acceptance of model Brown-headed Cowbird(Molothrus ater) eggs. This finding is contrary to re-sults of experimental parasitism on four other speciesof jays, also with little or no recent history of parasit-ism, which eject cowbird eggs. Given that Gray Jays

1 Department of Biological Sciences, University ofManitoba, Winnipeg, MB R3T 2N2, Canada.

2 Department of Biological Sciences, Western Illi-nois University, Macomb, IL 61455, USA.

3 RR #1, Oxtongue Lake Road, Dwight, ON P0A1H0, Canada.

4 Corresponding author; e-mail:[email protected]

nest in the boreal forest and earlier in the season thancowbirds initiate breeding, it may be that neither GrayJays nor their congeners have been parasitized regu-larly in their evolutionary histories, which may explainacceptance of cowbird parasitism. Received 22 Janu-ary 2008. Accepted 15 July 2008.

Gray Jays (Perisoreus canadensis) initiateclutches in late winter in boreal and sub-alpineforests of North America (Strickland andOuellet 1993) and are largely isolated frombrood parasitism by the Brown-headed Cow-bird (Molothrus ater), both spatially and tem-porally. In Ontario, where Gray Jays have


been studied most extensively, the mean dateof first eggs was 26 March (�9.4 days [SD])(Strickland and Ouellet 1993), whereas theearliest laying date for cowbirds there is 17April (Peck and James 1987). Not surprising-ly, no one has reported cowbird parasitism ofnests of Gray Jays (Friedmann and Kiff 1985,Strickland and Ouellet 1993, Ortega 1998).Gray Jays should be expected to accept for-eign eggs, if parasitism occurred naturally orexperimentally, because there has not been se-lective pressure for evolution of the ability todiscriminate and reject foreign eggs (Briskieet al. 1992, Peer et al. 2007).

We tested this hypothesis by introducing anon-mimetic model cowbird egg into GrayJay nests with the knowledge that all other jayspecies and most other corvids that have beentested in North America, and which also havelittle or no recent history of brood parasitism,reject eggs. Thus, this behavior appears to bedeeply rooted in this group and has been re-tained in the absence of brood parasitism(Bolen et al. 2000, Rothstein 2001, Fleischerand Woolfenden 2004, Underwood et al.2004, Peer et al. 2007). Gray Jays belong tothe Old World clade of corvids (Ericson et al.2005) and are closely related to New Worldjays (Bonaccorso and Peterson 2007). Rejec-tion of foreign eggs by Gray Jays would likelybe a result of past parasitism by Old Worldcuckoos (Cuculus spp.), as has been suggestedby Bolen et al. (2000) for Black-billed Magpie(Pica hudsonia) and Yellow-billed Magpie (P.nuttalli). This paper reports the results of test-ing whether Gray Jays are an accepter or re-jecter of foreign eggs.

METHODS

DS initiated a study of the population bi-ology of the Gray Jay �30 years ago (Rutter1969, Strickland and Ouellet 1993) in Algon-quin Park, Ontario, adjacent to ProvincialHighway 60 where it crosses the southwestcorner of the Park (45� 33� N, 78� 33� W). Thestudy site has been described elsewhere (Rut-ter 1969, Strickland and Ouellet 1993). DSconducted the experiment in March and earlyApril 1999, and in April 2000 under condi-tions of thick snow and low temperatures. Onemodel cowbird egg made of plaster-of-Parispainted with acrylic paint to match real cow-bird eggs (Rothstein 1975), warmed by hand

or in a vehicle, was introduced into accessiblenests from which Gray Jay females flushedfrom eggs. No jay egg was removed from anyclutch when the model was introduced. Eachnest was inspected the next day or as soon asconditions permitted. A model egg was con-sidered accepted if it and the jay’s eggs wereincubated at least 5 days, whereas rejectionwas recorded if the model disappeared and thejays continued to incubate (Rothstein 1975).All adults except the female tested in 2000were color-banded and there is no possibilitythe same female was tested twice. All trialswere restricted to exceptionally low nests thatcould be reached from the ground or with asmall ladder to avoid disturbance that never-theless occurred at nest #2.

Gray Jay eggs are larger than Brown-head-ed Cowbird eggs and have a different basecolor and pattern (Lowther 1993, Stricklandand Ouellet 1993). Mean (� SD) length andwidth of 16 Gray Jay eggs from AlgonquinPark were 28.30 � 1.6 � 20.5 � 1.0 mm,respectively, versus 21.5 � 1.10 � 16.5 �0.61 mm for 127 Brown-headed Cowbird (M.a. ater) eggs. Gray Jay eggs are pale greenishwhite, or gray, finely covered with irregulardots or flecks that range from dark olive torusty, whereas the model cowbird eggs werewhite or grayish white and heavily spottedwith brown or gray, usually slightly moredensely at the large end.

RESULTS

Five Gray Jay nests were tested, nests #1–3 in 1999 and nests #4 and #5 in 2000.

