|range of Screaming cowbird|
The screaming cowbird (Molothrus rufoaxillaris) is an obligate brood parasite belonging to the family Icteridae and is found in South America. It is also known commonly as the short billed cowbird.
The screaming cowbird has mildly iridescent black plumage; the lesser under-wing coverts are rufous. The female is slightly duller in colour than the male. The legs are black and the iris is reddish brown. Adult body length is 18–21 cm and mean adult weight is 58 g for males and 48 g for females. The call of the screaming cowbird was first described as "impetuous screaming notes"; however, a more useful description for field identification is noisy, explosive and piercing with rasp like calls also produced  Screaming cowbirds are mostly seen in pairs or small flocks.
Distribution and habitat
Within South America, the screaming cowbird is found in north east and central Argentina, south east Bolivia, central Brazil and throughout Paraguay and Uruguay. Its natural habitat is pastureland where it forages amongst grazing animals such as cows, hence the name "cowbird". Similar to other cowbirds, it forages predominantly on the ground, eating invertebrates that have been disturbed by grazing stock. The distribution of the screaming cowbird has increased significantly in recent decades due to habitat alteration caused by deforestation and by following its hosts into new areas. Both female and male screaming cowbirds show high fidelity in roosting locations.
The screaming cowbird is a specialist brood parasite, predominantly parasitizing the nests of the bay-winged cowbird (Agelaidoides badius [formerly Molothrus badius]). In 1874, W H Hudson was first to observe this parasitic relationship when he witnessed what he believed to be bay-winged cowbird chicks morph into screaming cowbird plumage. Bay-winged cowbirds have a clutch size of 3-4 eggs; they do not build their own nests and instead mostly use the old nests of other species.
The screaming cowbird also parasitizes the nests of the chopi blackbird (Gnorimopsar chopi) and the brown and yellow marsh bird (Pseudoleistes virescens). Parasitism of these other two species generally occurs in areas where the bay-winged cowbird is absent but can also occur in its presence. The three host species that can successfully raise screaming cowbirds are all cooperative breeders. "Helpers" at the nest provide assistance with chick feeding and with predator defense.
Screaming cowbirds are monogamous and form stable pairs for the duration of the breeding season. As obligate brood parasites, they do not build their own nests, and instead, parasitize the nests of other species, predominantly the bay-winged cowbird.
Most bay-winged cowbird nests are parasitized by the screaming cowbird with parasitism rates of 74-100% recorded. Parasitism rates of 5-20% have been recorded for the brown and yellow marsh bird  and 46% for the Chopi blackbird.
Screaming cowbird eggs are spotted like those of their main host, the bay-winged cowbird, but do vary in shape, background colour and markings. Although this may be obvious when they appear in the host nest during pre-laying, they can be difficult to detect in a nest full of eggs.
Screaming cowbirds can lay 6-20 eggs in a bay-winged cowbird nest but usually one pair will lay only 2 eggs in the host nest. Up to 12 female screaming cowbirds can parasitize the same bay-wing nest.
Screaming cowbird adults frequently pierce the eggs of their hosts as well as previously laid parasite eggs. One study found 22.5% of bay-winged cowbird eggs were punctured by the screaming cowbird. Screaming cowbirds can distinguish between their eggs and those of other species and laboratory trials have shown that screaming cowbirds will puncture shining cowbird (Molothrus bonariensis) eggs more frequently than their own. The purposes of egg puncture behavior are not clearly understood but may provide the parasite with information on the embryonic development of the host eggs and therefore whether or not to parasitize. Egg puncturing may also be practiced to reduce nestling competition and to enhance survival of parasitic offspring. Heavy predation in the form of egg punctures, which result in total nest failure and nest abandonment, may also create new opportunities for screaming cowbirds in the form of new nests to parasitize.
Screaming cowbirds deceive their main host, the bay-winged cowbird, with superb visual chick mimicry. In fact, screaming cowbirds are the only avian brood parasite to exhibit this trait. There are slight differences in skin and bill colour of nestlings but this is only present for the first 4–5 days. They then remain almost identical in size and appearance until they become nutritionally independent.
Screaming cowbird chicks also mimic the begging calls of their bay-winged cowbird nest mates and, in addition, beg for longer and at a higher intensity. This more intense begging does not reflect greater hunger demands; instead, reflects a hard-wired behavior to ensure adequate nourishment and survival. Due to the mimicry of nestlings and fledglings, it has been suggested that screaming cowbirds and bay-winged cowbirds are closely related; however, molecular research has shown the species are not each other's closest relatives.
