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Home , Brood (comics), Shiny cowbird

The Condor 96:716-721 0 The Cooper Ornithological Society 1994

BROOD PARASITISM OF THE SHINY , MOLOTHRUS BONARIENSIS, ON THE BROWN-AND-YELLOW MARSHBIRD, PSEUDOLEISTES VIRESCENS

MYRIAM E. MERMOZ AND JUAN C. REBOREDA Laboratorio de Biologiiade1 Comportamiento, Institute de Biologia y Medicina Experimental-CONICET, Vuelta de Obligado 2490, 1428 BuenosAires,

Abstract. We studied the relationship between a generalist , the Shiny Cowbird (Molothrus bonariensis)and one of its hosts, the Brown-and-Yellow Marshbird (Pseudoleistesvirescens). Seventy-four percent of the nestsfound were parasitized. Although the parasite lays both white and spotted eggs,most of the cowbird eggsfound in the nests of this host were spotted. Artificial parasitism experiments showed that the host rejected the cowbird white eggs.Shiny cowbird parasitism reducedthe nesting successof the Brown- and-Yellow Marshbird mainly by puncturesor cracksof the host eggs.The reduction of the nesting successof the parasite was due mostly to the loss of eggsin multiple parasitized nests. Shiny cowbird chicks were not outcompeted for food although they are smaller than the host chicks. The Brown-and-Yellow Marshbird appearsto be a very good host, capable of rearing up to four cowbird chicks in a nest. We compared the nesting successof the Shiny Cowbird in Brown-and-Yellow Marshbird nests with its nesting successin the other sym- patric hosts studied. Key words: Broodparasitism;nesting success; Shiny cowbird;Brown-and- YellowMarsh- ; Molothrus bonariensis;Pseudoleistes virescens.

INTRODUCTION Rufous-collared Sparrow, Zonotrichia capensis Interspecific brood parasitism is a breeding sys- (Sick 1958; Ring 1973; Fraga 1978, 1983), the tem in which the parasite lays its eggsin the nest Chalk-browed Mockingbird, Mimus saturninus of another species,the host, which performs all (Salvador 1984, Fraga 1985) the Yellow-shoul- the . It is found in about 1% of bird dered , Agelaius xanthomus (Post and species in seven different taxa. Those taxa are Wiley 1977) and the Yellow-hooded Blackbird, two subfamilies of (Cuculinae and Neo- Agelaius icterocephahts (Cruz et al. 1990). morphinae), two types of finches (Anomalospiza The Brown-and-Yellow Marshbird, Pseudole- imberbis and whydahs in the Viduinnae), the istes virescens (Icterinae), inhabits marshes and (Indicatoridae), the Black-headed moist grasslandsin the east-northeast of Argen- Duck (Anatidae), and the (Icterinae) tina, and adjacent areas of and (Payne 1977, Rothstein 1990). it is sympatric with the Shiny Cowbird in all its Within the cowbirds, the Shiny Cowbird, Mol- distribution (Ridgely and Tudor 1989). Although othrus bonariensis, is one of the most ubiquitous Brown-and-Yellow Marshbirds have been men- species.It is widespread through most of South tioned as highly parasitized by the Shiny Cow- America, with major gapsin the Amazonian for- bird (Gibson 19 18, Hudson 1920, Orians 1985), ests, High and the south of Patagonia there are no reports of this speciesrearing cow- (Friedmann 1929, Fraga 1985, Wiley 1985). Its bird chicks successfully. eggs have been found in the nests of 201 bird In eastern Argentina, southeast of Brazil and speciesand at least 53 of them have reared its Uruguay, Shiny Cowbird eggscan be either white- chicks successfully(Friedmann and Kiff 1985). immaculate or spotted (Hudson 1874, Fried- However, the impact of Shiny Cowbird par- mann 1929). Spotted eggshave a white, pale gray asitism on the nesting successof its hosts has or pale blue background color and a variable only been studied in detail in four species:the pattern of gray and reddish-brown spots.Brown- and-Yellow Marshbird eggshave also reddish- brown spots, similar to the Shiny Cowbird’s 1Received 15 November 1993. Accepted 18 April spotted morph. It has been suggestedthat this 1994. host acceptsthe spotted cowbird eggsbut rejects