Nest #1 (Mud Creek Territory): under con-struction on 23 February, the nest waschecked on 7 March and appeared completebut was empty. On 16 March the femaleflushed from three eggs, the model was intro-duced, and about 5 min later, the female re-turned to the nest, paused on the rim, lookedbriefly as usual into the nest, then hopped onand snuggled down on the eggs. Five minuteslater, the female stood up and put her headinto the cup, then sat back down on the eggs.The next day, the female flushed from threejay eggs plus the model, as she did also on 23and 31 March. The model egg was removedon 12 April when the three nestlings 10 daysof age were banded. Backdating revealed the


third (last) egg had been laid on 15 March,the day before the model had been introduced.

Nest #2 (Sim’s Pit Territory): the modelwas introduced on 23 March when the nestcontained one jay egg. A shower of snow wasaccidentally dislodged from upper branches ofthe nest tree when DS leaned the ladderagainst it; some snow fell into the nest andhad to be scooped out by hand before themodel was added. The female returned im-mediately to the nest and sat on the eggs. Thenext day the incubating female flushed fromtwo jay eggs plus the model, but by 28 Marchthe intact nest was empty likely due to pre-dation. Two days later the pair was building areplacement nest in which the female even-tually laid four eggs that produced threeyoung.

Nest #3 (East Beach Territory): found on 9March, the nest was checked on 13 March andagain on 20 March when the female was in-cubating three eggs. The nest contained twoeggs on 29 March when the model was added.Two jay eggs and the model were in the neston 30 March and 3 April. On 8 and 15 April,the nest contained the model and one nestling(banded on the latter visit).

Nest #4 (Whitefish Territory): this pair wasone of several in 2000 that apparently hadbarely enough fat reserves to support nestingand, which consequently started late andfailed early. The nest was found on 17 Marchbut no egg was detected until 3 April whenthe female was sitting on one egg and themodel was introduced. The next day, therewas still only one jay egg plus the model. On8 April the nest contained two jay eggs andthe model; the jays were nearby but showedno interest in the nest.

Nest #5 (Bell Telephone Territory): this nestcontained three eggs when the model was add-ed late in the incubation period, on 15 April2000. The next day, two young were drapedover the model egg and one jay egg was vis-ible. On 22 April, three begging jays obscuredthe model, but when the nest was visited on28 April to band the young, only the modelwas present.

Acceptances were recorded at nests #1 and#3 with models added during the jays’ incu-bation periods. Results at the other three nestssuggest acceptance but disruptions at two andhatching of jays at one leave the results only

suggestive. At nest #2, the model was accept-ed for 24 hrs after parasitism during laying,but when inspected on the fifth day, the nestwas empty and abandoned.

DISCUSSION

The results suggest that Gray Jays acceptmodel cowbird eggs, although a larger samplemight reveal a low level of ejection. Tests attwo nests resulted in acceptance and cowbirdegg models remained at least 24 hrs in twoother active nests, which suggest acceptancebecause ejection likely would have occurredon the first day of ‘‘parasitism.’’ One-dayejection is characteristic of most ejecter spe-cies and all cowbird eggs are usually eitherrejected or accepted (Peer and Sealy 2004,Sealy and Underwood 2004; but see Peer andSealy 2000). Nest #4 was deserted but birdsdesert nests for a variety of reasons and with-out carefully controlled experiments we donot know whether desertion was a response tothe parasitic egg rather than a response tosome other disturbance (Rothstein 1975)

Four other species of jay in North America,which experience little or no current broodparasitism, rejected most experimental eggswithin 24 hours (Rothstein 1975, Fleischerand Woolfenden 2004, Peer et al. 2007);hence, acceptance by Gray Jays is exception-al. Observations of natural cowbird parasitismby the Bronzed Cowbird (Molothrus aeneus),including multiple parasitism, of the GreenJay (Cyanocorax yncas; Collins et al. 1980,Carter 1986) suggest acceptance but experi-ments like those we conducted are requiredfor confirmation (Peer et al. 2000, Sealy andUnderwood 2004). Non-mimetic eggs, similarto those of the Brown-headed Cowbird,should be used because Bronzed Cowbirdeggs are immaculate, pale blue (Peer and Sea-ly 2004) and Green Jay eggs are pale whiteor green with some spots at the larger end(Gayou 1995); Bronzed Cowbird eggs may bedifficult to recognize.

