Brown-and-yellow Marshbird (<i>Pseudoleistes virescens</i>) as a host of the Screaming Cowbird (<i>Molothrus rufoaxillaris</i>): first evidence of fledglings’ attendance
Brown-and-yellow Marshbird (Pseudoleistes virescens) as a host of the Screaming Cowbird (Molothrus rufoaxillaris): first evidence of fledglings’ attendance
Instituto de Ecología, Genética y Evolución de Buenos Aires (IEGEBA-UBA-CONICET), Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina
Mermoz, Myriam E., and Emilio M. Charnelli. 2025. “Brown-and-Yellow Marshbird (Pseudoleistes Virescens) As a Host of the Screaming Cowbird (Molothrus rufoaxillaris): First Evidence of fledglings’ Attendance”. El Hornero 40 (2): 61-67. https://doi.org/10.56178/eh.v40i2.1523.
The Screaming Cowbird (Molothrus rufoaxillaris) is a highly specialized brood parasite that primarily parasitizes the Greyish Baywing (Agelaioides badius). Being parasitized at a markedly lower frequency, the Brown-and-yellow Marshbird (‘Marshbird’, Pseudoleistes virescens) has been reported as an ‘alternative host’. However, previous studies on this alternative host ended when Screaming Cowbird fledglings left the nest. In 2024, during a study on the breeding success and survival of fledglings of Marshbirds in General Madariaga (Buenos Aires province, Argentina), we found that 15.6% of nests (n = 10) were parasitized by Screaming Cowbirds; four of which reached the fledgling stage. We recorded the post-fledgling care of two Screaming Cowbird fledglings from one nest up to 20 days after they fledged. Given that Screaming Cowbird chicks remain in the nest until they are 7-12 days old, the total care time recorded by Marshbirds for these two chicks was 32 days. This value falls within the range of 30-40 days reported for the Greyish Baywing. The reported quality (Screaming Cowbird nestlings ready to fly) of alternative hosts, including Marshbirds, is similar to that of Greyish Baywing. Therefore, the Screaming Cowbird’s preference for Greyish Baywing could be related to interspecific competition for potential hosts with the generalist brood-parasite, the Shiny Cowbird (Molothrus bonariensis).
Antonson ND, Rubenstein DR, Hauber ME, Botero CA (2020) Ecological uncertainty favours the diversification of host use in avian brood parasites. Nature Communications 11(1):4185. https://doi.org/10.1038/s41467-020-18038-y
Chaneton EJ, Perelman SB, León RJC (2005) Floristic heterogeneity of Flooding Pampa grasslands: a multi-scale analysis. Plant Biosystems 139(3):245-254. https://doi.org/10.1080/11263500500340714
De Mársico MC, Reboreda JC (2008) Differential reproductive success favours strong host preference in a highly specialized brood parasite. Proceedings of the Royal Society of London B 275(1650):2499-2506. https://doi.org/10.1098/rspb.2008.0700
De Mársico MC, Mahler B, Chomnalez M, Di Giácomo AG, Reboreda JC (2010) Host use by generalist and specialist brood-parasitic cowbirds at population and individual levels. In: Macedo R (ed). Advances in the Study of Behavior (Vol 42), Academic Press, Burlington. Pp 83-121. https://doi.org/10.1016/S0065-3454(10)42003-3
Di Giacomo AG, Reboreda JC (2015) Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird. The Auk 132(1):16-24. https://doi.org/10.1642/AUK-13-008.1
Fraga RM (1979) Differences between nestlings and fledglings of Screaming and Bay-winged Cowbirds. The Wilson Bulletin 91(1):151-154
Fraga RM (1996) Further evidence of parasitism of Chopi Blackbirds (Gnorimopsar chopi) by the specialized Screaming Cowbird (Molothrus rufoaxillaris). The Condor 98(4):866-867. https://doi.org/10.2307/1369874
Fraga RM (1998) Interactions of the parasitic Screaming and Shiny Cowbirds (Molothrus rufoaxillaris and M. bonariensis) with a shared host, the Bay-winged Cowbird (M. badius). In: Rothstein, SI Robinson SK (eds) Parasitic birds and their hosts, studies in coevolution, Oxford University Press, New York pp 173-193