[7161 COWBIRD PARASITISM ON THE BROWN-AND-YELLOW MARSHBIRD 717 the white ones (Hudson 1874, Friedmann et al. if more than two adults performed active mob- 1977, Orians 1985). bing during our visits to the nest. As the nests In this paper,we estimatethe incidence of Shiny were not in clumps, it is unlikely that our activ- Cowbird parasitism on the Brown-and-Yellow ities elicited mobbing behavior by close neigh- Marshbird and analyze its effect on the nesting bors. successof the host. We also analyze the nesting In the artificial parasitism experiments, a nat- successof the Shiny Cowbird in Brown-and-Yel- ural cowbird white egg was added to complete low Marshbird nests and compare it with the clutches (3-6 host eggs)and was considered as nesting successin the other sympatric hostsstud- accepted if it remained in the nest for at least ied. five days after the experimental introduction (Rothstein 1975). Values with means are stan- METHODS dard error of the means. The study was performed near General Lavalle (36”25’ South, 56”55’ West), in the province of Buenos Aires, Argentina from October to De- RESULTS cember 1992. This is a flat, low, marshy region, The incidence of Shiny Cowbird parasitism on with little of the land rising more than 10 the Brown-and-Yellow Marshbird was 74.3% (55 above sea level. The native vegetation is com- out of 74 nests were parasitized). Twenty-two posed of short grassspecies with scatteredpatch- out of the 47 nests found during the egg stage es of woodland of Celtis tala and Jodina rhom- were multiple parasitized(10 with 2 cowbird eggs, bifolia trees in the higher areas. 7 with 3, 3 with 4 and 2 with 5). On average, the We followed the fatesof 74 Brown-and-Yellow number of eggslaid in parasitized nestswas 1.87 Marshbird nests. Sixty-six nestswere found dur- f 0.17. Most of the cowbird eggs found were ing the building or eggstage and eight during the spotted.We observed white cowbird eggsin only nestling stage.The nestswere found mainly along 3 of 47 nests and in one of these nests the white the sides of an unpaved road parallel with an egg disappeared later. The percentage of white artificial drainage canal (Canal 2). They were built eggsin the 47 nests was 4.5 (4 of 88 eggs). To in thistles (Cynara cardunculus, Carduus sp.), test if this low percentagecould be the result of black rushes (Juncus acutus), pampa grasses host rejection, we made artificial parasitism ex- (Cortaderia selloana) and cat-tails (Typha sp.) periments with white eggsin nests that had been between 0.5 and 1.5 m in height. In most cases, naturally parasitized with spotted eggs. In five nestswere not in close proximity to one another. out of six cases,the white egg was rejected (in We marked the nests with a coded tag and we three cases before 24 hr and in the other two visited them every other day when possible. The between the fourth and the fifth day), but in all eggswere checkedfor cracksor punctures,marked the casesthe spotted egg remained in the nest. with water proof and measured with calipers There were no differences in width and length (long and wide) to the nearest0.1 mm. The chicks between the white introduced eggsand the spot- were marked with waterproof ink or color bands ted eggsthat were in the nest. and were weighed with Pesola scalesto the near- We did not detect any cases of rejection of est 0.5 g until they fledged. All the nests were spotted eggs and they were accepted although checkeduntil they either fledgedyoung or failed. parasite eggswere shorter (22.7 ? 0.082 mm, n We considered as parasitized those nests that = 121 vs. 25.96 k 0.07, n = 302, t-test P = had cowbird eggs or nestlings at any stage. A 0.00 1) and narrower (18.07 * 0.05 mm, n = 12 1 parasite egg was considered as accepted if it vs. 19.09 + 0.04, n = 302, t-test P = 0.001) than hatched or if it stayed in the nest at least five the host eggs. days. On the other hand, it was considered as The main factor which affectedthe nesting suc- rejected if it disappeared from the nest without cessof Brown-and-Yellow Marshbirds was pre- any sign of disturbance (e.g., a new parasitic egg, dation. Only 22 (4 non-parasitized and 18 par- disappearanceof another egg, etc.). asitized) out of 74 nests (29.7%) produced Brown-and-Yellow Marshbirds have helpers fledglings. The other 52 nests were abandoned at the nest (Orians et al. 1977). We did not make and/or depredated during the egg (n = 42) or direct observations of nest-provisioning activi- nestling (n = 10) stage.There was no difference ties but we considered that the nest had helpers in the percentage of abandonment and/or pre- 718 MYRIAM E. MERMOZ ANDJUAN C. REBOREDA

