Molothrus Rufoaxillaris Research Articles

Is the Rufous Axillary Patch of the Screaming Cowbird a Secondary Sexual Character?

The screaming cowbird Molothrus rufoaxillaris is described as being sexually monochromatic, although it has a rufous axillary patch that shows inter-individual size variation. We investigated potential sex differences in patch size in 82 molecularly-sexed adults. All males exhibited large patches and 23 females had small patches; however, seven females lacked a patch and nine had large patches. Patch size was associated with sex and independent of body condition, location or date of sampling. We discuss several hypotheses that may explain the incidence of this male character in some females.

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

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.

Screaming Cowbird Parasitism of Nests of Solitary Caciques and Cattle Tyrants

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.

Reproductive Success and Nestling Growth of the Baywing Parasitized by Screaming and Shiny Cowbirds

Abstract 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 ...

Brood Parasitism Increases Mortality of Bay-Winged Cowbird Nests

Abstract. Brood-parasitic cowbirds (Molothrus spp.) can cause total nest failure directly by inducing nest desertion or by destroying the host's clutch or indirectly by facilitating nest predation. We examined the relationship between brood parasitism and nest survival in the Bay-winged Cowbird (Agelaioides badius), the primary host of the Screaming Cowbird (M. rufoaxillaris) and a secondary host of the Shiny Cowbird (M. bonariensis). We used the program MARK to model daily nest-survival rates, including hypothesized effects of intensity of parasitism, egg losses caused by cowbirds, and total clutch size. Support for each model was evaluated by an information-theoretic approach. More than 50% of the nests failed before incubation was completed, mainly because of the ejection or desertion of parasitized clutches. The model of daily nest survival with best support included the additive effects of intensity of parasitism and number of eggs lost, which were negatively related to nest survival. The model inclu...

Utilization of a new host in the screaming cowbird Molothrus rufoaxillaris, a host specialist brood parasite: host switch or host acquisition?

The screaming cowbird Molothrus rufoaxillaris has been long known as a host specialist brood parasite. However, in the past years, the utilization of two new hosts has been documented. We examined the variation in mitochondrial control region sequences from screaming cowbird chicks found in the nests of two hosts, the bay-winged cowbird (Agelaioides badius), which is its regular host, and the chopi blackbird (Gnorimopsar chopi), which is a new host, in Formosa Province, Argentina. If a group of females switched to this new host, we expected to find an association between host use and haplotype frequency distribution, indicating the presence of host-specific female lineages, whereas we expected no such association if the cowbird population incorporated this new host and females use both hosts simultaneously. Haplotype frequency distributions differed between cowbird chicks from the nests of both hosts. This indicates that nest choice by females of this brood parasite is not random and that they preferentially parasitize the nests of the same host species.

Conspecific and heterospecific social learning in shiny cowbirds

We tested whether shiny cowbirds, Molothrus bonariensis, can improve the acquisition of a feeding response by observing a demonstrator of their own species or a species with which they share roosts and foraging flocks. We trained birds to peck a transilluminated response key using a classical conditioning procedure (the key was transilluminated for 10 s and then food was delivered to a hopper). We compared the efficiency in acquiring the pecking response in four experimental groups: (1) birds that could observe a conspecific demonstrator peck the key and then eat the food, (2) birds that could observe a heterospecific demonstrator (a screaming cowbird, Molothrus rufoaxillaris) peck the key and then eat the food, (3) birds that could observe the apparatus working automatically (‘ghost’ demonstrator) and (4) birds that learned the task alone. Birds in the conspecific demonstrator group required fewer trials to learn to peck the key than did birds in the groups with a ghost demonstrator or without a demonstrator. Birds in the group with a heterospecific demonstrator also required fewer trials to learn the task than did birds in the group without a demonstrator. There were no differences between the groups with a conspecific and a heterospecific demonstrator. Our results show that shiny cowbirds can acquire behaviour by observing other individuals, and that they learn from heterospecifics with which they share roosts and foraging flocks as readily as they do from conspecifics.

Population dynamics and avian brood parasitism: persistence and invasions in a three‐species system

Summary Avian brood parasites include species that are host specialists and others that are generalists. The impact of each kind of parasite on the persistence of the host population is studied by means of a population dynamics model. Our model examines conditions for coexistence and invasions in a community of three South American cowbirds, the shiny cowbird Molothrus bonariensis (a generalist parasite), the screaming cowbird M. rufoaxillaris (a specialist parasite), and the bay‐winged cowbird Agelaiodes badius (a nonparasite that hosts the other two). Three biologically realistic characteristics not previously included in brood parasitism models are explored and shown to be crucial for the stability of the system. These characteristics are: (i) female parasites take at least a day to produce an egg and cannot store eggs for delayed laying – this is modelled by means of a type II functional response; (ii) parasites often remove or puncture (destroy) host eggs when visiting a nest; and (iii) hosts desert nests when the total clutch (host plus parasite) exceeds some threshold. These characteristics have a direct impact on parasite population renewal and reduce dramatically the stable coexistence conditions. Comparing the stability conditions of the host–specialist system with those of the three‐species system shows the impact of the arrival of a generalist parasite on the persistence of the host–specialist system when the three characteristics are present. The stability boundaries are restricted when the generalist is absent and change little for a realistic density of generalist. The study of invasion by a specialist into a host–generalist stable community shows that the parameter region for coexistence and invasion coincide. Comparison of our model against its precursors using field data for the parameters when available, shows that the three‐species cowbird model system is stable for empirically realized parameter values, unlike a previous model by May & Robinson (1985; American Naturalist, 126, 475–494) where none of the three characteristics were included.