Fraga RM (2008) Notes on the nesting of Chopi Blackbirds (Gnorimopsar chopi) in Argentina and Paraguay, with data on cooperative breeding and brood parasitism by Screaming Cowbirds (Molothrus rufoaxillaris). Ornitología Neotropical 19:299-303
Hudson WH (1874) Notes on the procreant instincts of the three species of Molothrus found in Buenos Ayres. Proceedings of the Zoological Society of London 42(1):153-174. https://doi.org/10.1111/j.1096-3642.1874.tb02466.x
Hudson WH (1920) Birds of La Plata. Dent and Sons Ltd, Toronto, Canada
Jaramillo A, Burke P (1999) New World blackbirds: the Icterids. Princeton University Press, Princeton, New Jersey
Lima RD (2021) An obligate brood parasite, the Screaming Cowbird (Molothrus rufoaxillaris), more than doubled its range expanding northeastward in the last decades. Ornithology Research 29(2): 63-67. https://doi.org/10.1007/s43388-021-00052-3
López-Lanús B (2008) Bird sounds from southern South America (DVD). Audiornis Producciones, Buenos Aires
Mermoz ME, Reboreda JC (1996) New host for a specialized brood parasite, the screaming cowbird. The Condor 98: 630-632
Mermoz ME, Fernández GJ (2003) Breeding success of a specialist brood parasite, the Screaming Cowbird, parasitizing an alternative host. The Condor 105(1):63-72. https://doi.org/10.1093/condor/105.1.63
Mermoz ME, Reboreda JC (2003) Reproductive success of Shiny Cowbird (Molothrus bonariensis) parasitizing the larger Brown-and-yellow Marshbird (Pseudoleistes virescens) in Argentina. The Auk 120(4):1128-1139. https://doi.org/10.1093/auk/120.4.1128
Mermoz ME, Depalma DM, Charnelli EM, Zilli C (2021a) First record of parasitism of Scarlet-headed Blackbird (Amblyramphus holosericeus) by the specialized Screaming Cowbird (Molothrus rufoaxillaris). El Hornero 36(1):91-100. https://doi.org/10.56178/eh.v36i1.434
Mermoz ME, Villarruel C, de la Colina A, Mahler B (2021b) Fledgling sex-ratio is biased towards the helping sex in a Neotropical cooperative breeder, the brown-and-yellow marshbird (Pseudoleistes virescens). Behaviour. 158(2):135-160. https://doi.org/10.1163/1568539X-bja10061
Mermoz ME, Svagelj WS, Fernández GJ (2025) Helpers-at-the nest mitigate the costs of cowbird brood parasitism in brown-and-yellow marshbirds. Ornithology Research 33(1):11. https://doi.org/10.1007/s43388-024-00218-9
Molina-Morales M, Martínez JG, Avilés JM (2016) Criteria for host selection in a brood parasite vary depending on parasitism rate. Behavioural Ecology 27(5):1441–1448. https://doi.org/10.1093/beheco/arw066
Orians GH, Orians CE, Orians KJ (1977) Helpers at the nest in some Argentine blackbirds. In: Stonehouse B Perrins C. (eds) Evolutionary Ecology. Palgrave, London. https://doi.org/10.1007/978-1-349-05226-4_13
Orians GH (1980) Some adaptations of marsh-nesting blackbirds. Vol.14 Princeton University Press, Princeton
Pantoja V, Medrano F, Tejeda I (2023) Range expansion of the Screaming Cowbird (Molothrus rufoaxillaris) mediated by a new brood parasite-host interaction in central Chile. Gayana 87(1):18-24. https://doi.org/10.4067/S0717-65382023000100018
Rothstein SI (1990) A model system for coevolution: avian brood parasitism. Annual Review of Ecology and Systematics 21:481-508
Reboreda JC, Fiorini VD, De Mársico MC, Gloag R, Scardamaglia RC (2018) Parasitic Behaviour of Interspecific Brood Parasitic Females. In: Soler, M. (eds) Avian Brood Parasitism. Fascinating Life Sciences. Springer, Cham. https://doi.org/10.1007/978-3-319-73138-4_18. Pp 325-342
Sick H (1985) Ornitologia brasileira: uma introducçao. University of Brasilia, Brasilia
Soler M (2018) Brood Parasitism in Birds: A Coevolutionary Point of View. In: Soler, M. (eds) Avian Brood Parasitism. Fascinating Life Sciences. Springer, Cham. https://doi.org/10.1007/978-3-319-73138-4_1. Pp 1-19
Spottiswoode CN, Kilner RM, Davies NB (2012) Brood parasitism. In: Royle NJ, Smiseth PT, Kölliker M (eds) The evolution of parental care. Oxford University Press, Oxford
Ursino CA, Facchinetti C, Reboreda JC (2012) Preformative molt in brood parasitic Screaming (Molothrus rufoaxillaris) and Shiny (M. bonariensis) Cowbirds. Ornitología Neotropical 23:150-168