Rejection of cowbird eggs or models hasbeen recorded experimentally in the Blue Jay(Cyanocitta cristata) (Rothstein 1975; S. G.Sealy, unpubl. data), Florida Scrub-Jay(Aphelocoma coerulescens) (Fleischer andWoolfenden 2004), Island Scrub-Jay (A. in-sularis), and Western Scrub-Jay (A. califor-nica) (Peer et al. 2007). In other North Amer-


ican corvids, Black-billed Magpies and Yel-low-billed Magpies eject nonmimetic eggs(Bolen et al. 2000, Underwood et al. 2004),whereas the American Crow (Corvus bra-chyrhynchos) accepts them (Underwood et al.2004). Eurasian Magpies (Pica pica) rejectnon-mimetic eggs in the absence of cuckooparasitism (Soler et al. 1999) but other OldWorld corvids that have been tested acceptnon-mimetic eggs (Red-billed Chough [Pyr-rhocorax pyrrhocorax], Western Jackdaw[Corvus monedula], Carrion Crow [C. coro-ne], and Northern Raven [C. corax]) (Yom-Tov 1976, Soler 1990).

Rejection likely has evolved more thanonce in corvids because species of magpie(Pica) and jay (Aphelocoma, Cyanocitta),which reject cowbird eggs, are in separateclades (Ericson et al. 2005) and the formeralso contain accepter species (AmericanCrow, Red-billed Chough, Western Jackdaw,Carrion Crow, Northern Raven). Alternative-ly, rejection could have evolved once but waslost in some species. Rejection may be lostthrough random genetic drift (Peer et al. 2007)or, if a host has a high level of intraclutch eggvariation, it may eject its own oddly coloredeggs. Thus, selection may act directly againstrejection (Rothstein 2001, Peer and Sealy2004, Peer et al. 2007). Most corvids, but ap-parently not Gray Jays, demonstrate intra- andinterclutch egg variation (Strickland and Oul-let 1993, Verbeek and Caffrey 2002, Peer etal. 2007), and this may have affected retentionof rejection. The core distribution of jays ofthe genus Perisoreus is in the Old World,which suggests these species originated thereand then colonized the New World (Ericsonet al. 2005). Unlike other Old World speciesthat have apparently retained rejection fromOld World cuckoo parasitism (Bolen et al.2000, Rothstein 2001, Underwood et al.2004), this does not appear to be the case forthe Gray Jay. The Gray Jay and the other spe-cies of Perisoreus, Siberian Jay (P. infaustus)and Sichuan Jay (P. internigrans), also are ap-parently not regularly parasitized by cuckoos(Johnsgard 1997, Payne 2005). Both nest ear-ly and cuckoo parasitism has not been record-ed in either species (Blomgren 1971, Lindgren1975, Ekman et al. 1994; Jing Yu, pers.comm.). This is not to suggest that rejectionhas been lost in the absence of parasitism,

rather it may be more likely that Gray Jaysand their congeners were not parasitized intheir evolutionary histories, given their pre-ferred breeding habitat, early onset of nestingseasons, and that rejection evolved later inother species in the Corvidae. More speciesneed to be tested for acceptance or rejectionto understand better the evolution of this be-havior in Corvidae, especially species of theNew World jay clade, all of which tested sofar are rejecters.

ACKNOWLEDGMENTS

We thank S. I. Rothstein and an anonymous review-er for comments on the manuscript. Funds for ongoingstudies of the interactions between brood parasites andtheir hosts have been provided by Discovery Grantsfrom the Natural Sciences and Engineering ResearchCouncil of Canada to SGS.

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UNDERWOOD, T. J., S. G. SEALY, AND C. M. MCLAREN.2004. Experiments on egg discrimination in twoNorth American corvids: further evidence for re-tention of egg ejection. Canadian Journal of Zo-ology 82:1399–1407.

VERBEEK, N. A. M. AND C. CAFFREY. 2002. AmericanCrow (Corvus brachyrhynchos). The birds ofNorth America. Number 647.

YOM-TOV, Y. 1976. Recognition of eggs and young bythe Carrion Crow (Corvus corone). Behaviour 59:247–251.


Repeated Brown-headed Cowbird Parasitism of an Artificial Nest

Bryan R. Coppedge1

ABSTRACT.—On 27 May 2007 I discovered twoBrown-headed Cowbird (Molothrus ater) eggs in anartificial nest in a wooded fence line on the TallgrassPrairie Preserve, Osage County, Oklahoma. The arti-ficial nest was one of 20 placed along a transect 10days earlier and baited with two infertile Blue Quail(Coturnix adansonii) eggs during a study on nest de-tection and predation. Brown-headed Cowbird parasit-ism of artificial open-cup nests in the absence of hostnesting activity is rare, but has been reported. Thisoccurrence of superparasitism represents the first rec-

1 Science and Mathematics Division, Tulsa Com-munity College, Tulsa, OK 74107, USA; e-mail:[email protected]

ord wherein more than one cowbird egg was depositedin an artificial nest, indicating that either a single fe-male repeatedly parasitized the nest, or that a secondfemale contributed an egg to the previously parasitizednest. Received 28 January 2008. Accepted 2 June2008.

