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MCARTHLJR,P. D. 1982. Mechanisms and development Departmentof BiologicalSciences, North Arizona Univer- of parent-youngvocal recognition in the Pinyon Jay sity, Flagstax Arizona 86011. Received 2 1 January 1985. (Gymnorhinuscyanocephalus). Anim. Behav. 30:62- Final acceptance15 July 1985. 14.

The Condor87:561-562 being fed by other passerine species,but has seen none 0 The CooperOrnithological Society 1985 associatedwith SageThrashers. We experimentally parasitized SageThrashers to deter- SAGE THRASHERS REJECT mine their responsesto cowbird eggs.We wanted to de- termine whether few casesof parasitism are reported be- COWBIRD EGGS causeSage Thrashers are not parasitizedor, in part at least, becausethey remove cowbird eggsbefore observersfind them. The differing appearances of cowbird and Sage TERRELL RICH Thrasher eggsindicate that a bird could distinguish be- AND tween them easily. Cowbird eggsare white with numerous small brown and gray spots, whereas Sage Thrasher eggs STEPHEN I. ROTHSTEIN are blue-greenwith red-brown blotches. We located SageThrasher nests in basin big sagebrush (Artemisia tridentatatridentata) habitat in Blaine County, Actual and potential hosts of brood parasitism by Brown- Idaho, during April and May, 1984. Nest contents were headedCowbirds (Molothrus ater) can be classifiedas either maninulated between 08:OOand 14:O0.with most manio- rejecters or accepters of cowbird eggs (Rothstein 1975, ulations being performed between 09:OOand 10:OO.At 1982a,b). Few North American speciesshow intermediate each nest, we exchangedor added an artificial cowbird egg responses(Rothstein 1982a). Rothstein (1982b) reasoned quickly and then left the area soas to minimize disturbance that once the rejection behavior appears in a ‘species, it of adult thrashers.These eggswere made of plasterof Paris has such a high adaptive value that it is rapidly fixed. and measured 21.1 x 16.3 mm. They were identical to Whatever the cause,this dichotomy betweenaccepters and eggsin Rothstein’s (1975) studyand closelyresembled real rejectersmakes it possibleto determine the responseof a cowbird eggsfound in southernIdaho (Rich, pers.observ.). given speciesby experimentally manipulatingthe contents Nests were checkedbetween 1 h and severaldays after the of relatively few nests. manipulation to determine responses.Unless noted oth- It is desirableto continue accumulatingevidence on the erwise, all nests were subjectedto only one experimental responsesto brood parasitism of untestedspecies because manipulation. no absolute criteria that explain why some species are In ten nests, we removed a thrasher eggand replacedit rejecters and others accepters have yet been identified with an artificial cowbird egg. Five of these nests were (Rothstein 1975). Although Rothstein (1975) failed to find found during egg-laying,and five were found during in- a strong relationship between taxonomy and responseto cubation. Although most natural cowbird parasitism oc- non-mimetic parasitic eggs, knowledge of the host re- cursduring the host’s egg-layingperiod, nest stagehas little sponsein all members of a family would be usefulbecause or no relation to responsein most rejecter species(Roth- many variablesrelated to morphology,behavior, and evo- stein 1976, 1977), a trend also indicated by our results. lutionary history would be somewhat controlled. Nine of the 10 eggswere ejected. The tenth egg remained The Mimidae have severalfeatures that make the family in the nest, which was deserted.The last nest was the only a good group for study of responseto brood parasitism. one where there had been only a singleegg laid at the time In particular, they have at least four of the six character- of manipulation. At an eleventh nest, we added an artificial istics suggestedby Rothstein (1975) as contributing to the cowbird egg to a clutch of four eggs. This egg was also formation of rejection behavior. Their eggsare unlike cow- ejected along with two thrasher eggs, and the nest was bird eggs,their beak is large, their nest is large and easily subsequentlydeserted. found, and