Obligate Brood Parasites Show More Functionally Effective Innate Immune Responses: An Eco-immunological Hypothesis

Immune adaptations of obligate brood parasites attracted interest when three New World cowbird species (Passeriformes, Icteridae, genus Molothrus) proved unusually resistant to West Nile virus. We have used cowbirds as models to investigate the eco-immunological hypothesis that species in parasite-rich environments characteristically have enhanced immunity as a life history adaptation. As part of an ongoing program to understand the cowbird immune system, in this study we measured degranulation and oxidative burst, two fundamental responses of the innate immune system. Innate immunity provides non-specific, fast-acting defenses against a variety of invading pathogens, and we hypothesized that innate immunity experiences particularly strong selection in cowbirds, because their life history strategy exposes them to diverse novel and unpredictable parasites. We compared the relative effectiveness of degranulation and oxidative burst responses in two cowbird species and one related, non-parasitic species. Both innate immune defenses were significantly more functionally efficient in the two parasitic cowbird species than in the non-parasitic red-winged blackbird (Icteridae, Agelaius phoeniceus). Additionally, both immune defenses were more functionally efficient in the brown-headed cowbird (M. ater), an extreme host-generalist brood parasite, than in the bronzed cowbird (M. aeneus), a moderate host-specialist with lower exposure to other species and their parasites. Thus the relative effectiveness of these two innate immune responses corresponds to the diversity of parasites in the niche of each species and to their relative resistance to WNV. This study is the first use of these two specialized assays in a comparative immunology study of wild avian species.     

Egg Discrimination and Sex-Specific Pecking Behaviour in Parasitic Cowbirds

We studied egg‐pecking behaviour in males and females of three cowbird species: the shiny cowbird (Molothrus bonariensis), a host generalist brood parasite, the screaming cowbird (M. rufoaxillaris), a host specialist brood parasite, and the bay‐winged cowbird (Agelaioides badius), a non‐parasitic species. We conducted three experiments in which we offered each bird an artificial nest with two plaster eggs and recorded whether egg pecking occurred and the number of pecks on each egg. In expt 1, we tested if there were species and sex differences in egg‐pecking behaviour by offering the birds two spotted eggs of similar pattern. Shiny and screaming cowbirds responded in 40.3% and 44% of the trials, respectively, with females and males presenting similar levels of response. In contrast, bay‐winged cowbirds did not show any response. In expt 2, we tested if shiny cowbirds responded differentially when they faced a choice between one host and one shiny cowbird egg, while in expt 3, we tested if screaming cowbirds responded differentially when they faced a choice between one shiny and one screaming cowbird egg. Shiny cowbirds pecked preferentially host eggs while screaming cowbirds pecked more frequently shiny cowbird eggs. Our results show that egg‐pecking behaviour is present in both sexes of parasitic cowbirds, but not in non‐parasitic birds, and that parasitic cowbirds can discriminate between eggs of their own species and the eggs of their hosts or other brood parasites.     

Do shiny cowbird females adjust egg pecking behavior according to the level of competition their chicks face in host nests?

Interspecific brood parasites, like the shiny cowbird (Molothrus bonariensis), lay eggs in nests of other species. Shiny cowbird females peck and puncture eggs when they parasitize host nests. This behavior increases the survival of cowbird chicks when they have to compete for food with larger nestmates. However, cowbird chicks may benefit from smaller nestmates as they increase food provisioning by parents and the cowbird chicks secure most extra provisioning. We investigated whether egg-pecking behavior by female shiny cowbirds might be adjusted to the competition that their chicks face in host nests. We found that more host eggs are destroyed per cowbird egg laid in a larger-bodied host (chalk-browed mockingbird, Mimus saturninus, 70-75 g) than a smaller-bodied host (house wrens, Troglodytes aedon, 12-13 g). We also tested egg-pecking preferences in choice experiments with female cowbirds in captivity and found cowbirds presented with eggs in artificial nests pecked first and more frequently, and punctured more frequently the larger egg when this was a host egg, but not when this was a cowbird egg. Our results are partially consistent with the hypothesis that shiny cowbird females adaptively adjust their egg pecking behavior according to the competition that their chicks face in host nests.     

Differences in morphology and colour pattern of shiny cowbird (Molothrus bonariensis) eggs found in nests of two hosts

Genetic differentiation among shiny cowbird (Molothrus bonariensis) females that use different hosts indicates that in this brood parasite, host use is not random at an individual level. We tested whether there exist differences in morphology and coloration between eggs of shiny cowbirds laid in the nests of two different hosts, the chalk‐browed mockingbird (Mimus saturninus) and the house wren (Troglodytes aedon). We took morphometric measures of shiny cowbird eggs found in nests of mockingbirds and wrens and analysed their coloration using digital photography and reflectance spectrometry. We found that shiny cowbird eggs found in mockingbird nests were wider and more asymmetric than those found in wren nests. In addition, cowbird eggs coming from mockingbird nests were brighter and had higher relative red reflectance than those coming from wren nests. Our results show that shiny cowbird eggs laid in nests of two different hosts vary in shape and background colour, but not in spotting pattern. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 838–845.     