Brown-headed Cowbirds (Molothrus ater)are obligate brood parasites known to lay eggsin nests of �220 host species (Friedmann andKiff 1985). Cowbird reproductive success isdependent on the females’ ability to not onlyfind suitable host nests in the proper stage of


clutch development, but also on the tendencyof the host to not reject cowbird eggs (Peer etal. 2000). Host activity and behavior are im-portant cues in the nest parasitism process, al-though nests can be located and parasitized inthe absence of adult hosts (Thompson andGottfried 1981).

Some researchers have used artificial neststo study mechanisms by which cowbirds findand select host nests. These studies have gen-erally used one of two methodologies; eitherall eggs in the clutch are added to the nestsimultaneously, or eggs are added sequentiallyone per day to mimic the hosts’ normal egg-laying pattern. No cowbird parasitism hasbeen observed in studies where clutches wereadded simultaneously (Thompson and Gott-fried 1976, 1981). Studies in which host eggswere added sequentially have reported threeinstances of parasitism where a single cowbirdegg was deposited in the nest (Lowther 1979,Thompson and Gottfried 1981). Some studiesusing artificial nests to investigate predation,rather than parasitism, have also reported in-cidents of cowbird parasitism. Jobin and Pic-man (1994) reported parasitism of an artificialnest in marsh habitat that had been baited witha single King Quail (Coturnix chinensis) egg.Thorington et al. (2007) reported parasitism ofan artificial nest in suburban habitat in Floridathat had been baited with a Japanese Quail(Coturnix japonica) egg and a tethered clay‘‘sham’’ egg. In both instances, a single cow-bird egg was found in the artificial nest, whichlacked any of the cues used by cowbirds tolocate suitable hosts (adult activity, sequentialegg-laying, or vocalizations) (Clotfelter1998). Thus, cowbird parasitism of artificialnests is quite rare with only five reported cas-es from �300 studies using artificial nestmethodology (Moore and Robinson 2004). Ireport the first instance of parasitism in whichtwo cowbird eggs were deposited in an arti-ficial nest lacking simulated host nesting ac-tivity.

OBSERVATIONS

I began an experimental nest study in April2007 to examine nest detection and predationat the Tallgrass Prairie Preserve (TGPP) inOsage County, Oklahoma (36� 50� N, 96� 25�W). This 15,500-ha prairie site is owned andmanaged by The Nature Conservancy and

dominated by perennial tallgrass species in-cluding big bluestem (Andropogon gerardii),switchgrass (Panicum virgatum), indiangrass(Sorghastrum nutans), little bluestem (Schi-zachryriam scoparium), and tall dropseed(Sporobolus asper). Woody vegetation is lim-ited by topoedaphic factors and fire withdraws dominated by Crosstimbers forestscomprised of blackjack oak (Quercus mari-landica), post oak (Q. stellata), and blackhickory (Carya texana). Scattered areas withcottonwood (Populus deltoides), black willow(Salix nigra), and American elm (Ulmusamericana) occurred along larger drainages.

I set out a replicate of 20 artificial nestsalong a wooded fence line on 18 May 2007that dissected a lightly wooded riparian area.The artificial nests were constructed of grassstems and leaves woven into a cup shape.Nests had been aired outdoors immersed inherbaceous material collected on site for 4weeks prior to deployment to minimize anyunusual odors that might attract potentialpredators (Whelan et al. 1994). Nests wereplaced 30–50 cm aboveground in shrubs andvines to simulate the natural nests of locallyabundant species including Red-wingedBlackbird (Agelaius phoeniceus) and Dickcis-sel (Spiza americana). Each nest was attachedwith a wire to a supporting branch to preventwind dislodgment, baited with two fresh in-fertile Blue Quail (Coturnix adansonii) eggs,and checked every fifth day for a 15-day pe-riod.

I checked a nest 45 cm above ground in abuckbrush (Symphoricarpos orbiculatus)shrub at 0800 hrs CDT on 27 May 2007 anddiscovered that, in addition to the two quaileggs, the nest also contained two Brown-head-ed Cowbird eggs (Fig. 1) deposited sometimeafter 22 May 2007 (the previous nest check).The eggs and nest were collected to avoidpossible depredation of the nest contents,measured and weighed the following day. Thenest cup diameter was 7.5 cm, 3.3 cm deepwith an overall diameter of 10.7 cm. The cow-bird eggs were smaller (21.4 � 15.8 and 20.5� 15.9 mm, respectively) than the quail eggs(24.3 � 19.4 and 24.3 � 19.7 mm). The cow-bird eggs weighed 2.94 and 2.81 g, roughly athird less than the quail eggs (4.49 and 4.54g). The cowbird eggs’ base color was creamywhite with dark brown speckling concentrated


FIG. 1. Artificial nest collected on 27 May 2007 in Osage County, Oklahoma containing two Brown-headedCowbird eggs (upper pair of speckled eggs) and two Blue Quail eggs.

at the larger end. The quail eggs were solidtan with faint brown speckling (Fig. 1).