lining is reddish. Shiny cowbird nestlings have 0 Non-parasitized flesh-colored skin too, but with scatteredtufts of blackish down. The oral flangesrange from white 4- sj Parasitized to yellow and the mouth lining is reddish. The T host and the parasite chicks stayed in the nest during 11-13 days and their mass at day nine $ 3- was 45.1 f 1.24 (range 33-58, II = 23) and 38 f + 1.2 g (range 3 1.5-47, n = 14) respectively. The 2 2- mass of an adult Brown-and-Yellow Marshbird is approximately 80 g whereas the mass of an l- adult Shiny Cowbird (Molothrus bonariensis)is: males = 55.5 + 0.88 g (n = 21), females = 45.6 f 0.69 g(n = 31). 0’ The main factors that affectedthe nesting suc- Eggs Nestlings Ili!iFledglings cess of the parasite in the 18 parasitized nests that produced fledglings were the loss of eggsin FIGURE 1. Number (mean * SEM) Oir Brown-and- multiple parasitized nestsand the failure of some Yellow Marshbirdeggs at the time of hatching,chicks eggsto hatch. The number of eggsat the time of hatchedand fledglingsproduced in non-parasitized hatching, the number of chicks that hatched and (whitebars) and parasitized(stripped bars) nests. Sam- ple sizesfor the non-parasitizedgroup are 6, 6, and 4 the number of fledglings produced were smaller nestsand for the parasitizedgroup are 26, 26, and 18 than the number ofeggslaid (P < 0.05, Friedman nests. test and multiple comparisons). There was no difference between the number of chicks that hatched and the number of chicks that fledged dation between parasitized and non-parasitized (Fig. 2). Hatching failures were similar to the nests. observedfor the host eggsin non-parasitized nests Although the number of eggslaid by Brown- (20.6%). and-Yellow Marshbirds did not differ between We observed brood reduction in five out of 18 parasitized and non-parasitized nests (3.88 f parasitized nests and in two out of six non-par- 0.14, n = 33 vs. 4.25 + 0.3, n = 12, Mann- asitized nests.Brood reduction always took place Whitney two-tailed test, P = 0.26), the number before the fifth day after hatching. Therefore, to of eggspresent in the nest at the time of hatching analyze brood reduction we considered the nests was smaller in parasitized nests (Mann-Whitney that had not been depredated up to that day and two-tailed test, P = 0.029). Similarly, the number where at least two chicks had hatched. In the of chicks that hatched and the number of fledg- parasitized nestsbrood reduction affectedmainly lings produced was smaller in parasitized nests the host chicks. All five parasitized nestslost host (Mann-Whitney two-tailed tests, P = 0.017 and chicks (9 out of 10 chicks) but only one nest lost P = 0.032 respectively) (Fig. 1). Hatching failures parasite chicks (2 out of 11 chicks) (Fisher Exact in non-parasitized and parasitized nests were two-tailed test, P c 0.001). 21.1% and 43.1% respectively (Mann-Whitney Eight of 22 nests (2 non-parasitized and 6 par- two-tailed tests, P = 0.17). asitized) had helpers at the nest. Nests with help- Parasitism took place mainly during the laying ers produced more fledglings (host and parasite) period of the host (21 out of 28 cases). Shiny than the ones without helpers (3.25 + 0.37, n = Cowbird chicks hatched after 1 l-l 3 days of in- 8 vs. 2 f 0.36, n = 14, Mann-Whitney two-tailed cubation compared to 14-15 days for the host test, P = 0.04). All the nestsin which we observed chicks. Consequently, in most cases (18 out of brood reduction did not have helpers. 22) cowbird chicks hatched before host chicks. The nestlings of the host and the parasite are DISCUSSION clearly different in their general aspect. Brown- Several studies have analyzed the host selection and-Yellow Marshbird nestlings have flesh-col- by Shiny Cowbirds (Mason 1986b, Wiley 1988) ored skin with scatteredtufts of yellowish down, and the quality of its hosts(Sick 1958; Ring 1973; being these particularly conspicuous above the Post and Wiley 1977; Fraga 1978, 1985; Wiley eyes. The oral flanges are white and the mouth 1985, 1986; Mason 1986a). COWBIRD PARASITISM ON THE BROWN-AND-YELLOW MARSHBIRD 719