Host switching in cowbird brood parasites: how often does it occur?

Avian obligate brood parasites lay their eggs in nests of host species, which provide all parental care. Brood parasites may be host specialists, if they use one or a few host species, or host generalists, if they parasitize many hosts. Within the latter, strains of host‐specific females might coexist. Although females preferentially parasitize one host, they may occasionally successfully parasitize the nest of another species. These host switching events allow the colonization of new hosts and the expansion of brood parasites into new areas. In this study, we analyse host switching in two parasitic cowbirds, the specialist screaming cowbird (Molothrus rufoaxillaris) and the generalist shiny cowbird (M. bonariensis), and compare the frequency of host switches between these species with different parasitism strategies. Contrary to expected, host switches did not occur more frequently in the generalist than in the specialist brood parasite. We also found that migration between hosts was asymmetrical in most cases and host switches towards one host were more recurrent than backwards, thus differing among hosts within the same species. This might depend on a combination of factors including the rate at which females lay eggs in nests of alternative hosts, fledging success of the chicks in this new host and their subsequent success in parasitizing it.     

Host–parasite coevolution beyond the nestling stage? Mimicry of host fledglings by the specialist screaming cowbird

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.     

BEHAVIORAL DEFENSES AGAINST AVIAN BROOD PARASITISM IN SYMPATRIC AND ALLOPATRIC HOST POPULATIONS

The brown-headed cowbird (Molothrus ater) is a widespread, obligate brood parasite of North American passerine birds. In southern Manitoba, where hosts are sympatric with cowbirds, American robins (Turdus migratorius) ejected parasitic eggs from all experimentally parasitized clutches (N = 25) and no eggs were accepted for more than four days. In contrast, robins in northern Manitoba, an area where cowbirds do not breed, accepted parasitic eggs in 33% of nests (N = 18) for at least five days. Acceptance of experimental cowbird eggs by a second host, the yellow warbler (Dendroica petechia), was similar in allopatric (100% of 20 nests) and sympatric (88.6% of 35 nests) populations, but models of a female cowbird elicited greater nest defense by warblers in the area of sympatry. Neither host rejected eggs of conspecifics, thus, rejection of cowbird eggs was not an epiphenomenon of conspecific brood parasitism. These results support the hypothesis that recognition of cowbirds and their eggs evolved as adaptations to counter cowbird parasitism and not some other selection pressure. The expression of anti-parasite defenses by some individuals within allopatric populations further suggests these traits may be controlled genetically but persist in such areas either through the continued introgression of rejecter genes from sympatric populations or because of the low cost of rejection behavior when parasitism is absent or rare.     

Small hosts infrequently disrupt laying by Brown-headed Cowbirds and Bronzed Cowbirds

ABSTRACT Brood parasites often must overcome host defenses that may include behaviors that serve other functions, such as deterrence of predators and nest attendance during laying and incubation. Host use by brood parasites may also be influenced by competitors in areas where more than one parasitic species occurs. We identified the degree to which behavior of potential hosts and potential competitors affected laying by Brown‐headed Cowbirds (Molothrus ater) and Bronzed Cowbirds (M. aeneus) at a site in south Texas where they co‐occur. We watched potential host nests during the presunrise period to record cowbird laying and document nest visitation, laying, cowbird‐host encounters, and nest attentiveness by hosts. Hosts were frequently at their nests when cowbirds laid eggs (83% of 121 watches among nests of five host species) and cowbirds regularly encountered hosts (43–74% and 40–77% of watches per species of host for Brown‐headed and Bronzed cowbirds, respectively). Host nest defense infrequently interfered with cowbird laying and cowbirds rarely interacted with one another during laying. Overall, 12% of the 42 cowbird laying attempts that elicited host nest defense failed, resulting in cowbird eggs either laid atop hosts as they sat in nests or laid outside the nest cup. We clearly documented that relatively small hosts can thwart parasitism by cowbirds. Thus, the potential for successful defense of nests should be considered when assessing the evolution of behaviors to deter the removal of host eggs by cowbirds and mechanisms leading to nest abandonment. Regarding the latter, the presence of a cowbird at a nest would be a poor indicator for parasitism as some laying attempts were thwarted and unparasitized broods were reared at those nests. Despite the potential for nest defense to affect host use by cowbirds, we did not detect an effect of nest defense. Because most host defense was ineffective, we examined hypotheses for the timing of cowbird laying and host nest attendance. Our analysis of time of day of laying by Brown‐headed Cowbirds at our site and data compiled from the literature suggests that laying time is best predicted by the time of civil twilight (first light) rather than sunrise.     