DISCUSSION

The cowbird eggs I discovered are withinthe size and weight range of eggs producedby captive female cowbirds on subsequentdays (Ankney and Johnson 1985). Further-more, variability in egg weight from a singlefemale is similar to that of eggs collected fromdifferent females (Ankney and Johnson 1985).Thus, egg size and weight cannot be used re-liably to distinguish between female cowbirds.Baicich and Harrison (1997:316) state that‘‘Usually only one egg is laid in a nest, butseveral eggs may be laid by different cowbirdfemales.’’ In their study of female cowbirdhost use strategies, Strausberger and Ashley(2005) reported that a single female cowbirdlaid two eggs each in two Field Sparrow (Spi-zella pusilla) nests. Other studies report fre-quent cowbird superparasitism, wherein mul-tiple females lay eggs in previously parasit-ized natural nests (Strausberger and Ashley2003, Ellison et al. 2006). Based on egg size,shape, and coloration, and without direct ob-servation of the event, I cannot distinguishwhether this superparasitism event arose froma single female cowbird repeatedly parasitiz-

ing the nest, or from a second female contrib-uting an egg to the previously parasitized nest.

This incident of brood parasitism is consis-tent with previous studies in this grassland re-gion reporting that nests close to woodedfences and roadsides experience high rates ofcowbird parasitism (Patten et al. 2006). Fe-male cowbirds often use elevated perches toobserve the behavior of nearby breeding birdsto find suitable hosts (Freeman et al. 1990). Idid not quantify nest concealment, but the nestwas not easily visible to the eye and, in ad-dition to the buckbrush shrub in which thenest was located, several blackberry (Rubusoklahomus) vines and western ironweed (Ver-nonia baldwinii) stems growing through thebuckbrush visually concealed the nest bothfrom above and laterally. Several trees werenearby, including a large American elm 15 msouthwest of the nest location. The relativeabundance and proximity of perch sites fromwhich to scan for nests in this grassland areamay have been a contributing factor in thisunusual instance of cowbird parasitism of anartificial nest.

ACKNOWLEDGMENTS

Research support was provided by the OklahomaOrnithological Society. I thank The Nature Conservan-


cy and R. G. Hamilton for access to TGPP and logis-tical support during field research.

LITERATURE CITED

ANKNEY, C. D. AND S. L. JOHNSON. 1985. Variation inweight and composition of Brown-headed Cow-bird eggs. Condor 87:296–299.

BAICICH, P. J. AND C. J. O. HARRISON. 1997. A guideto the nests, eggs, and nestlings of North Ameri-can Birds. Academic Press, San Diego, California,USA.

CLOTFELTER, E. D. 1998. What cues do Brown-headedCowbirds use to locate Red-winged Blackbirdhost nests? Animal Behaviour 55:1181–1189.

ELLISON, K., S. G. SEALY, AND H. L. GIBBS. 2006. Ge-netic elucidation of host use by individual sym-patric Bronzed Cowbirds (Molothrus aeneus) andBrown-headed Cowbirds (Molothrus ater). Cana-dian Journal of Zoology 84:1269–1280.

FREEMAN, S., D. F. GORI, AND S. ROHWER. 1990. Red-winged Blackbirds and Brown-headed Cowbirds:some aspects of a host-parasite relationship. Con-dor 92:336–340.


JOBIN, B. AND J. PICMAN. 1994. Artificial nest parasit-ized by a Brown-headed Cowbird, Molothrus ater.Canadian Field-Naturalist 108:482–484.

LOWTHER, P. E. 1979. Nest selection by Brown-headedCowbirds. Wilson Bulletin 91:118–122.

MOORE, R. P. AND W. D. ROBINSON. 2004. Artificial

bird nests, external validity, and bias in ecologicalfield studies. Ecology 85:1562–1567.

PATTEN, M. A., E. SHOCHAT, D. L. REINKING, D. H.WOLFE, AND S. K. SHERROD. 2006. Habitat edge,land management, and rates of brood parasitismin tallgrass prairie. Ecological Applications 16:687–695.


STRAUSBERGER, B. M. AND M. V. ASHLEY. 2003. Breed-ing biology of brood parasitic Brown-headedCowbirds (Molothrus ater) characterized by par-ent-offspring and sibling-group reconstruction.Auk 120:433–445.

STRAUSBERGER, B. M. AND M. V. ASHLEY. 2005. Hostuse strategies of individual female Brown-headedCowbirds Molothrus ater in a diverse avian com-munity. Journal of Avian Biology 36:313–321.

THOMPSON, C. F. AND B. M. GOTTFRIED. 1976. How docowbirds find and select nests to parasitize? Wil-son Bulletin 88:673–675.

THOMPSON, C. F. AND B. M. GOTTFRIED. 1981. Nestdiscovery and selection by Brown-headed Cow-birds. Condor 83:268–269.

