Brood Parasitism in a Host Generalist, the Shiny Cowbird: Ii

Home , Saffron finch, The Tyrant (House)

BROOD PARASITISM IN A HOST GENERALIST, THE SHINY COWBIRD: II. HOST SELECTION

]•AUL MASON • Departmentof Zoology,University of Texas,Austin, Texas 78712 USA

AI•STI•ACT.--Hostselection by brood parasiticShiny Cowbirds (Molothrusbonariensis) was studiedat two sitesin BuenosAires Province,Argentina. The eggsof the parasiteare either spottedor immaculate,so host selectionwas studiedwith respectto egg type as well as to site. Immaculate eggs were rare at both sites. Cowbirds in this region prefer to parasitize nestsof birds larger than themselves.This preferencecontrasts with that of almostall other brood parasites,and even that of Shiny Cowbirds in other parts of the species'range. One large species,the Chalk-browedMockingbird (Mimussaturninus) was used frequently and consistentlyat both sites.At least two other large species(thrushes) were used at Site I, but neither was present at Site II. Cowbirds were more specializedon large hostsat Site I: only 9% of the spottedeggs were laid in nestsof small birds at Site I, whereas 35% were laid in suchnests at Site II. Small species,such as the Rufous-collaredSparrow (Zonotrichiacapensis), were usedto a greaterextent at Site II than Site I (where someof the samespecies were not usedat all). The shift to smallerhosts is probablya responseto a changein the structureof the community;large host speciesare relatively lessabundant at Site II. Rejecterspecies are large, and all were parasitizedwhile many smaller accepterspecies were unmolested.Surprisingly, more immaculateeggs were laid in nestswhere they had little chanceof successfulincubation, rather than in nestsof accepters.No evidencesuggests that host races("gentes") are formed. To the contrary, female cowbirdslaying different egg types apparently selecthosts in the same manner. Received8 August1984, accepted 25 July 1985.

SPECIESthat use a wide variety of resources plete descriptor of parasitism in this case. A are known as generalists.The brood parasitic fourth bias concernsparasitism of speciesthat Shiny Cowbird (Molothrusbonariensis) is an ex- reject cowbird eggs. Becauserejecter species treme generalist with respect to the "host generally remove cowbird eggsin a very short niche": its eggs have been found in nests of time, parasitism of rejecters is unlikely to be 201 species(Friedmann and Kiff 1985). observed(Rothstein 1971, 1975, 1977). Experi- Our knowledge of how female cowbirds se- ments simulating parasitism must have been lect hostsis incompleteand biased.First, data performedpreviously to identify rejecters. on parasitismare often gatheredindirectly by This study describesand interpretsthe con- studying a particular host species.Information trasting patterns of host selection by Shiny about the compositionof the host community Cowbird females at two sites in Buenos Aires and the rates of parasitismof other speciesis Province, Argentina. Geographic variation in frequently unavailable.A secondsource of bias host selection remains unexplained. For ex- concernsthe breeding seasonsof host and par- ample,parasitism of the Rufous-collaredSpar- asite. In Tucum•n Province, Argentina, only row (by the nominate subspeciesof the cow- early nestsof the Rufous-collaredSparrow (Zo- bird) varies from 15 to 77% according to site notrichiacapensis) escaped parasitism: all late (Sick 1958;King 1973;Fraga 1978, 1983;Goch- nestswere parasitized(King 1973). Third, sev- feld 1979; Salvador 1983). eral Shiny Cowbirds often parasitize a single I also presentdata on host selectionwith re- nest (Friedmann 1929, 1963; Friedmann et al. gard to egg color. Almost all eggs can be de- 1977). The proportion of nestsparasitized, the scribedas spotted or immaculate in easternAr- most frequently reported statistic,is an incom- gentina, Uruguay, and portions of Brazil. Intermediate eggswith a few fine spotsare rare (Friedmann 1929; Fraga 1978, 1983, 1985; Goch- •Present address:Department of BiologicalSci- feld 1979;Salvador 1983). Some host species, ences,University of California, Santa Barbara,Cali- dual accepters,incubate both morphs. Other fornia 93106 USA. hosts,dual rejecters,remove all cowbird eggs 61 The Auk 103: 61-69. January 1986 62 PAUl,MASON [Auk, Vol. 103 from the nest. One major host (Chalk-browed parasitized) are reported because many nests Mockingbird, Mimussaturninus) is a