The screaming cowbird frequently parasitizes its main host, the bay-winged cowbird, during the pre-laying period. Screaming cowbirds lay 31% of their eggs before the first bay-winged cowbird egg but most of the eggs laid are ejected, and often within 24 hours. By ejecting parasitic eggs with their feet, bay-winged cowbirds can reduce the parasitic egg load by 75%. Host species can eject an entire clutch and commence egg laying again in the same nest or abandon the nest and start afresh. Chopi blackbirds and brown and yellow marsh birds have not been observed to eject screaming cowbird eggs.
The pre-laying period (the time between nest making and egg laying) of the bay-winged cowbird exhibits great variability in length (1–19 days). It is suggested that this behavior may act as an antiparasitic/defense mechanism to reduce the chances of successful parasitism. Such behaviours are a fascinating element to the co-evolutionary arms race that exists between avian brood parasites and their hosts.
The main host, the bay-winged cowbird, can successfully fledge 1 screaming cowbird for 3 of its own. Reproductive success, as the number of fledgling per egg laid, has been recorded to be 0.14 for the screaming cowbird when hosted by the bay-winged cowbird. When hosted by the chopi blackbird, a reproductive success rate of 0.17 was found  In addition, the brown and yellow marsh bird is also able to successfully rear screaming cowbird chicks.
The main host species, the bay-winged cowbird, clearly suffers losses through intense parasitism by the screaming cowbird; however, they are able to successfully raise their young with little overall impact in terms of hatching success, survival of nestlings and fledgling body mass.
- BirdLife International (2012). "Molothrus rufoaxillaris". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Fraga, R. M. (2011). Family Icteridae (New World Blackbirds). In Handbook of the Birds of the World, vol 16: Tanagers to New World Blackbirds (J. del Hoyo,. A. Elliot, and D. A. Christie, Editors). Lynx Edicions, Barcelona, Spain. pp. 684-807
- Davies, N. B. (2000). Cuckoos, Cowbirds and Other Cheats. Academic Press, London
- Hudson, W. H.; Grönvold, Henrik (1920). Details - Birds of La Plata, by W. H. Hudson, with twenty-two coloured illustrations by H. Gronvold ... - Biodiversity Heritage Library. doi:10.5962/bhl.title.13416. hdl:2027/uc2.ark:/13960/t2m61dc1r.
- Hoy, Gunnar; Ottow, Johann (1964). "Biological and Oological Studies of the Molothrine Cowbirds (Icteridae) of Argentina". The Auk. 81 (2): 186–203. doi:10.2307/4082768. JSTOR 4082768.
- Di Giacomo, Alejandro G.; Reboreda, Juan C. (2014). "Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird". The Auk. 132 (1): 16–24. doi:10.1642/AUK-13-008.1. hdl:11336/19670. ISSN 0004-8038.
- Scardamaglia, R.C.; Kacelnik, A.; Reboreda, J.C. (2018). "Roosting behaviour is related to reproductive strategy in brood parasitic cowbirds". Ibis. 160 (4): 779–789. doi:10.1111/ibi.12587.
- de Mársico, María C.; Mahler, Bettina; Reboreda, Juan C. (2010-08-24). "Reproductive Success and Nestling Growth of the Baywing Parasitized by Screaming and Shiny Cowbirds". The Wilson Journal of Ornithology. 122 (3): 417–431. doi:10.1676/09-140.1. ISSN 1559-4491.
- de Mársico, María C.; Reboreda, Juan C. (2010-05-01). "Brood Parasitism Increases Mortality of Bay-Winged Cowbird Nests" (PDF). The Condor. 112 (2): 407–417. doi:10.1525/cond.2010.090118. ISSN 0010-5422.
- Fraga, R. M. (1979). Differences between nestlings and fledglings of Screaming and Bay-winged cowbirds. Wilson Bulletin 91, 151-154
- De Mársico, María C.; Reboreda, Juan C. (2014). "High frequency but low impact of brood parasitism by the specialist Screaming Cowbird on its primary host, the Baywing". Emu. 114 (4): 309–316. doi:10.1071/mu14008.
- Fraga, R. M. (1996). Further evidence of parasitism of Chopi Blackbirds (Gnorimopsar chopi) by the specialized Screaming Cowbird (Molothrus rufoaxillaris). The Condor 98:866-867
- Mahler, B., Sarquis Adamson, Y., Di Giacomo, A. G., Confalonieri, V. A. and Reboreda, J. C. (2009). Utilization of a new host in the Screaming Cowbird Molothrus rufoaxillaris, a host specialist brood parasite: Host switch or host acquisition? Behavioral Ecology and Sociobiology 63:1603-1608
- Mermoz, Myriam E.; Reboreda, Juan C. (1996-08-01). "New Host for a Specialized Brood Parasite, the Screaming Cowbird". The Condor. 98 (3): 630–632. doi:10.2307/1369576. JSTOR 1369576.