THORINGTON, K. K., R. BOWMAN, AND R. FLEISCHER.2007. Molothrus ater (Brown-headed Cowbird)lays egg in artificial nest in Highlands County,Florida. Southeastern Naturalist 6:559–563.

WHELAN, C. J., M. L. DILGER, D. ROBSON, N. HALLYN,AND S. DILGER. 1994. Effects of olfactory cues onartificial-nest experiments. Auk 111:945–952.


Proximate Mechanisms of Parasite Egg Rejection byNorthern Mockingbirds

John Quinn1,2,3 and Kim Marie Tolson1

ABSTRACT.—Northern Mockingbirds (Mimus po-lyglottos) are known to reject parasitic eggs at an in-termediate rate. However, proximate mechanisms ofrejection remain unexplored. Our objectives were toexamine the rejection behavior of Northern Mocking-birds in northeast Louisiana, explore if nesting date oregg color of parasitic eggs influence the rate thatNorthern Mockingbirds reject Brown-headed Cowbird(Molothrus ater) eggs, and compare results to those ofprevious studies. Northern Mockingbirds rejected 68%

1 Department of Biology, University of Louisiana atMonroe, Monroe, LA 71209, USA.

2 Current address: University of Nebraska–Lincoln,School of Natural Resources, 3310 Holdrege, Lincoln,NE 68583, USA.

3 Corresponding author; e-mail: [email protected]

of artificial eggs. Early nesting individuals rejected50% of the model eggs and late nesters rejected 82%.Early nesting Northern Mockingbirds rejected 72% oflight eggs compared to 27% of dark eggs. Color of theparasitic egg and date of parasitism may influence re-jection rates of Northern Mockingbirds. Received 28January 2008. Accepted 2 June 2008.

Brown Thrasher (Toxostoma rufum), North-ern Mockingbird (Mimus polyglottos), andEastern Meadowlark (Sturnella magna) havedocumented intermediate egg rejection rates(Haas and Haas 1998, Peer et al. 2000, Peerand Sealy 2004). Their rejection rates mayvary as the costs of rejection and acceptance


change. Evolutionary hypotheses explainingthis variation have been presented (Winfree1999), but more focus is needed on proximatehypotheses. We present two proximate mech-anisms of parasite egg rejection by NorthernMockingbirds: egg coloration and nestingdate.

Friedmann’s (1934) early compilation ofBrown-headed Cowbird (Molothrus ater)hosts lists the Northern Mockingbird as an ac-cepter species. Rothstein (1975a) reports a re-jection rate of 25%, but with a limited samplesize of four. Friedman and Kiff (1985) reportdata showing this host rejected 50% ofBrown-headed Cowbird eggs placed in nests.The most robust survey, presented by Peer etal. (2002), reported Northern Mockingbird re-jection of Brown-headed Cowbird eggs at60% (n � 30) and of Bronzed Cowbird (Mol-othrus aeneus) eggs at 69% (n � 32) over 2years in Texas.

Our objectives were to: (1) examine the eggrejection behavior of Northern Mockingbirdsin northeast Louisiana, (2) identify proximatemechanisms involved in Northern Mocking-bird response to Brown-headed Cowbird eggs,specifically if host nesting date and parasiteegg coloration influence the probability of re-jection, (3) and compare results of previousstudies to results obtained.

METHODS

Northern Mockingbird nests were artificial-ly parasitized during 2006 within OuachitaParish, northeast Louisiana (32� 30� N, 92�05� W). The parish (� county) is a mix ofagriculture, forest, residential, mixed urban,and developed property. Sites for searchingwere chosen based on presence of NorthernMockingbird nesting and foraging habitat inaddition to site accessibility. Nests were lo-cated through observations and systematicnest searching.

Model cowbird eggs, because of the diffi-culty of observing natural rejection, wereplaced in nests of laying and brooding North-ern Mockingbirds to provoke rejection. Thereis no documented difference in host responseto real cowbird eggs and mimetic eggs createdin the laboratory (Rothstein 1975a, Prochazkaand Honza 2004). Model mimetic cowbirdeggs were made of plaster of Paris, coatedwith acrylic paint, and glazed to mimic eggs

of Brown-headed Cowbirds (Rothstein1975a). A base coat of titanium white acrylicpaint (Winsor and Newton Acrylic) was ap-plied to each egg. Brown spots (Rust-oleumAmerican Accents 7642 Satin Finish-Nutmeg)were applied on the egg in two spotting pat-terns, heavy (dark) and light, mimicking nat-ural variation of cowbird eggs. Spots covered�60% of the surface on heavily marked eggsand �30% of the lightly spotted eggs.

Nests were randomly assigned as a receiverof dark or light artificial eggs. Variation in eggcolor allowed us to test if heavily spotted ordarker model eggs, more similar to NorthernMockingbird eggs, would be rejected at thesame or different frequency than light coloredmodel Brown-headed Cowbird eggs that aredissimilar to Northern Mockingbird eggs.

