Brown-and-yellow Marshbird (<i>Pseudoleistes virescens</i>) as a host of the Screaming Cowbird (<i>Molothrus rufoaxillaris</i>): first evidence of fledglings’ attendance

The Screaming Cowbird (Molothrus rufoaxillaris) is a specialized brood parasite that primarily parasitizes the Bay-winged Cowbird, (Agelaioides badius; Screaming Cowbirds parasitize 80–100% of this species' nests). In contrast, the Shiny Cowbird (M. bonariensis) parasitizes more than 200 hosts. According to the differential reproductive success hypothesis, we expect that Screaming Cowbirds would have a lower reproductive success than Shiny Cowbirds when parasitizing other hosts. We assessed the breeding success of the Screaming Cowbird using an alternative host, the Brown-and-yellow Marshbird (Pseudoleistes virescens). This species is a common host of the Shiny Cowbird with 60–70% of nests parasitized, and is also regularly parasitized by the Screaming Cowbird but with lower frequency (6–20% of the nests). We compared the breeding success of Screaming and Shiny Cowbirds parasitizing this host species. No differences were found in the number of fledglings produced per egg laid between cowbird spec...

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.

The Screaming Cowbird (Molothrus rufoaxillaris) is the most specialized brood-parasitic cowbird, relying almost entirely on the Bay-winged Cowbird (Agelaioides badius) as host. Recently, Screaming Cowbirds have expanded their range to areas where Bay-winged Cowbirds are absent, and they are exploiting the Chopi Blackbird (Gnorimopsar chopi). Interactions between Screaming Cowbirds and Chopi Blackbirds are largely unexplored, as is the reproductive success of the parasite in this host. Screaming Cowbirds, Chopi Blackbirds, and Bay-winged Cowbirds coexist in northeastern Argentina, providing an ideal system to explore interactions between a specialist brood parasite and an alternative host and to compare the reproductive success of the parasite in its main host and in an alternative host. Screaming Cowbirds parasitized both hosts throughout their breeding seasons (Chopi Blackbirds, mid-October to mid-January; Bay-winged Cowbirds, mid-November to mid-March). Frequency of parasitism was lower in Chop...

The Screaming Cowbird (Molothrus rufoaxillaris) is one of the most specialized brood parasites with only three known hosts: Baywing (Agelaioides badius), the main host throughout most of its range, and two alternative hosts in some areas of its distribution, Chopi Blackbird (Gnorimopsar chopi) and Brown-and-yellow Marshbird (Pseudoleistes virescens). We studied Screaming Cowbird parasitism in northeast Argentina where this parasite uses Baywings and Chopi Blackbirds as hosts. We monitored 69 nests of Baywings, 251 of Chopi Blackbirds, 31 of Solitary Caciques (Cacicus solitarius), and 30 of Cattle Tyrants (Machetornis rixosa). The frequency of Screaming Cowbird parasitism on Baywing nests was 80% and was 46% for Chopi Blackbirds. We recorded one event of Screaming Cowbird parasitism on one nest of Solitary Caciques and three events of Screaming Cowbird parasitism on one nest of Cattle Tyrants. The identities of parasitic eggs in both hosts were confirmed by sequencing the mtDNA control region. We propose these events of parasitism resulted from recognition errors by Screaming Cowbird females that regularly parasitize Baywings and Chopi Blackbirds. The nest of Solitary Caciques had been frequently visited by a pair of Baywings before Screaming Cowbird parasitism occurred, and the nest of Cattle Tyrants was near an active Chopi Blackbird nest that had been previously parasitized by Screaming Cowbirds.

Social monogamy predominates in avian mating systems, but this strategy seems unlikely among brood parasites, where the emancipation from parental care should lead to an increase in numbers of mates for both sexes. Despite this, there is great diversity in mating systems in brood parasitic birds. The Screaming CowbirdMolothrus rufoaxillarisis a brood parasite that specializes almost exclusively on one host, the Greyish BaywingAgelaioides badius. Field observations and a radio telemetry study have indicated that male–female pairs spend a great deal of time together through the day, suggesting that they may be socially monogamous. We tested the hypothesis of social monogamy by radio‐tracking male and female Screaming Cowbirds using an array of automated radiotracking receivers to collect data on spatial behaviour and social mating system. Monogamous pair bonds were recorded through social network analysis. These bonds endured throughout the entire breeding season, with males and females remaining associated throughout the day. In accordance with the hypothesis, paired Screaming Cowbirds spent significantly more time in proximity to their mates than to other conspecifics of the opposite sex, and had similar home‐ranges. Social monogamy in the Screaming Cowbird differed markedly from the social polygyny/promiscuity exhibited by the sympatric congener Shiny CowbirdMolothrus bonariensis, a generalist brood parasite.