differential were multiply parasitized.Eggs laid in peculiar accepterthat ejectsimmaculate eggs but accepts circumstances(such that they could not be spotted ones (Fraga 1980, 1985; Mason 1986). scored for acceptanceor rejection), and eggs Severalother important host speciesprobably found in nestsbut later rejectedare included. are differential accepters of cowbird eggs Only 15 immaculateand 6 intermediateeggs (Friedmann et al. 1977, Fraga 1985). These with a few fine spots were seen. Intermediate speciesare particularly important becausethey eggs appear immaculate from a distance of representstrong sources of natural selectionon about 1 m or more. The Chalk-browed Mock- egg morphology and host selection. The eco- ingbird, the only speciesknown to responddif- logicalsituation could potentially result in the ferentially to different egg morphs(Fraga 1985, evolutionof hostraces or "gentes"(Payne 1977). Mason 1986), apparently perceivesintermedi- ate eggsas immaculate(see below). METHODS Comparisonsbetween sites.--Immaculate and intermediate eggs were rare at both sites. At One seasonwas spent at each of two sites located Site I, I found 4 immaculate eggs, 0 interme- 17.6 km apart near Magdalena, Buenos Aires Prov- diate eggs,and 8 spottedeggs in nestsof dual ince, Argentina: Site I was EstanciaSan Isidro and accepters.At Site II, I found 0 immaculateeggs, Site II was Estancia E1 Talar. Further details, includ- 1 intermediate egg, and 53 spotted eggsin such ing the relative abundanceof passefinespecies, are nests. presentedelsewhere (Mason 1985). The first season lastedfrom 20 October 1977 to 24 February 1978,and At both sites,species larger than the cowbird the second from 21 September 1978 to 10 February were preferred as hosts (Site I: G = 57.6, 7 df, 1979. Very little laying by any passerinespecies oc- P < 0.001; Site II: G = 34.0, 7 df, P < 0.001). curs outside this interval (see below and Mason 1985). The mockingbird was the only large species Nests were observedrepeatedly until young fledged present at both sites and representedby sam- or activity at the nest ceased.Observations were car- pies sufficient for statisticalcomparison; it was ried out daily, although most nests were not ob- parasitizedequally at each site (G = 13.3, 9 df, servedwith suchfrequency. Shiny Cowbird eggswere P > 0.1). All thrush nests were parasitized. In scored for color (spotted, immaculate, or intermedi- addition to the 5 nests found in the egg stage ate), date, and host. A cohort of nests of the same (see Appendix), an additional nest of each speciesat each site was then characterizedby a distribution describing the frequency of nests with a specieseach contained 2 cowbird young. certain number of cowbird eggs of a specific type. At Site I, large, abundant passerines(exclud- Distributions obtained in this manner were com- ing the mockingbird) were: Rufous Hornero (a pared using the heterogeneityG-test (Sokal and Rohlf dual rejecter),Great Kiskadee(response to spot- 1981). Other statisticaltests vary accordingto the par- ted cowbird eggs uncertain; Mason 1986), Ru- ticular comparison(see below). The diversity of host fous-bellied Thrush, White-rumped Swallow use at each site was described by calculating the (body size considerablysmaller than that of the Shannon-Weaver index (Peet 1974, Whittaker 1975). cowbird, but included because of its wing- Scientificnames of speciesare given in the Appen- length), and the Brown-and-yellowMarshbird dix. [Pseudoleistesvirescens; no nestsfound, but a fre- quent hostchoice in BuenosAires (Gibson1918; RESULTS Hudson 1920; Friedmann 1929, 1963; Fried- mann et al. 1977)]. At Site II, the thrush was The data base consists of 360 nests (7 nest absent and the Brown-and-yellow Marshbird boxes)of 28 passerinespecies. I found 233 Shiny rare (Mason 1985). Excluding the small-bodied Cowbird eggs in nests of 13 species.The dis- swallow, only 1 large accepterof spotted eggs tribution of 212 spotted Shiny Cowbird eggsin (the mockingbird) was abundant at Site II, as 333 nestsdiscovered in the egg phase is given opposedto at least2 (mockingbird,thrush) but in the Appendix. Speciesare listed by decreas- probably 3 (including the marshbird) such ing winglength of adult females (Mason 1980) speciesat Site I. becausemore accurate data for body size (i.e. The sites differed markedly in that species weight) are lacking. Both intensity (mean num- smaller than the cowbird were