they practicegood nest sanitation (Bent 1948). Cowbird eggswere usually ejected quickly, but not al- Yet, three North American mimids accept eggs of the waysimmediately; artificial cowbird eggswere still present Brown-headed Cowbird: Northern Mockingbird (Mimus at one nest after 2 h and at another after 3 h. The earliest polyglottos;Rothstein 1975) Le Conte’s Thrasher (Tox- known ejections occurred within 1, 2, and 3 h, and two ostomalecontei), and California Thrasher (T. redivivum; within 4 h, althoughin no casedid we watch a bird remove Rothstein, pers. observ.). Among North American mim- an egg. Only one of 17 ejected eggswas found, and that ids, three specieseject cowbird eggsfrom their nests:Gray was at a distance of 3 m from the nest. The egg bore no Catbird (Dumetella carolinensis).Brown Thrasher (T. ru- evidence of pecking, thereby matching previous findings fim; Rothstein 1975, 1982a), and Crissal Thrasher (T. that most speciesthat eject eggsdo so by holding eggsin dorsale;Finch 1982). A neotropical mimid, the Chalk- their bills, rather than by spiking them (Rothstein 1975). browed Mockingbird (Mimus saturninus),is also known We performed nine additional experiments with two to eject parasitic eggs(Mason 1980, Fraga 1982). types of eggs intermediate between cowbird and Sage The Sage Thrasher (Oreoscoptesmontanus) may have Thrasher eggsto get some indication of the factors that accepteda cowbird egg in the only known case of para- SageThrashers use to distinguishamong eggtypes. Single, sitism (Friedmann 1963), but most of the evidence sug- real, thrasher eggsthat were painted to resemble cowbird geststhat this speciesmay be a rejecter. Rich (1978) found eggswere ejected from four nests. These results suggest no cowbird eggs in 21 Sage Thrasher nests in an area that the difference in size between thrasher (24.8 x 16.8 frequentedby cowbirds,where at leasttwo other sympatric mm, Bent 1948, p. 429) and cowbird eggs(21.8 x 16.8 specieswere parasitized. Also, Rich (pers. observ.) has mm, Bent 1958, p. 451) is not a necessaryreleaser for examined about 40 other thrasher nests in Bingham and rejection behavior. Blame counties, Idaho, and found neither cowbird eggs Artificial eggsidentical in size to the artificial cowbird nor nestlings.In Mono County, California, both cowbirds eggs,but colored an immaculate blue (identical to eggtype and Sage Thrashers are locally common and sometimes “s” in Rothstein 1982a).were accemedat two offive nests. forage at the same horse corrals (Rothstein et al. 1980). Birds at three of thesenests were incubatingand had eject- Rothstein has seen a large number of fledgling cowbirds ed artificial cowbird eggs44 to 48 h before the blue egg 562 SHORT COMMUNICATIONS was added. Blue eggswere acceptedduring incubation at This note was improved by the constructivecomments one nest where thrashershad ejected a cowbird eggearlier of H. Mayfield, D. Finch, and an anonymous reviewer. and during egg-layingat another nest where thrashershad no known prior experiencewith foreign eggs. Becausea blue eggwas acceptedin two of five cases,it LITERATURE CITED appearsthat SageThrashers, like other rejecters(Rothstein 1982a),will accept some eggtypes that are distinguishable BENT, A. C. 1948. Life histories of North American nut- from their own eggs.Such tolerance may be adaptive in hatches,wrens, thrashers,and their allies. U.S. Natl. that it reduces the chancesof a host’s rejecting unusual Mus. Bull. 195. eggsof its own. Indeed, Sage Thrashers occasionallylay BENT, A. C. 1958. Life histories of North American immaculate blue eggs(Rich, pers. observ.) which are sim- blackbirds,orioles, tanagers, and allies.U.S. Natl. Mus. ilar to the experimental blue egg, except for the former’s Bull. 211. slightly larger size. Ejection of three artificial blue eggs FINCH,D. M. 1982. Rejection of cowbird eggsby Crissal shows that SageThrashers are capable of subtle discrim- Thrashers. Auk 991719-724. ination. Becauseof the small sample size, however, more FRAGA,R. M. 1982. Host-brood interactions between tests with blue eggsare necessarybefore conclusionscan Chalk-browed