This chapter deals with the triangular interactions of three avian species currently classified in the genus Molothrus, a system consisting of two brood parasites sharing a host. The host species is the bay-winged cowbird (Molothrus badius), quite different from the other cowbirds because it is nonparasitic, has extensive parental care, and lives in permanent groups that show cooperative breeding (Fraga 1972, 1991, 1992). The screaming cowbird (Molothrus rufoaxillaris) is one of the most specialized avian brood parasites and has M. badius as its only host over most of its range. Lastly, the shiny cowbird (M. bonariensis) is a generalist avian parasite with hundreds of hosts (Friedmann and Kiff 1985) that regularly parasitizes M. badius. A recent phylogeny based on mitochondrial DNA (Lanyon 1992) suggests that M. badius is not closely related to the parasitic cowbirds.

We studied the breeding biology of the Baywing (Agelaioides badius), a shared host of Screaming (Molothrus rufoaxillaris) and Shiny (M. bonariensis) cowbirds. We monitored 193 nests from December 2002 to March 2007 in the Province of Buenos Aires, Argentina. Baywings used a wide variety of nesting sites, mainly old nests of furnarids. Their breeding season lasted from late November to February and was closely matched by that of Screaming Cowbirds. The breeding season for Shiny Cowbirds started in late September but overlapped that of Baywings. Frequency and intensity of Screaming Cowbird parasitism were 93% and 5 eggs per parasitized nest, while for Shiny Cowbirds they were 16% and 1.4 eggs. Host clutch size was 4.0 ± 0.1 eggs and did not vary with time of breeding. Weight at hatching and age of maximum growth were similar for host and Screaming Cowbird nestlings. Shiny Cowbird nestlings had higher weight at hatching and lower age of maximum growth than the other two species. Screaming and Shiny ...

Nestlings of obligate brood parasites must obtain resources from heterospecific hosts that are attuned to the solicitation behaviours of their own progeny. Failing to match the appropriate stimuli may result in suboptimal provisioning or even the starvation of parasite young. Parasitic nestlings could overcome it by sharing the nest with host young, as long as they are able to compete efficiently for parental feedings. Here, we examined if non-mimetic shiny cowbird (Molothrus bonariensis) nestlings reared alone fail to elicit sufficient parental care from the grayish baywing (Agelaioides badius), a host that discriminates between their own and parasitic nestlings based on species-specific begging cues. We manipulated baywing broods to assess the ability of shiny cowbird nestlings to elicit parental provisioning and survive without host nest mates. Host provisioning rates and survival did not differ between shiny cowbirds reared alone and host nestlings in singleton broods. Also, growth patterns of experimental nestlings reared alone were similar to those reported for shiny cowbirds reared alongside baywing young. Hence, we did not find support for the hypothesis that non-mimetic nestlings require the assistance of host nest mates to elicit sufficient parental care from its baywing host. Furthermore, we found that shiny cowbirds that were reared alone continued receiving care from baywings after fledging, unlike shiny cowbirds from mixed broods. Our results add evidence to the idea of opposing selective pressures on the evolution of nest mate acceptance in cowbirds and suggest context-dependent host’s discrimination abilities that deserve further investigation. The evolution of nest mate killing versus tolerance strategies is a long-standing question in the study of brood parasitism. Parasitic nestlings may benefit from sharing the nest with host young if they collectively induce more parental provisioning and the parasite can benefit from it. Host nest mates could also confer protection against discrimination when hosts respond preferentially to stimuli of their own kind. We examined this idea in a generalist brood parasite, the shiny cowbird (Molothrus bonariensis), and a host able to discriminate species-specific begging cues. The results showed that shiny cowbirds reared alone performed well and circumvent host discrimination also after leaving the nest. Our study supports the existence of trade-offs on the evolution of nest mate killing behaviours and adds evidence that context-dependent host’s discrimination abilities similar to those observed towards parasitic eggs can be at play towards parasitic fledglings.

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.