used to a much ber of cowbird.eggs per parasitized nest) and greater extent at Site II. The Rufous-collared frequency of parasitism(percentage of nests Sparrow was used significantly more at Site II January1986] HostSelection by ShinyCowbirds 63 than at Site I (G = 9.7, 3 df, P < 0.01). Two oth- T^I•LE1. Diversity in host selection by site.a er species(Yellow-browed Tyrant, House Wren) were ignored by cowbirds at Site I, but both -pilog Pl were parasitizedat Site II, althoughsample sizes are small. The tyrant, wren (natural nest sites), Host diversity measuredby species and sparrow were used equally at Site II (G = Site I 4.4, 6 df, P > 0.50). In addition, the Bay-winged Rufous Hornero 0.013 0.024 Cowbird was used more at Site II than at Site Cattle Tyrant 0.013 0.024 Chalk-browed Mockingbird 0.759 0.091 I, and a single spotted cowbird egg was laid in Rufous-bellied Thrush 0.127 0.114 a nest of the Freckle-breasted Thornbird at Site Creamy-bellied Thrush 0.013 0.024 II the day following loss of that nest. Only 1 Bay-wingedCowbird 0.025 0.040 small species(Saffron Finch) was parasitized at Saffron Finch 0.013 0.024 Rufous-collaredSparrow 0.038 0.054 Site I but not Site II. When frequencies for all H' = 0.395 b small specieswith data from both sites were Site II pooled, smaller specieswere parasitized signif- Rufous Hornero 0.022 0.037 icantly more often (G = 16.9, 5 df, P < 0.005). Freckle-breasted Thornbird 0.007 0.016 House Wrens at Site II nesting in boxes were Yellow-browed Tyrant 0.030 0.045 more heavily parasitizedthan those nesting in Cattle Tyrant 0.007 0.016 natural sites (G = 17.30, 5 df, P < 0.05). A pair Fork-tailed Flycatcher 0.052 0.067 of wrens used a nest box at Site I, but this nest Wkite-rumpedSwallow 0.037 0.053 House Wren 0.059 0.073 was not parasitized. The only other use of a House Wren (nest boxes) 0.141 0.120 nest box was by a pair of Saffron Finches (Site Chalk-browed Mockingbird 0.533 0.146 II). Bay-wingedCowbird 0.015 0.027 Cowbirds selected a more diverse array of Rufous-collaredSparrow 0.096 0.098 hostsat Site II than at Site I (measuredby the H' = 0.698 Shannon-Weaver index; Peet 1974, Whittaker Host diversity measuredby sizec 1975), regardlessof whether hosts were char- Site I acterized by speciesor by size (Table 1). The Large 0.91 ! 0.037 latter analysis was done because a cowbird's Small 0.089 0.093 decision to parasitize a nest may be based on H' = 0.130 some general aspect,such as size, rather than Site II on speciesidentity (Rothstein 1976). Large 0.644 0.123 Parasitismof rejecters.--AlthoughI could not Small 0.356 0.160 estimatethe true frequencyof parasitismof re- H' = 0.283 jecters, I observed some cowbird eggs in nests ap's represent the proportionof spottedeggs found of the RufousHornero and the Fork-tailed Fly- in nestsof different species. catcher(both dual rejecters;Fraga 1980,Mason bHost diversity measured using the Shannon- 1986). Responsescould not be scoredin 3 cases Weaver index (Whittaker 1975):H' = -• pilog Pl of parasitismof the hornero becauseI acciden- cLarge and small reckonedusing winglength relative to the Shiny Cowbird. tally destroyed one nest, or the cowbird egg was laid extremely late in the incubation or nestling period. Two spottedeggs were reject- hornero and the flycatcher,generally rejects ed and later found, one in the entrance of a immaculateeggs. Thirteen immaculateand in- nest and the other directly below a nest. A bro- termediateeggs were observedin 10 mocking- ken eggshell was found below another nest, bird nests.Three were accepted,7 ejected,and probably from an egg laid and rejected be- the remainder could not be scored(predation tween observations. Four nests of the Fork- or collection). At one nest where acceptance tailed Flycatchercontained cowbird eggs.Two was observed,an immaculateegg was rejected nestshad 3 cowbird eggseach (1 of which was the previousday. intermediate), and all eggs were ejected.Nei- Rejecter speciesof South America resemble ther nest was ever known to contain host eggs. North Americanrejecters in being large.Rejec- Two other nestseach received 1 spotted egg, ters (of at least one morph) are, on average, which was ejectedin both cases. larger than accepterspecies (Table 2; P < 0.05, The Chalk-browedMockingbird, unlike the Mann-Whitney U = 5; Siegel 1956). This test, 64 PAU•MA$O• tAuk, Vol. 105

TABLE2. Passefinesize (estimatedby winglength) and responseto cowbirdeggs.