In the area where we worked, other hosts that 3 have been previously studied are the Rufous- collared Sparrow and the Chalk-browed Mock- ingbird. The incidence of parasitismin the Brown- and-Yellow Marshbird (74.3%) was similar to 2 that reported for the Rufous-collared Sparrow (72.5%, Fraga 1978) and the Chalk-browed E Mockingbird (72.5%, Fraga 1985). However, our z ’ value is probably an underestimate becausesome 1 nestsrecorded as non-parasitized could have been parasitized with white eggsthat were quickly re- jected. The 4.5% of cowbird white eggsthat we found in Brown-and-Yellow Marshbird nestswas clear- 0 ly smaller than the 50% observed in nests of the Eggs (1) IEggs (2) Nestlings Fledglings Rufous-collared Sparrow, a speciesthat accepts both morphs (Fraga 1978). The rejection ofwhite FIGURE 2. Number (mean k SEM) of Shiny Cow- bird eggs laid (Eggs l), eggs at the time of hatching eggsshown by the Brown-and-Yellow Marshbird (Eggs2), chickshatched and fledglingsproduced in the could account for this result. However, we can- 18 parasitizedBrown-and-Yellow Marshbird neststhat not exclude the possibility that the low percent- reached the fledgling stage. age of white eggsfound in the nests of this host was also due to the fact that the Shiny Cowbird females that lay white eggsavoid parasiting this host, as suggestedby Friedmann (Friedmann et et al. 1988) and therefore its eggscould crack the al. 1977). We did not observe white cowbird eggs eggs of the host during the laying or the incu- in the ground near the nests as mentioned by bation (Blankespoor et al. 1982, Roskaft et al. Hudson (1920). The percentageof cowbird white 1990). In addition, parasitic eggscould reduce eggsin Brown-and-Yellow Marshbird nests was the hatching successof the host and parasitic similar to the one observed in nestsof the Chalk- chicksmay outcompete some or all the host nest- browed Mockingbird (4.9%, Fraga 1985), anoth- lings for food. er rejecter of the cowbird white eggs(Fraga 1985, The reduction of the nesting successof the Mason 1986). Brown-and-Yellow Marshbird by Shiny Cow- The rejection of cowbird eggsby Brown-and- bird parasitism was mainly due to loss of eggs Yellow Marshbird was apparently elicited by the in parasitized nests. These lossescould be a con- difference in color between its eggsand the eggs sequenceof either the removal of intact eggsby of the parasite. As noted above Brown-and Yel- the parasite or the removal by the host of eggs low Marshbird eggsare spotted, similar to the that had been punctured or cracked by the par- Shiny Cowbird spotted morph. Cowbird spotted asite. We did not detect any effect of parasitism eggswere accepted although they were shorter on either the hatching successof the remaining and narrower than the host eggs. This suggests host eggsor the fledglingsuccess ofthe host chicks. that Brown-and-Yellow Marshbirds do not dis- Although brood reduction in parasitized nests criminate by size as does the Rufous Homero, affectedmainly the host chicks, we also observed Furnarius rufus(Mason and Rothstein 1986). We brood reduction in non-parasitized nests. There- recorded only one case of rejection of a spotted fore, brood reduction appears to happen inde- eggin one nest in which the host female laid eggs pendently of parasitism and could be related to with few, pale brown spots.In this nest, the hosts the presenceof helpers at the nests. accepted one cowbird white egg. The Brown-and-Yellow Marshbird appearsto Several studies have shown that cowbird par- be a very good host, capable of rearing up to four asitism reduces the nesting successof the host. cowbird chicks in a nest. The main reduction of The female can remove or puncture one or more the nesting successof the parasite was due to the eggs of the host (Payne 1977, Rothstein 1990, loss of eggs,probably by punctures of other cow- Sealy 1992). They also have a thicker egg shell in multiple parasitized nests. It is interest- than their host (Spaw and Rohwer 1987, Rahn ing to point out that, although cowbird chicks 720 MYRIAM E. MERMOZ AND JUAN C. REBOREDA