Sixty Screaming Cowbirds (Molothrus rufoaxillaris) and 150 Shiny Cowbirds (M. bonariensis) were trapped and banded in Buenos Aires Province, Argentina. Analysis of recaptures provided statistical evidence for pair formation in Screaming but not Shiny cowbirds, a result supported by observations of free-living birds. Pairs of Screaming Cowbirds were stable throughout the breeding season. They are probably monogamous. Shiny Cowbirds showed no pattern of association between the sexes. This is consistent with a promiscuous mating system, although this conclusion is tentative. I also present ancillary data on sexual size dimorphism, sex ratio, and related aspects of behavior. Both species are dimorphic in size to a similar extent, although the Screaming Cowbird is slightly larger. Neither species departs from an adult (quaternary) sex ratio of unity. Pairs of Screaming Cowbirds are conspicuous near host nests. Female Shiny Cowbirds are not accompanied by males at nests, but may be accompanied by other females. Shiny Cowbird females were surreptitious near nests. Differences in host selection behavior may have profound effects on other aspects of the species breeding biology.

Interspecific brood parasites exploit the parental care of host species. This exploitation entails fitness costs for the hosts, which favor the evolution of antiparasitic defenses. Host defenses select for counter-defenses in the parasite, which in turn select for improved host defenses; this results in a coevolutionary arms race that may operate at each stage of the nesting cycle of the host. Most studied examples of the coevolutionary arms race in brood parasites are restricted to the egg stage, with relatively few studies showing coevolution between hosts and parasites at the nestling or fledgling stages; studies on the whole set of host defenses and potential parasite counter-defenses at each stage of the breeding cycle are still lacking. Systems in which parasites are host specialists are particularly well suited to an examination of the pairwise coevolutionary interactions before, during and after host egg-laying, and how these interactions have shaped host resistance or tolerance of parasitism. The Screaming Cowbird (Molothrus rufoaxillaris) is one of the most specialized brood parasites, and mainly parasitizes nests of a single host species, the Grayish Baywing (Agelaioides badius). Parasitism rates of Grayish Baywing nests are extremely high, with most nests parasitized several times. Several traits of this host-parasite system suggest ancient coevolutionary relationships encompassing the entire nesting cycle. In this paper we summarize the main results of a long-term study on the costs of Screaming Cowbird parasitism on the Grayish Baywing’s reproductive success and how these costs have favored reciprocal adaptations and counter-adaptations at each stage of the nesting cycle.

Brood-parasitic cowbirds are hypothesized to search for and locate host nests within a relatively constant area, as this is presumed to facilitate the monitoring of nests over time and the synchronization of parasitism with host laying. We tested this hypothesis in Shiny Cowbirds (Molothrus bonariensis) and Screaming Cowbirds (M. rufoaxillaris), two cowbird species that differ in host specificity and, apparently, in social mating system, by radio-tracking females and males for 3–6 consecutive days and determining individual daily morning ranges and cumulative morning ranges. In Shiny Cowbirds, the mean size of morning daily ranges and cumulative morning ranges was larger for males than for females, but we did not find a difference between the sexes in range size for Screaming Cowbirds. In both species, there was extensive overlap in the morning ranges of individual females between consecutive days, and the addition of new area to their ranges decreased over time. For both Shiny and Screaming cowbirds, morning ranges of conspecific females radio-tracked the same day overlapped, indicating lack of territoriality. Male and female Screaming Cowbirds that were trapped together were also spatially associated during radio-tracking, indicating social monogamy. Most radio-tracked Shiny and Screaming cowbirds used mainly one roost, relatively close to their morning ranges, which was maintained throughout the breeding season. Our results show that Shiny and Screaming cowbird females use relatively constant areas for nest searching and that Screaming Cowbirds are socially monogamous.

Avian obligate brood parasites exploit the parental care of individuals of other species (hosts) that rear the parasitic offspring at the expense of their own reproductive success. The costs of parasitism select for antiparasitic defences in host populations (i.e., adaptations that reduce the impact of parasitism). This, in turn, may favour counteradaptations in the parasite population, leading to a coevolutionary arms race between parasite and host. We review the reciprocal adaptations between a specialist brood parasite, the Screaming Cowbird (Molothrus rufoaxillaris), and its primary host, the Baywing (Agelaioides badius). The defences of the Baywing include the rejection of parasitic females, a little predictable egg-laying behaviour, the rejection of whole “superparasitized” clutches, and discrimination against juveniles that do not resemble their own. These defences are partially countered by the Screaming Cowbird through an elusive behaviour and rapid egg-laying, a close monitoring of host nesting activities and the evolution of visual and vocal mimicry of host young in the parasite juveniles. These results suggest an escalated arms race between the Screaming Cowbird and the Baywing through the entire nesting cycle. Future studies of the interactions between these species before, during and after parasitism might improve the understanding of host-parasite coevolutionary processes and the evolution of specialization in host use in brood parasitic birds.