Mockingbirds and Shiny Cowbirds. be drawn. Ph.D. diss., Univ. of California, Santa Barbara. SageThrashers clearly reject cowbird eggsby eiection. FRIEDMANN.H. 1963. Host relationsofthe oarasiticcow- The SageThrasher has ai least five of the sixch&acieristics birds. U.S. Natl. Mus. Bull. 233. _ that Rothstein(1975) identified asbeing in common amone. GRINNELL,J. 1909. A new cowbird of the genus Mol- rejectors:(1) its eggs’areunlike cowbird eggsin size, cola< othrus.Univ. Calif. Publ. Zool. 5:275-281. and maculation; (2) it practices nest sanitation, as nests MASON,P. 1980. Ecologicaland evolutionary aspectsof are completely free of excrement and other extraneous host selectionin cowbirds. Ph.D. diss., Univ. of Tex- material (Rich, pers. observ.); (3) thrasher nests are large as, Austin. and relatively easy to find (The mean dimensions of nine REYNOLDS,T. D., ANDT. D. RICH. 1978. Reproductive newly built nests were: outside diameter at rim, 20.8 + ecologyof the SageThrasher (Oreoscoptesmontanus) 1.O cm, and depth, 12.2 & 1.8 cm.); (4) although the Sage on the Snake River Plain in southcentralIdaho. Auk Thrasher is the smallest mimid, its beak is large enough 95:580-582. to manipulate eggsat least as large as its own; and, (5) RICH, T. D. 1978. Cowbird parasitismof Sageand Brew- with territories of about 1.O ha (Reynolds and Rich 1978) er’s sparrows. Condor 80:348. and the large range of the species(Bent 1948), population ROTHSTEIN,S. I. 1975. An experimental and teleonomic size must be consideredas moderate to large. investigation of avian brood parasitism. Condor 77: The sixth characteristic,history of sympatry, may also 250-271. favor the evolution of rejection in Sage Thrashers. Al- ROTHSTEIN,S. I. 1976. Experiments on defensesCedar though some cowbirds occur in the continuous expanses Waxwings use against cowbird parasitism. Auk 93: of sagebrushoccupied by SageThrashers, they are today 675-691. most common in more favorable islands of habitat, such ROTHSTEIN,S. I. 1977. Cowbird parasitism and eggrec- as riparian growth and livestock pastures,that exist within ognition of the Northern Oriole. Wilson Bull. 89:21- sagebrush.Cowbirds thus occur with Sage Thrashers at 32. numerouspoints within the latter’s range. This co-occur- ROTHSTEIN,S. I. 1982a. Mechanisms of avian egg rec- rence in the Great Basin is probably not recent (Grinnell ognition: which egg parameters elicit responsesby 1909). Before settlement of the West by Europeans,cow- rejecter species?Behav. Ecol. Sociobiol. 11:229-239. birds in the Great Basin may have been sustainedin part ROTHSTEIN,S. I. 1982b. Successesand failures in avian by the foraeing affordedbv large herds of bison. Two other egg and nestling recognition with comments on the speciesthat a& broadly simp&ic with SageThrashers in utility of optimalitv reasoning. Am. Zool. 22:547- sagebrushhabitat, the Sage Sparrow (Amphispiza belli) 560. - - - and the Brewer’s Sparrow (Spizella breweri), may reject ROTHSTEIN,S. I., J. VERNER,AND E. STEVENS.1980. Range cowbird eggsthrough desertion (Rich 1978). Experimental expansion and diurnal changesin dispersion of the verificationof sucha responseis necessary(Rothstein 1975), Brown-headed Cowbird in the Sierra Nevada. Auk however, becausedesertion does not provide clear evi- 97~253-267. denceof a responseto cowbird parasitism. Thus, the three passerinesmost closelytied to sagebrushhabitatmay have U.S. Bureau of Land Management, ShoshoneDistrict, had long periods during which they could have evolved P.O. Box 2B, Shoshone,Idaho 83352; Department of Bi- host defenses. ologicalSciences, University of California, Santa Barbara, It would be valuable to determine the responsesto cow- California 93106. Presentaddress of$rst author: U.S. Bu- bird enesof those mimids so far untested.We would then reau of Land Manazement, P.O. Box 1229, Dickinson, be ableto compare all the speciesin a family whose char- North Dakota 58602. Received 1 February 1985. Final acteristicssuggest that they should be largely preadapted acceptance12 July 1985. to rejectcowbird eggs, but which have had varying amounts of historical contact with cowbirds.