40 for cowbirds than the other sympatric hosts stud- A ied. The high quality as a host could be related g 35- to the presence of helpers at the nests that would E allow the Brown-and-Yellow Marshbird to rear .o, !$ 30- successfully both, the host and the parasitic chicks. =!? u” 25- ACKNOWLEDGMENTS a, LL We thank Mario Beade, Mariano Merino and Alejan- 20 I I I dro Serret from Fundacion Vida Silvestre Argentina for logisticalsupport during the field work and Gustav0 J. Femandez, Bruce Lyon and Pablo L. Tubaro for helpful discussionand comments on earlier versions of the manuscript. We also wish to thank Steve Roth- B 0.1 stein and an anonymous reviewer for helpful sugges- 2 0.08 tions. MEM was supportedby a predoctoralfellowship of the Consejo National de InvestigacionesCientificas i 0.06 y Tecnicasde la RepublicaArgentina (CONICET). JCR is Research Fellow of the CONICET. This work was 9 0.04 supportedby a CONICET grant (PID 3 180/92) to JCR.

LITERATURE CITED 0 PseudoleistesZonotrjchja Mimus BLANKE~FOOR,G. W., J. OOLMAN,AND C. UTHE. 1982. virescens capensis saturninus Eggshellstrength and cowbirdnarasitism of Red- winged Blackbirds. Auk 993363-365. FIGURE 3. Mean weight of Shiny Cowbird chicks CRUZ, A.. T. D. MANOLIS. ANDR. W. ANDREWS.1990. at day nine (A) and number of fledglingsproduced per Reproductive interactions of the Shiny Cowbird Shiny Cowbird egg laid (B) in three hosts, the Brown- Molothrusbonariensis and the Yellow-Hooded and-Yellow Marshbird Pseudoleistesvirescens, the Ru- BlackbirdAgelaius icterocephalus in Trinidad. Ibis fous-collaredSparrow Zonotrichiacupensis (data from 132:436444. Fraga 1978) and the Chalk-browed Mockingbird Mi- FRAGA,R. M. 1978. The Rufous-collared Sparrow mussaturninus (data from Fraga 1985). as a host of the Shiny Cowbird. Wilson Bull. 90: 27 l-284. FRAGA,R. M. 1983. Parasitism0de cria de1renegrido were smaller than Brown-and-Yellow Marshbird (Molothrusbonariensis) sobre el chingolo (Zono- chicks, they were not outcompeted for food as it trichiacapensis): nuevas observacionesy conclu- siones. Homer0 12:245-255. happens in nests of the Chalk-browed Mocking- FRAGA, R. M. 1985. Host-parasite interactions be- bird (Fraga 1985). We only observed two cow- tween Chalk-browed Mockingbirds and Shiny bird chicks that starved and that happened in a Cowbirds. In P. A. Buckley, M. S. Foster, E. S. nest which had five chicks, two of the host and Morton, R. S. Ridgely, and F. G. Buckley [eds.], three of the parasite. In the other nests that pro- Neotropical ornithology. Ornithological Mono- graphs No. 36. duced fledglings all the cowbird chicks fledged. FRIEDMANN,H. 1929. The cowbirds. A study in the This result contrasts with that reported for the biology of social parasitism. Charles C. Thomas other sympatric cowbird hosts that have been Publisher, Baltimore, MD. studied. Starvation of cowbird chicks is common FRIEDMANN,H., AND L. F. Kmn. 1985. The parasitic cowbirdsand their hosts.Proc. West. Found. Vert. in the nests of both, Rufous-collared Sparrow Zool. 2:226-302. and Chalk-browed Mockingbird (Fraga 1978, FRIEDMANN,H., L. E. KIFF, ANDS. I. ROTHSTEIN.1977. 1985). In the nests of these speciesthe weight of A further contribution to the knowledgeofthe host the cowbird nestlings at day nine (asymptotic relationsofthe parasiticcowbirds. Smithson. Con- weight) is smaller than in the nestsof the Brown- trib. Zool. No 235:1-75. GIBSON,E. 1918. Further ornithological notes from and-Yellow Marshbird (Fig. 3a). The nesting suc- the neighbourhoodof cape San Antonio, province cess of cowbirds, measured as fledglings pro- of Buenos Aires. Part. I, passeres.Ibis 10th Ser. duced per egg laid, is higher in the nests of the 6:363-415. Brown-and-Yellow Marshbird than in the nests HUDSON,W. H. 1874. Notes on the procreantinstinct of the three speciesof Molothrusfound in Buenos of the Rufous-collared Sparrow or the Chalk- Ayres. Proc. Zool. Sot. London. XI: 153-174. browed Mockingbird (Fig. 3b). Thus, the Brown- HUD&N, W. H. 1920. Birds of La Plata. J. M. Dent and-Yellow Marshbird has a higher “host value” and Sons, London. COWBIRD PARASITISM ON THE BROWN-AND-YELLOW MARSHBIRD 721