Response to eggsø Fig. 1. Datesthat cowbirdeggs were laid or found. Wing- Immac- lengtha Species Spotted ulate 138 Brown-chested Martin Acc Acc The breeding seasonsof the Chalk-browed 129 Great Kiskadee c ? Acc Mockingbird and the Rufous-collaredSparrow 126 Chalk-browedMockingbird Acc Rej were completelywithin the breeding seasonof 125 Rufous-belliedThrush c Acc? Rej? the cowbird. I divided the breeding seasonsof 123 Creamy-belliedThrush Acc Acc thesetwo hostsinto quartersand tested for het- 119 White-rumped Swallow Acc Acc 112 Fork-tailedFlycatcher Rej Rej erogeneity among the subsamples. For the 107 RufousHornero Rej Rej mockingbird, I pooled both sites because the 104 Shiny Cowbird distributionswere equivalent. For the sparrow, 99 Cattle Tyrant Acc Acc parasitismonly from Site II was analyzed.The 98 Blue-and-yellowTanager Acc Acc subsamplesprovided no evidence of hetero- 93 Bay-wingedCowbird Acc Acc 90 Yellow-browed Tyrant Acc Acc geneityin either species(mockingbird: G = 27.4 85 Firewood-gatherer Acc Acc for 27 df, NS, 0.25 < P < 0.5; sparrow:G = 9.2 81 Vetmillion Flycatcher Acc Acc for 9 df, NS, 0.25 < P < 0.5). 73 House Sparrow Acc Acc 71 Grassland Yellow-Finch Acc Acc DISCUSSION 70 Freckle-breasted Acc Acc Thornbird 67 Rufous-collaredSparrow Acc Acc The extent of generalizationby Shiny Cow- 66 Saffron Finch Acc Acc birds varied between two sites. Most large 64 Tufted Tit-Spinetail Acc Acc speciesapparently were used consistently,re- 63 Wren-like Rushbird Acc Acc gardlessof responseto cowbird eggs. Use of 63 Little Thornbird Acc Acc 57 GrasslandSparrow Acc Acc smaller specieswas variable, but more preva- 51 House Wren Acc Acc lent at Site II. The Chalk-browed Mockingbird 51 Masked Gnatcatcher Acc Acc was parasitizedconsistently and heavily at both 48 White-crestedTyrannulet Acc Acc sites. Other large hosts (thrushes) were para- aRejecters are significantly larger than accepters sitized at Site I, but were absent from Site II. (Mann-Whitney U-test, P < 0.05, U = 5). Large and The Brown-and-yellow Marshbird, probably a small reckonedrelative to the Shiny Cowbird. common host in Buenos Aires Province (Hud- bAcc = accepts,Rej = rejects(Mason 1986). son 1874, 1920; Sclater and Hudson 1888; Gib- cNot included in the analysisbecause response to cowbird eggsnot verified experimentally. son 1918;see reviews by Friedmann 1929, 1963, and Friedmann et al. 1977), was abundant at Site I but rare at Site II. based on winglength, is conservativebecause The Shannon-Weaver indices (Table 1) de- it includes as large birds two Hirundinidae scribe the width of the "host niche" and illus- (White-rumped Swallow, Brown-chestedMar- trate differencesin site-specifichost selection. tin), birds with extremely long wings relative Although the measurementswere determined to body size. in part by my ability to find and examine nests, Seasonality.--Thefirst cowbird egg was laid the occasionalor rare parasitismof a few species on 7 October and the last on 19 January (Fig. not censused would result in only slight 1). Three species,all furnariids (Rufous Hor- changes.If I missedcommon, heavily parasit- nero, Firewood-gatherer, Freckle-breasted ized species,the indiceswould be substantially Thornbird), completeda portion of their breed- altered.The only specieslikely in this category ing before the cowbird (Mason 1985). The last is the Brown-and-yellowMarshbird at Site I. If two species accept all cowbird eggs (Mason this specieswas heavily parasitized, the index 1986), but only a single nest of the thornbird (reckoned by species)would increaseat Site I, was parasitized (see Appendix). Nests of the decreasing the difference between sites. If Rufous Hornero were ranked by date: parasit- reckoned by size, however, the index would ized nests occurred later in the season (Mann- increase the contrast because the marshbird is Whitney U = 8, P < 0.025; Siegel 1956). a large species(Mason 1980). January1986] HostSelection by ShinyCowbirds 65