- Mermoz, M. E., and Fernandez, G. J. (2003). Breeding success of a specialist brood parasite, the Screaming Cowbird, parasitizing an alternative host. The Condor 105:63-72
- Svagelj, Walter S.; Fernández, Gustavo J.; Mermoz, Myriam E. (2009-03-01). "Effects of nest-site characteristics and parental activity on cowbird parasitism and nest predation in Brown-and-yellow Marshbirds". Journal of Field Ornithology. 80 (1): 9–18. doi:10.1111/j.1557-9263.2009.00200.x. ISSN 1557-9263.
- Di Giacomo, Alejandro G.; Reboreda, Juan C. (2014-10-01). "Reproductive success of the specialist brood parasite Screaming Cowbird in an alternative host, the Chopi Blackbird". The Auk. 132 (1): 16–24. doi:10.1642/AUK-13-008.1. hdl:11336/19670. ISSN 0004-8038.
- Ursino, Cynthia A.; Mársico, María C. De; Sued, Mariela; Farall, Andrés; Reboreda, Juan C. (2011-08-18). "Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird". Behavioral Ecology and Sociobiology. 65 (12): 2279–2286. doi:10.1007/s00265-011-1238-7. ISSN 0340-5443.
- Mason, P. (1987). Pair formation in cowbirds; evidence found for Screaming but not Shiny Cowbirds. Condor 89:349-356.
- Hoy, Gunnar; Ottow, Johann (1964-04-01). "Biological and Oological Studies of the Molothrine Cowbirds (Icteridae) of Argentina". The Auk. 81 (2): 186–203. doi:10.2307/4082768. JSTOR 4082768.
- Davies, N. B. (2000). Cuckoos, Cowbirds and Other Cheats. Academic Press, London.
- Llambías, Paulo E.; Ferretti, Valentina; Reboreda, Juan C. (2006-11-01). "Egg Discrimination and Sex-Specific Pecking Behaviour in Parasitic Cowbirds". Ethology. 112 (11): 1128–1135. doi:10.1111/j.1439-0310.2006.01272.x. ISSN 1439-0310.
- Massoni, Viviana; Reboreda, Juan Carlos (1998-04-01). "Costs of brood parasitism and the lack of defenses on the yellow-winged blackbird - shiny cowbird system". Behavioral Ecology and Sociobiology. 42 (4): 273–280. doi:10.1007/s002650050439. ISSN 0340-5443.
- Mársico, María C. De; Gantchoff, Mariela G.; Reboreda, Juan C. (2012-09-07). "Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird". Proceedings of the Royal Society of London B: Biological Sciences. 279 (1742): 3401–3408. doi:10.1098/rspb.2012.0612. ISSN 0962-8452. PMC 3396896. PMID 22648157.
- Kruger, O. (2007). Cuckoos, cowbirds and hosts: Adaptations, trade-offs and constraints. Philosophical Transactions of the Royal Society of London, Series B 362:1873-1886.
- Lichtenstein, Gabriela (2001-06-01). "Selfish begging by screaming cowbirds, a mimetic brood parasite of the bay-winged cowbird". Animal Behaviour. 61 (6): 1151–1158. doi:10.1006/anbe.2000.1688.
- Lanyon, Scott M. (1992-01-03). "Interspecific Brood Parasitism in Blackbirds (Icterinae): A Phylogenetic Perspective". Science. New Series. 255 (5040): 77–79. Bibcode:1992Sci...255...77L. doi:10.1126/science.1553533. JSTOR 2876063. PMID 1553533.
- De Marsico, M. C., and Reboreda, J. C. (2008). Differential reproductive success favours strong host preference in a highly specialized brood parasite. Proceedings of the Royal Society of London, Series B 275:24499-2506
- Mársico, María C. De; Gloag, Ros; Ursino, Cynthia A.; Reboreda, Juan C. (2013-06-23). "A novel method of rejection of brood parasitic eggs reduces parasitism intensity in a cowbird host". Biology Letters. 9 (3): 20130076. doi:10.1098/rsbl.2013.0076. ISSN 1744-9561. PMC 3645041. PMID 23485877.
- REBOREDA, Juan C.; FIORINI, Vanina D.; MARSICO, María C. De (2013-01-01). "Antiparasitic defenses in hosts of South American cowbirds". Chinese Birds. 4 (1): 57. doi:10.5122/cbirds.2013.0003.
- Fraga, Rosendo M. (September 1984). "Bay-Winged Cowbirds (Molothrus badius) Remove Ectoparasites From Their Brood Parasites, the Screaming Cowbirds (M. rufoaxillaris)". Biotropica. 16 (3): 223–226. doi:10.2307/2388055. JSTOR 2388055.
- Mermoz, M. E., and Fernandez, G. J. (2003). Breeding success of a specialist brood parasite, the Screaming Cowbird, parasitizing and alternative host. The Condor 105:63-72.