Eggs were placed in the nest within 5 hrsafter sunrise. No host eggs were removed.Nests were checked at 2 and 5 days post-par-asitism to learn the fate of the mimetic egg.An egg was considered rejected if it vanishedfrom a nest within 5 days. An egg was con-sidered rejected only if the host’s eggs orfledglings were present in the nest. Any eggthat remained in the nest after 5 days was con-sidered accepted (Rothstein 1975a). Occur-rence of natural parasitism, date of artificialparasitism, clutch size, and model egg colorwere recorded for each nest parasitized.

The nesting season was subdivided into twoperiods, early (Mar–Apr) and late (May–Jul).A Chi-square test of independence was usedto compare rejection frequencies with colora-tion and nesting period. Fisher’s exact test wasused when expected values were �5. A Chi-square Goodness of Fit test was used to com-pare results obtained in northeast Louisianawith those reported by Peer et al. (2002). Alltests were two-tailed with an alpha level of0.05. Data were analyzed using SPSS v.12.0(SPSS Institute Inc. 2006).

RESULTS

Northern Mockingbird nests (n � 48) wereparasitized with model eggs between 15March and 15 July 2006. The hosts rejected68% of model eggs. Rejection frequency byNorthern Mockingbirds differed between ear-ly (Mar–Apr) (n � 22) and late breeding sea-son (May–Jul) (n � 26) (�2 � 5.850, df � 1,P � 0.02). Individuals nesting early in the


season rejected 50% of the model eggs, whilethose nesting late rejected 82%. Early in thebreeding season rejection frequency differedbetween light and dark colored eggs (�2 �4.545, df � 1, P � 0.03). Early breedingmockingbirds rejected 72% of light coloredeggs compared to 27% of the dark eggs. Re-jection rate by late nesting mockingbirds didnot differ between egg color (Fisher’s exacttest, P � 0.13). Egg rejection rate for the en-tire season did not differ between egg color(�2 � 0.048, df � 1, P � 0.83). The rejectionrate of Northern Mockingbirds in OuachitaParish did not differ from that (60%) reportedby Peer et al. (2002) (�2 � 0.527, df � 1, P� 0.47). We found no Brown-headed Cowbirdeggs in nests of Northern Mockingbirds in2006. Cowbirds and evidence of cowbird par-asitism were recorded in Ouachita Parish.

DISCUSSION

Early nesting (Mar and Apr) NorthernMockingbirds rejected foreign eggs less fre-quently than late nesting hosts and rejecteddissimilar model eggs more frequently thansimilar eggs. Our results indicate that, early inthe breeding season, Northern Mockingbirdsmay not be as adept at recognizing foreigneggs. Early in the breeding season the risk ofrejecting one’s own egg may outweigh thebenefits of rejecting a foreign egg. Toleranceto foreign eggs reduces the likelihood thathosts will reject their own atypical eggs(Rothstein 1982). However, the host’s toler-ance of foreign eggs may change as costs andbenefits of rejection shift within the breedingseason.

Victoria (1972) and Rothstein (1975a) sug-gested that egg size, color, or pattern may bea factor in the evolution of egg rejection. Oth-er studies reported it is difficult for host spe-cies, whose eggs resemble cowbirds, to dif-ferentiate between their eggs and those ofcowbirds (Rothstein 1975b, Peer and Sealy2000). The physical appearance of NorthernMockingbird and Brown-headed Cowbirdeggs is similar (Friedmann et al. 1977, Peerand Sealy 2004). Rejecter species have beendocumented to show varied tolerance towardsparasitic eggs based on similarity to its (thehost) own eggs (Rothstein 1982).

Our results suggest a greater level of dis-crimination by Northern Mockingbirds versus

other mockingbird species. Peer et al. (2002)reported that spotless eggs of Bronzed Cow-birds (Molothrus aenous) were rejected morefrequently than spotted Brown-headed Cow-bird eggs. Fraga (1985) reported the Chalk-browed Mockingbird (Mimus saturninus),which lays spotted eggs, accepted markedeggs but rejected immaculate eggs of theShiny Cowbird (Molothrus bonariensis).

Early nesting Northern Mockingbirds mayhave not learned the coloration and pattern oftheir eggs. Rothstein (1974) suggested thathost species may learn the pattern and designof their own eggs only after examination oftheir first clutch. Peer et al. (2000) proposedthat meadowlarks (Sturnella spp.) in thenorthern Great Plains might not reject cowbirdeggs laid early during the laying period be-cause they mistake them for their own. More-over, they suggest this may be the same forBrown Thrashers, a member of the same fam-ily as Northern Mockingbirds.