Conspicuousnessmight predisposea nest to while the small Rufous-collaredSparrow is parasitism (Brown-headed Cowbird, Molothrus parasitizedless. During the last half of a 10~yr ater:Nice 1937,Rothstein 1975; Shiny Cowbird: study of the sparrow at Lobos, Buenos Aires Friedmann 1929, 1963; Gochfeld 1979), but the (Fraga 1978, 1983),parasitism dropped signifi- contrast between sites in parasitism of the Ru- cantly from 72.5 to 43.5%.This coincidedwith fous-collaredSparrow and other small species changesin local agriculturethat attractedthe arguesagainst the importanceof conspicuous- White-browed Blackbird(Sturnella superciliaris), ness.The elaboratenests of furnariids(e.g. the a bird much larger than the sparrow.Two of 6 hornero, Firewood-gatherer,and thornbirds) nestscontained cowbird eggs, but the species are conspicuousto the human observer, but was seen tending cowbird fledglings (Fraga theseare not equally likely to be parasitized. 1985).Parasitism of the mockingbird remained Birds that nest in old hornero nests (Tufted Tit- high (above that of the sparrow;Fraga 1985) Spinetail, White-rumped Swallow, Saffron and unchanged. In Villa Maria, C6rdoba Prov- Finch, House Sparrow) were likewise not ince, the mockingbirdwas usedmore than any equally parasitizeddespite the control for nest other local host (86.9%of nestsparasitized; Sal- site (Appendix; Mason 1985). The strongest vador 1983). Other large hosts also were used, support for the conspicuousnessargument is but only 8 of 22 Rufous-collaredSparrow nests the significantdifference (at Site II) in parasit- held cowbird eggs.With regard to the parasitism of wrens nesting in boxesas opposedto ism of this host, Salvador wrote: "at this site, it natural nesting sites. [the sparrow]was not a host of great impor- A more satisfyingexplanation attributes the tance,if we compareit with Mimussaturninus" local differences to the operation of the same [Salvador1983:155 (trans. by PM)]. host-selection mechanism in environments of- Gochfeld (1979) found heavy use (23/24 fering differing arrays of resources(hosts). nests)of the Long-tailedMeadowlark (Sturnella Preference measures have different formal de- loyca)but no use (0/11) of the highly similar scriptions(Ivlev 1961, Murdoch 1969, Rapport Lesser Red-breasted Meadowlark (Sturnella de- and Turner 1970, Chesson 1978, Jaenike 1980), filippi)in BahiaBianca, southern Buenos Aires but nonecan be calculatedhere becauseprecise Province.The low frequencyof parasitismof numericaldensities of host and parasitepopu- the Rufous-collaredSparrow (2/13 nests) relations are unavailable. Nonetheless, three dis- semblesmy resultsat Site I, but 0 of 4 nestsof tinct lines of evidenceprovide qualitativesup- mockingbirdscontained cowbird eggs (Goch- port that large hostsare preferred:(1) among feld pets.comm.). Both types of eggsoccurred accepters,larger hosts were parasitized more at the site. The absenceof parasiticeggs in frequently and to a greater extent than were mockingbird and Lesser Red-breasted Mea- smaller hosts;(2) all known rejecter species dowlark nestsmay possiblyreflect rejection be- (which are large)were parasitizedwhile many havior of the hostsrather than avoidanceby accepterswere left unmolested;and (3) when cowbirds. larger hostswere lessabundant or absent,cow- The preferencefor large hosts,a trait unusu- birds used smaller hosts. al in any parasiticbird (Payne 1977), may be a Niche expansionand inclusionof specieslike phenomenonrestricted to the Rio de La Plata the sparrowis probablyadaptive. Pecking of basinand surroundingarea. In the West Indies, host eggsby cowbirdsis a density-dependent there is no clearrelationship between host size form of mortality that sometimesterminates and preferenceranking by Shiny Cowbirds.In nestingattempts. Pecking depressesthe survi- Trinidad the diminutive House Wren may be vorship of mockingbirdnests to a level indis- the mosthighly preferredhost (Manolis 1982), tinguishable from that of the Rufous-collared while in Puerto Rico both large and small hosts Sparrow (Mason 1986). Presumably, if niche may be heavily used(P•rez Rivera 1983,Wiley width remained constant in the two environ- 1985). ments,nest losses to peckingwould further in- Hostselection by femaleslaying contrasting egg creaseat Site II, and cowbirdswould experi- types.