Shiny CowbirdsMolothrus bonariensisand Screaming CowbirdsMolothrus rufoaxillarisare closely related brood parasites but the former is socially polygynous or promiscuous and an extreme host generalist, whereas the latter is socially monogamous and parasitizes almost exclusively one host. Females of both species lay in relative darkness, before dawn, relying for host nest location on previous days’ prospecting activity, or possibly on following better‐informed roost associates. We studied the temporal and spatial patterns of roosting behaviour in these species to test the hypothesis that roosting behaviour of cowbirds is related to their breeding strategy (brood parasitism) and reflects differences in strategies between species. We recorded fidelity to a roost, location fidelity within a roost, inter‐individual spatial associations and timing of roost departures and parasitic events, using tagged individuals. Female Shiny Cowbirds and both sexes of Screaming Cowbirds showed marked fidelity in roosting location, and roost departures occurred both during and after the known time window for parasitism, with earlier departures probably corresponding to laying days. Screaming Cowbird females and males that were trapped together and showed high levels of association during the day, also showed high levels of association in the roost. We describe the spatial and temporal patterns of a relatively poorly known aspect of avian ecology in general and the behaviour of brood parasites in particular.

The Screaming Cowbird (Molothrus rufoaxillaris) is a specialized brood parasite that primarily parasitizes the Bay-winged Cowbird, (Agelaioides badius; Screaming Cowbirds parasitize 80–100% of this species' nests). In contrast, the Shiny Cowbird (M. bonariensis) parasitizes more than 200 hosts. According to the differential reproductive success hypothesis, we expect that Screaming Cowbirds would have a lower reproductive success than Shiny Cowbirds when parasitizing other hosts. We assessed the breeding success of the Screaming Cowbird using an alternative host, the Brown-and-yellow Marshbird (Pseudoleistes virescens). This species is a common host of the Shiny Cowbird with 60–70% of nests parasitized, and is also regularly parasitized by the Screaming Cowbird but with lower frequency (6–20% of the nests). We compared the breeding success of Screaming and Shiny Cowbirds parasitizing this host species. No differences were found in the number of fledglings produced per egg laid between cowbird species. About 8–10% of cowbird eggs produced fledglings. The daily survival rate of Screaming Cowbird eggs was higher than daily survival rates for Shiny Cowbird eggs, but no differences were detected in the nestling daily survival rates. Moreover, we could not detect any difference in the hatching success (nestlings per egg), fledging success (fledglings per nestling), or growth rates of the two parasitic cowbird chicks. Furthermore, breeding success and growth rates of Screaming Cowbird chicks were similar to those previously reported while parasitizing the Bay-winged Cowbird. Our results are not consistent with the differential reproductive success hypothesis proposed as an explanation for the specialized parasitism of Screaming Cowbirds.Éxito Reproductivo del Parásito de Cría Especialista, Molothrus rufoaxillaris, Parasitando un Hospedador AlternativoResumen. Molothrus rufoaxillaris es un parásito de cría especialista que parasita principalmente a Agelaioides badius (80–100% de los nidos son parasitados). Contrariamente, Molothrus bonariensis parasita más de 200 especies. Basándonos en la hipótesis del éxito reproductivo diferencial, esperamos que M. rufoaxillaris tenga un menor éxito reproductivo que M. bonariensis al parasitar a otros hospedadores. En este trabajo evaluamos el éxito reproductivo de M. rufoaxillaris parasitando un hospedador alternativo, Pseudoleistes virescens. Esta especie es un hospedador común de M. bonariensis con 60–70% de los nidos parasitados, y es también regularmente parasitado por M. rufoaxillaris pero con menor frecuencia (6–20% de los nidos). Comparamos el éxito reproductivo de M. rufoaxillaris y M. bonariensis parasitando esta especie. No encontramos diferencias en el número de volantones producidos por huevo puesto entre ambos parásitos. Alrededor del 8–10% de los huevos puestos por los Molothrus produjeron volantones. La tasa de supervivencia diaria de los huevos de M. rufoaxillaris fue más alta que la de huevos de M. bonariensis. Tampoco encontramos diferencias en el éxito de eclosión (pichón por huevo), éxito de emplumamiento (volantón por pichón), ni en las tasas de crecimiento de los pichones de Molothrus. Además, el éxito reproductivo de M. rufoaxillaris y las tasas de crecimiento de sus pichones fueron similares a las previamente descritas cuando parasita a A. badius. En consecuencia, nuestros resultados no avalan la hipótesis del éxito reproductivo diferencial para explicar la especialización en el parasitismo de M. rufoaxillaris.