KING, J. R. 1973. Reproductive relationships of the R~SKAFT,E., G. H. O~ANS,AND L. D. BELFTSKY.1990. Rufous-collared Sparrow and the Shiny Cowbird. Why do Red-winged Blackbirds accept eggs of Auk 90: 19-34. Brown-headed Cowbird? Evol. Ecol. 4:35-42. MAWN, P. 1986a. Brood parasitism in a host gen- ROTHSTEIN,S. I. 1975. An experimental and teleon- eralist, the Shiny Cowbird: I the quality ofdifferent omit investigation of avian brood parasitism. speciesas hosts. Auk 103:52-60. Condor 771250-271. MASON, P. 1986b. Brood parasitism in a host gen- ROTHSTEIN,S. I. 1990. A model system for coevo- eralist, the Shiny Cowbird: II host selection. Auk lution: avian brood parasitism. Ann. Rev. Ecol. 103:61-69. Syst. 21:481-508. MASON, P., AND S. I. ROTHSTEIN.1986. Coevolution SALVADOR,S. A. 1984. Estudio de1 parasitism0 de and avian brood parasitism: cowbird eggs show cria de1renegrido (Molothrus bonariensis)en cal- evolutionary response to host discrimination. andria (Mimes saturninus),en Villa Maria, C6r- Evolution 40: 1207-l 2 14. doba. Hornero 12:141-149. O~ANS, G. H. 1985. Blackbirds of the Americas. SEARLY,S. G. 1992. Removal of Yellow Warbler egg Univ. of Washington Press, Seattle. in association with cowbird parasitism. Condor ORIANS.G. H.. C. E. 0~s. ANDK. J. 0~s. 1977. 94140-54. Helpers at the nest in some Argentine blackbirds, SICK, H. 1958. Notas biol6gicas sobre o gauderio, p. 137-l 5 1. In B. Stonehouseand C. Perrins [eds.], “Molothrus bonariensis” (Gmelin) (Icteridae, Evolutionary ecology.The Macmillan PressLTD, Aves). Rev. Brasil. Biol. 18:41743 1. London. SPAW,C. D., AND S. ROHWER. 1987. A comparative PAYNE,R. B. 1977. The ecologyof brood parasitism study of eggshellthickness in cowbird and other in birds. Ann. Rev. Ecol. Svst. 8: l-28. uasserines.Condor 89:307-3 18. POST,W., ANDJ. W. WILEY. 1977. Reproductive in- W&Y, J. W. 1985. Shiny Cowbird parasitism in two teractions of the Shiny Cowbird and the yellow- avian communities in . Condor 87: shoulderedblackbird. Condor 79: 176-l 84. 165-174. RAHN, H., L. CURRAN-Evaanrr, AND D. T. BOOTH. WILEY, J. W. 1986. Growth of Shiny Cowbird and 1988. Eggshelldifferences between parasitic and host chicks. Wilson Bull. 98: 126-l 3 1. nonparasitic Icteridae. Condor 90~962-964. WILEY,J. W. 1988. Host selectionby the Shiny Cow- RIDGELY,R. S., AND G. TUDOR. 1989. The birds of bird. Condor 90:289-303. . Vol. I, The oscinepasserines. Ox- ford University Press, Oxford, U.K.