Breeding Success of a Specialist Brood Parasite, the Screaming Cowbird, Parasitizing an Alternative Host

AbstractThe 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.

BREEDING SUCCESS OF A SPECIALIST BROOD PARASITE, THE SCREAMING COWBIRD, PARASITIZING AN ALTERNATIVE HOST

Abstract 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...

Selfish begging by screaming cowbirds, a mimetic brood parasite of the bay-winged cowbird

Screaming cowbirds, Molothrus rufoaxillaris, are specialist parasites that almost exclusively parasitize bay-winged cowbirds, Molothrus badius. Parasitic chicks are almost identical to the host young in their appearance and vocalizations until the chicks attain nutritional independence. This system provides an excellent opportunity to study the relationship between begging behaviour and hunger level for parasitic chicks and host young. Using video analysis of laboratory experiments, I compared the begging behaviour of screaming and bay-winged cowbird nestlings under controlled levels of food deprivation with and without a nestmate. The results of these analyses indicate that: (1) the begging behaviour of parasitic chicks and host young are influenced by their respective hunger levels; (2) the begging intensity of a cowbird chick is not influenced by the begging intensity of its nestmates; and (3) parasitic chicks beg more intensely than host young for the same degree of hunger. The greater selfishness of the parasitic chicks is what would be expected from theory, because, unlike host chicks, their greed will not be constrained by kinship with their nestmates or with the host parent's future broods. Alternative hypotheses explaining intense begging in terms of parasitic chicks being more hungry, or having to compensate for their alien appearance can be rejected, as the parasitic chicks and host young are similar in size and appearance.

Sexual dimorphism and species differences in HVC volumes of cowbirds.

Cowbirds exhibit extensive variation in their social, territorial, and reproductive behaviors. Nissl-stained brain sections of specimens from a previous study (J. C. Reboreda, N. S. Clayton, & A. Kacelnik, 1996) were used to study the gross anatomy of a song control nucleus in 3 South American cowbirds (bay-winged, Molothrus badius; shiny, M. bonariensis; and screaming, M. rufoaxillaris). Cowbird high vocal center (HVC) volumes were consistently higher in males than in females in all 3 species. The largest HVC size of females found in bay-winged cowbirds is consistent with observations that females of this species, but not of the other 2 species, occasionally sing. The extent of the sexual dimorphism of relative HVC size was highest for the sexually dichromatic and promiscuous shiny cowbirds and smaller for the monochromatic and monogamous bay-winged and screaming cowbirds, suggesting that selection pressures associated with morphological traits and social systems are reflected in brain architecture.

New Host for a Specialized Brood Parasite, the Screaming Cowbird

New Host for a Specialized Brood Parasite, the Screaming Cowbird

Pair Formation in Cowbirds: Evidence Found for Screaming but Not Shiny Cowbirds

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.

Bay-Winged Cowbirds (Molothrus badius) Remove Ectoparasites From Their Brood Parasites, the Screaming Cowbirds (M. rufoaxillaris)

Bay-winged Cowbirds (Molothrus badius) remove ectoparasitic larvae of botflies from their nestlings and those of their brood parasite, the Screaming Cowbird (Molothrus rufoaxillaris). The host,also removes other ectoparasites like mites. This removal behavior may be one of the factors selecting for the host specific behavior of Screaming Cowbirds, as ectoparasitism by flies and mites reach lethal levels at times.

The eggs of the parasitic Screaming Cowbird(Molothrus rufoaxillaris) and its host, the Baywinged Cowbird(M. badius): is there evidence for mimicry?

Measurements and descriptions of 239 eggs of Baywinged Cowbirds(Molothrus badius) and of 263 eggs of Screaming Cowbirds(M. rufoaxillaris) are presented; egg identification is ultimately based on nestling identification. Statistical analyses of host and parasite eggs showed no significant differences in measurements, weight and index of size. An index of shape was more useful for egg identification; eggs ofM. rufoaxillaris were more spherical, but their has been found considerable overlap. Background color and presence/absence of thick dark lines (“scrawls”) were the most useful criteria for the identification of host and parasite eggs. In this host-parasite system there is no evidence of egg mimicry, and the behaviour ofM. badius is not selecting for egg mimicry.

Further Information of the Host Relations of the Parasitic Cowbirds

THIS paper brings together a number of recent records involving additional hosts of some of the species of cowbirds, and some that amplify earlier data on previously known victims. Some of these have been gathered from the widely scattered literature of the past few years; others are unpublished records generously made available to me by their reporters. The most important single source of new data is the material collected in Oaxaca, Mexico, in 1966 by the late J. Stuart Rowley, who suggested that I use his records and add to them any other information that I might have. At this time I wish to pay my respects to this indefatigable and enthusiastic field worker, whose tragic and untimely death in Oaxaca cut short further contributions to our knowledge of Mexican birds. All the eggs, nests, and birds Rowley collected are in the museum of the Western Foundation of Vertebrate Zoology, under whose auspices his field work was accomplished. A number of other interesting records in the collection of the Western Foundation, kindly called to my attention by Lloyd F. Kiff, are also included in this report. I am indebted to the Western Foundation, and particularly to its president, Ed N. Harrison, for permission to utilize Rowley's new information and these other records in this paper. For still other host data I am indebted to the Pacific Nest Records Scheme at the University of British Columbia and the North American Nest-Record Program at Cornell University. The details that follow are best treated separately for each of the three species of parasitic cowbirds involved. No new information has come to my attention for the other two species (Molothrus rufoaxillaris and Psomocolax oryzivorus).