--Only 5 immaculate and intermediate ence still lower success. eggs were laid in nests of accepters,whereas Other studies share a common feature that 16 were laid in nestsof speciesthat normally supportsthe claim of a preferencefor larger rejectthese morphs. Females that lay theseeggs hosts:at least one large host is heavily used, clearlydo not placethem preferentiallyin nests 66 PAULMASON [Auk,Vol. 103 where the eggswill be accepted.No evidence ACKNOWLEDGMENTS suggeststhat host selection varies between females. M. Christie helped get my researchprogram start- ed. J. M. Gallardo (Director) and J. R. Navas (Jefe, Fraga (1985) also describedimmaculate eggs Divisi6n de Ornitologia) give me free accessto the laid in and rejected from mockingbird nests. collectionsof the Museo Argentino de CienciasNat- He estimated that at least one-third of the nests urales, Buenos Aires. I am indebted to the estanci- were parasitizedwith white eggs.Descriptions eros, E. M. and D. Earnshaw (EstanciaSan Isidro) and of immaculateeggs below, but spottedeggs in- A. Landa(Estancia E1 Talar), for allowing me to work side, nestsof the Brown-and-yellow Marshbird on their land. They and their families showed me (Hudson 1874, 1920; Sclater and Hudson 1888) the greatesthospitality. R. M. Fragaoffered helpful suggestthat this host is a differential accepter suggestionsand encouragementat all stagesof the research.M. Gochfeldprovided a list of nestshe found like the mockingbirdand that cowbirdsbehave at Bahia Bianca. Earlier versions of the manuscript the same toward the marshbird and the mock- were read and criticized by A.M. Dufty, Jr., R. M. ingbird. This pattern of host selectionis appar- Fraga,M. Gochfeld, W. W. Murdoch, S. I. Rothstein, ently stable because Hudson's observations and S.S. Sweet. Their commentshave considerably were conductedmore than a century ago. improved the final version. The Frank M. Chapman Parasitismof rejecters.--Parasitismof rejecter Fund of the American Museum of Natural History speciesposes a significantand interestingevo- provided partial financial support. I was supported lutionary problem.Selection penalizes females by NSF grant BSR 82-14999 to S. I. Rothstein during that lay either egg morph when they lay in the preparationof the manuscript.Finally, I would nestsof dual rejecters.However, ovipositionin like to acknowledge H. Friedmann, whose 1929 monographand subsequentpublications served as nests of differential accepterspenalizes only my initial inspirationto undertakethis project. certain females.The situation is complex because selection occurs on a phenotype (the LITERATURE CITED morphologyof the egg) that is a productof the maternal genotype. CHESSON,J. 1978. Measuring preference in selective The frequencyof ovipositionin nestsof re- predation. Ecology59: 211-215. jecterssuggests that host race formation does FRAGA,R.M. 1978. The Rufous-collaredSparrow as a host of the Shiny Cowbird. Wilson Bull. 90: not occur in Shiny Cowbirds. Femalesthat lay 271-284. in nests of rejecters almost certainly did not 1980. The breeding of Rufous Horneros fledge from nestsof thosespecies. Similarly, it (Furnariusrufus). Condor 82: 58-68. seemsunlikely that all femaleslaying immac- 1983. Parasitismo de cria del renegrido ulate eggsin mockingbirdnests hatched from (Molothrusbonariensis) sobre el chingolo (Zono- spotted eggs laid in mockingbird nests, al- trichiacapensis): nuevas observaciones y conclu- though this possibility cannot be ruled out. In siones. Hornero 12, No. Extraord.: 245-255. addition, productionof cowbird offspringmay 1985. Host-parasite interactions between be highestfor hostsof low-preferenceranking. Chalk-browed Mockingbirds and Shiny Cow- Fraga(1985) presenteddata that more cowbirds birds. Pp. 829-844 in Neotropicalornithology (P. A. Buckley, M. S. Foster, E. S. Morton, R. S. fledged from nests of Rufous-collaredSpar- Ridgely, and F. G. Buckley, Eds.). Ornithol. rows than of Chalk-browed Mockingbirds,and Monogr. No. 36. he argued that most females laying in mock- FRIEDMANN,H. 1929. The cowbirds:a study in the ingbird nests probably fledged from nests of biology of social parasitism. Baltimore, C. C. other species. Thomas. The host selectionmechanism is clearly un- 1963. Host relations of the parasiticcow- der natural selection,but responseto selection birds. U.S. Natl. Mus. Bull. 233: 1-276. requires(among other things) that there exist , & L. F. KIFF. 1985. The parasitic cowbirds and their hosts. Proc. West. Found. Vert. Zool. 2: proximatecues for ultimate success.Apparent- 226-302. ly, these cuesare often lacking, at least in this --, & S. I. ROTHSTEIN. 