Decreased discrimination and rejection ear-ly in the breeding season may be a result ofa seasonal threshold. The breeding season ofthe parasite and host must overlap for a spe-cies to encounter the pressure of parasitism(Ortega and Cruz 1991, Peer and Bollinger1997). The first Northern Mockingbird nestwe found was on 6 March 2006. The nestingseason of Northern Mockingbirds in Louisianacan begin as early as February and continueinto August (Lowery 1974). Lowther (1993)reported that Brown-headed Cowbirds beginlaying in mid to late April continuing untilmid-July. It may be more costly to reject eggsif the likelihood of parasitism is low, reducingan individual’s sensitivity to foreign eggs ear-ly in the breeding season. If female Brown-headed Cowbirds only lay eggs during themiddle and end of the season, Northern Mock-ingbirds may be less likely to reject eggs earlyin the season because cowbird parasitismpressure is lower.

Rejection behavior should maximize thebenefits and minimize the costs. We do notknow if the ability to reject is a learned be-havior, influenced by other eggs in the nest,or an inherited picture based on the expectedappearance of the egg of the host. If recog-nition of eggs is a learned behavior, it may beless costly to accept a parasitic egg early inthe breeding season rather than risk a recog-


nition error. It would be valuable to conducta multi-year study with Northern Mocking-birds using banded individuals to confirm ifthe rejection rate of this species varies by ageor experience of the individual in addition toegg color.

ACKNOWLEDGMENTS

We thank L. D. Hayes, D. S. Roane, C. E. Quinn,B. D. Peer, and one anonymous reviewer for usefulcomments on the research process and manuscript. Wethank the Houston Museum of Natural History and theWestern Foundation of Vertebrate Zoology for provid-ing access to their egg collections. We thank the bird-ing community in northeast Louisiana, Black BayouLake National Wildlife Refuge, and numerous privatelandowners for allowing us on their property.

LITERATURE CITED

FRAGA, R. M. 1985. Host-parasite interactions betweenChalk-browed Mockingbirds and Shiny Cowbirds.Ornithological Monographs 36:829–844.

FRIEDMANN, H. 1934. Further additions to the list ofbirds victimized by the cowbird. Wilson Bulletin46:25–36.


FRIEDMANN, H., L. F. KIFF, AND S. J. ROTHSTEIN. 1977.A further contribution to knowledge of the hostrelations of the parasitic cowbirds. SmithsonianContributions to Zoology 235:1–75.

HAAS, C. A. AND K. H. HAAS. 1998. Brood parasitism byBrown-headed Cowbirds on Brown Thrashers: fre-quency and rates of rejection. Condor 100:535–540.

LOWERY, G. H. 1974. Louisiana birds. Kingsport Press,Kingsport, Tennessee, USA.

LOWTHER, P. E. 1993. Brown-headed Cowbird (Molothrusater). The birds of North America. Number 47.

ORTEGA, C. P. AND A. CRUZ. 1991. A comparativestudy of cowbird parasitism in Yellow-headed

Blackbirds and Red-wing Blackbirds. Auk 108:16–24.

PEER, B. D. AND E. K. BOLLINGER. 1997. Explanationsfor the infrequent cowbird parasitism on CommonGrackles. Condor 99:151–161.

PEER, B. D. AND S. G. SEALY. 2000. Conspecific broodparasitism and egg rejection in Great-tailed Grack-les. Journal of Avian Biology 31:271–277.

PEER, B. D. AND S. G. SEALY. 2004. Correlates of eggrejection in hosts of the Brown-headed Cowbird.Condor 106:580–599.

PEER, B. D., K. S. ELLISON, AND S. G. SEALY. 2002.Intermediate frequencies of egg ejection by North-ern Mockingbirds (Mimus polyglottos) sympatricwith two cowbird species. Auk 119:855–858.


PROCHAZKA, P. AND M. HONZA. 2004. Egg discrimina-tion in the Yellowhammer. Condor 106:405–410.

ROTHSTEIN, S. I. 1974. Mechanisms of avian egg rec-ognition: possible learned and innate factors. Auk91:796–807.

ROTHSTEIN, S. I. 1975a. An experimental and teleo-nomic investigation of avian brood parasitism.Condor 77:250–271.

ROTHSTEIN, S. I. 1975b. Evolutionary rates and hostdefenses against avian brood parasitism. AmericanNaturalist 109:161–176.

ROTHSTEIN, S. I. 1982. Mechanisms of avian egg rec-ognition: which egg parameters elicit responses byrejecter species. Behavioral Ecology and Socio-biology 11:229–239.

SPSS INSTITUTE INC. 2006. SPSS for Windows. Version12.0.1. SPSS Institute Inc., Chicago, Illinois,USA.

WINFREE, R. 1999. Cuckoos, cowbirds, and the persis-tence of brood parasitism. Trends in Ecology andEvolution 14:338–343.

VICTORIA, J. K. 1972. Clutch characteristics and eggdiscrimination ability of the African VillageWeaverbird Ploceus cucullatus. Ibis 114:367–376.

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Proximate Mechanisms of Parasite Egg Rejection by Northern Mockingbirds

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