The Condor 96:716-721 0 The Cooper Ornithological Society 1994 BROOD PARASITISM OF THE SHINY COWBIRD, 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, Argentina Abstract. We studied the relationship between a generalist brood parasite, 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- bird; 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 parental care. It is found in about 1% of bird dered Blackbird, Agelaius xanthomus (Post and species in seven different taxa. Those taxa are Wiley 1977) and the Yellow-hooded Blackbird, two subfamilies of cuckoos(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 honeyguides (Indicatoridae), the Black-headed moist grasslandsin the east-northeast of Argen- Duck (Anatidae), and the cowbirds (Icterinae) tina, Uruguay and adjacent areas of Brazil 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 Andes 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 m 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 ink 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.