1977. A further environment. Some features of Shiny Cowbird contribution to knowledge of the host relations parasitism,such as inclusion of the sparrow in of the parasitic cowbirds. Smithsonian Contr. the array of host choices,appear to be subtle Zool. 235: 1-75. adaptations(Mason 1986).Other features,such GIBSON,E. 1918. Further ornithological notes from as the parasitismof rejecters,remain perplex- the neighbourhoodof Cape San Antonio, Prov- ing. ince of BuenosAyres. Ibis 4, 4th Ser.: 1-38. January1986] HostSelection byShiny Cowbirds 67

GOCHFELD,M. 1979ß Broodparasite and host coevo- PEET,R. K. 1974. The measurement of speciesdi- lution: interactionsbetween Shiny Cowbirdsand versity.Ann. Rev. Ecol.Syst. 5: 285-307. two speciesof meadowlarks. Amer. Natur. 113: PgREZRIVERA, R. A. 1983. An account of bird para- 855-870. sitismby the Shiny Cowbirdin PuertoRico. E1 HUDSON,W. H. 1874. Notes on the procreant in- C6ndor: revista de la Universidad del Turabo 8: stinctsof the three speciesof Molothrusfound in 57-71. BuenosAyres. Proc.Zool. Soc.London: 153-174. RAPPORT,D. J., & J. E. TURNER. 1970. Determination ß 1920. Birds of La Plata, vol. I. New York, E. of predatorfood preferences.J. Theoret.Biol. 26: P. Dutton. 365-372. IVLEV,V.S. 1961. Experimentalecology of the feed- gOTHSTEIN,S.I. 1971. Observationand experiment ing of fishes(D. Scott,trans.). New Haven, Con- in the analysisof interactionsbetween brood necticut, Yale Univ. Press. parasitesand their hosts.Amer. Natur. 105:71- JAENIKE,J. 1980. A relativistic measure of variation 74. in preference.Ecology 61: 990-997. 1975. An experimental and teleonomic in- KING,J. R. 1973. Reproductiverelationships of the vestigationof avianbrood parasitism. Condor 77: Rufous-collaredSparrow and the Shiny Cow- 250-271. bird. Auk 90: 19-34. 1976. Cowbird parasitism of the Cedar MANOLIS,T.D. 1983. Host relationshipsand repro- Waxwingand its evolutionaryimplications. Auk ductivestrategies of the Shiny Cowbird in Trin- 93: 498-500. idad and Tobago. Unpublished Ph.D. disserta- 1977. Cowbird parasitism and egg recog- tion, Boulder, Colorado, Univ. Colorado. nition of the Northern Oriole. Wilson Bull. 89: MASON,P. 1980. Ecologicaland evolutionary as- 21-32. pectsof hostselection in cowbirds.Unpublished SALVADOR,S. A. 1983. Parasitismo de crla del re- Ph.D. dissertation, Austin, Texas, Univ. Texas. negrido (Molothrusbonariensis) en Villa Maria, 1985. The nesting biology of some passer- C6rdoba,Argentina (Aves:Icteridae). Hist. Nat. ines of BuenosAires, Argentina. Pp. 954-972 in 3: 149-158. Neotropical ornithology (P. A. Buckley, M. S. SCLATER,P. L., & W. H. HUDSON. 1888. Argentine Foster, E. S. Morton, R. S. Ridgely, and F. G. ornithology.Vol. I, Passeres.London, R. H. Por- Buckley,Eds.). Ornithol. Monogr. No. 36. ter. ß 1986. Brood parasitismin a host generalist, SICK,H. 1958. Notas bio16gicass6bre o gaud6rio, the Shiny Cowbird: I. The quality of different "Molothrusbonariensis" (Gmelin) (Icteridae, Aves). speciesas hosts.Auk 103: 52-60. Rev. Brasil. Biol. 18: 417-431. MURDOCH,W. W. 1969. Switchingin generalpred- SIEGEL,S. 1956. Nonparametricstatistics for the be- ators: experiments on predator specificity and havioral sciences. New York, McGraw-Hill. stability of prey populations.Ecol. Monogr. 39: SOKAL,R. R., & F. J. ROHLF.1981. Biometry,2nd ed. 335-354. San Francisco, W. H. Freeman. NICE,M.M. 1937. Studiesin the life history of the WHITTAKER,R. H. 1975. Communities and ecosys- Song Sparrow, part I. Trans. Linnaean Soc. New tems, 2nd ed. New York, Macmillan Publ. Co. York 4: 1-247. WILEY,J.W. 1985. Shiny Cowbird parasitismin two PAYNE,R. B. 1977. The ecologyof brood parasitism avian communities in Puerto Rico. Condor 87: in birds. Ann. Rev. Ecol. Syst.8: 1-28. 165-176. 68 P^ut.M^SON [Auk,Vol. 103

APPENDIX. Parasitizedand unparasitizednests.

No. of cowbird eggs per nestb Wing- length* Species 0 1 2 3 4 5 6 7 8 9 10 Freq.c Int. • A. Distribution, frequency,and intensity of parasitism 126 Chalk-browed Mockingbird (Mimus saturninus) Site I (n = 30) 8 8 6 5 0 0 0 1 1 0 1 0.73 2.73 Site II (n = 38) 10 9 6 6 3 3 1 0.74 2.57 125 Rufous-bellied Thrush (Turdusrufiventris) Site I (n = 4) 0 2 1 0 0 1 1.00 2.25 Site II Speciesabsent 123 Creamy-belliedThrush (T. amaurochalinus) SiteI(n= 1) 0 1 1.00 1.00 Site II Speciesabsent 119 White-rumped Swallow (Tachycinetaleucorrhoa) SiteI(n= 1) 1 0.00 -- Site II (n = 9) 5 3 1 0.44 1.25 112 Fork-tailedFlycatcher (Tyrannussavana)' Site I No nests examined Site II (n = 7) 3 2 1 1 0.57 1.75 107 Rufous Hornero (Furnariusrufus)' SiteI(n= 1) 0 1 1.00 1.00 Site II (n = 16) 13 3 0.19 1.00 104 Shiny Cowbird (Molothrusbonariensis) 99 Cattle Tyrant (Machetornis rixosus) SiteI(n= 1) 0 1 1.00 1.00 Site II (n = 2) 1 0.50 1.00 93 Bay-wingedCowbird (Molothrus badius) Site I (n = 10) 8 2 0.20 1.00 Site II (n = 5) 3 2 0.40 1.00 90 Yellow-browed Tyrant (Satrapaicterophrys) Site I (n = 2) 2 0.00 -- Site II (n = 8) 4 4 0.50 1.00 70 Freckle-breasted Thornbird ( Phacellodomusstriaticollis ) Site I (n = 3) 3 0.00 -- Site II (n = 4) 3 1 0.25 1.00 67 Rufous-collaredSparrow (Zonotrichiacapensis ) Site I (n = 32) 29 3 0.09 1.00 Site II (n = 19) 11 5 1 2 0.42 1.63 66 Saffron Finch (Sicalisfiaveola ) Site I (n = 13) 12 1 0.08 1.00 Site II (n = 22) 22 0.00 -- January 1986] Host Selectionby ShinyCowbirds 69

APPENDIX. Continued.

No. of cowbird eggs per nestb Wing- length' Species 0 1 2 3 4 5 6 7 8 9 10 Freq.c Int. a

51 House Wren (Troglodytesaedon) Natural nest sites Site I (n = 3) 3 0.00 -- Site II (n = 8) 3 3 1 1 0.63 1.60 Nest boxes SiteI(n= 1) 1 0.00 -- Site II (n = 5) 0 0 2 0 2 0 0 1 1.00 3.80 B. Speciesnot observedto be parasitizedf 138 Brown-chestedMartin (Phaeoprognetapera) (0, 1) 129 Great Kiskadee(Pitangus sulphuratus) ø (0, 7) 85 Firewood-gatherer(Anumbius annumbi) (0, 11) 81 Vetmillion Flycatcher(Pyrocephalus rubinus) (0, 22) 73 House Sparrow (Passerdomesticus) (0, 3) 71 Grassland Yellow-Finch (Sicalisluteola) (0, 3) 69 Hooded Siskin (Carduelismagellanica) (0, 7) 64 Tufted Tit-Spinetail (Leptasthenuraplatensis) (1, 3) 63 Wren-like Rushbird (Phleocryptesmelanops) (0, 22) 63 Little Thornbird (Phacellodomussibilatrix) (1, 0) 57 GrasslandSparrow (Ammodramushumerails) (0, 1) 51 MaskedGnatcatcher (Polioptila dumicola) (0, 1) 48 White-crestedTyrannulet (Serpophagasubcristata) (1, 3) ? TropicalKingbird (Tyrannusmelancholicus) (1, 0) ? Pipit (Anthussp.) (0, 1) aSpecies listed by decreasingwinglength (measuredfrom skins of adult femalescollected in BuenosAires Province; Mason 1980). bSpotted eggsonly. cFreq. = frequency(proportion of nestsparasitized). a Int. = intensity (mean number of eggsper parasitizednest). • Tyrannussavana and Furnariusrufus rejected cowbird eggs.Pitangus sulphuratus may show intermediate levelsof rejection.Some birds were not testedfor responseto cowbirdeggs (Mason 1986). eSample sizesfor sitesI and II in parentheses.