Partial host fidelity in nest selection by the shiny cowbird (Molothrus bonariensis), a highly generalist avian brood parasite

Obligate avian brood parasites can be host specialists or host generalists. In turn, individual females within generalist brood parasites may themselves be host specialists or generalists. The shiny cowbird Molothrus bonariensis is an extreme generalist, but little is known about individual female host fidelity. We examined variation in mitochondrial control region sequences from cowbird chicks found in nests of four common Argentinean hosts. Haplotype frequency distributions differed among cowbird chicks from nests of these hosts, primarily because eggs laid in nests of house wrens Troglodytes aedon differed genetically from those laid in nests of the other three hosts (chalk-browed mockingbird Mimus saturninus, brown-and-yellow marshbird Pseudoleistes virescens, and rufous-collared sparrow Zonotrichia capensis). These differences in a maternally inherited marker indicate the presence of a nonrandom laying behaviour in the females of this otherwise generalist brood parasite, which may be guided by choice for nest type, as house wrens nest in cavities whereas the other three species are open cup nesters.     

Molecular tracking of individual host use in the Shiny Cowbird – a generalist brood parasite

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120‐bp fragment of the mtDNA control region, we reconstructed full‐sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk‐browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.     

Do Hosts Discriminate between Sexually Dichromatic Male and Female Brown‐headed Cowbirds?

Female brood parasites are recognized as threats to reproductive success by many host species. Male brood parasites may accompany females while they search for nests to parasitize and males depredate nests throughout the nesting cycle. Hence, selection may also favour recognition of males. We examined whether two common host species perceive male brown-headed cowbirds ( Molothrus ater ) as brood parasites, as nest predators, or neither. We quantified visits of male cowbirds to nests of yellow warblers ( Dendroica petechia ) and red-winged blackbirds ( Ageliaus phoeniceus ) to assess the frequency with which these host species interact with male cowbirds. Males were observed near nests during hosts' laying and incubating stages, although less frequently than female cowbirds. No visits by cowbirds occurred while parents cared for nestlings. We then presented models of male and female cowbirds plus a non-threatening control to yellow warblers and red-winged blackbirds during laying and nestling periods. If hosts perceive males and females similarly, they should respond more intensely to the cowbird models during the laying period, when nests are most likely to be parasitized. Both species responded similarly to male and female cowbird models during laying, which suggests that hosts view cowbirds of both sexes as threats. The responses of yellow warblers with nestlings to male cowbirds were strongly influenced by the order of model presentation. Warblers first presented with the male cowbird gave much reduced anti-parasite responses than those that first interacted with the female then the male cowbird. These results suggest that yellow warblers recognized male vs. female cowbirds, but that discrimination was not expressed during laying. By contrast, red-winged blackbirds did not discriminate between male and female cowbirds at either nesting stage.     

Shiny cowbird Molothrus bonariensis egg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size affects chick growth and survival and it is an important component of reproductive success. The shiny cowbird Molothrus bonariensis is an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that differ markedly in body size: the chalk‐browed mockingbird Mimus saturninus (70–75 g), and the house wren Troglodytes aedon (12–13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were five days of age, but there was no association between egg size and growth rate, or asymptotic mass. There were no sexual differences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Differences in egg size between hosts and the effect of egg size on body mass after hatching support the hypothesis that different females are specialized in the use of hosts that differ in body mass.     

Does Nest Defence Deter or Facilitate Cowbird Parasitism? A Test of the Nesting-cue Hypothesis

The nesting-cue hypothesis poses that avian brood parasites use nest-defence responses directed toward them by hosts as cues to locate nests to parasitize. Hosts that respond more intensely to brood parasites should provide more cues about nest location than those hosts giving lower intensity responses. Thus, the nesting-cue hypothesis predicts that within a species parasitized nest owners should respond more intensely than unparasitized nest owners to cowbirds perched near and away from nests. This assumes that hosts respond to cowbirds when they are encountered away from the nest and that host responses gradually increase in intensity as the cowbird nears the nest. The nesting-cue hypothesis, its assumptions and prediction were tested using six host species of the brown-headed cowbird (Molothrus ater, Icterinae). We presented a female cowbird model at three distances from host nests and compared the responses elicited. All species responded to the cowbird at all distances, which supports the first assumption. Some of the rarely elicited responses (e.g. contacts) and the proximity of the host to the model varied significantly with distance, which suggests that cowbirds could use nest defence by the host as cues to the location of an active nest. However, parasitized nest owners did not respond more intensely than unparasitized nest owners to the cowbird positioned at any of the distances from the nest, which does not support the nesting-cue hypothesis itself. Further considerations are discussed that suggest that nest defence is not likely to be used as a nest-location cue.     

Multiple effects of brood parasitism reduce the reproductive success of prothonotary warblers, Protonotaria citrea

Avian brood parasitism often has multiple negative effects on the reproductive success of the host. Most studies have focused on one or two of these effects, but rarely have they all been studied simultaneously for one species. I studied prothonotary warblers to quantify the effects of different intensities of (i.e. multiple) brood parasitism by brown-headed cowbirds, Molothrus ater, on the production of host and cowbird young and on the between-year returns of adult warblers. Host clutch size decreased with an increase in the number of cowbird eggs laid in nests. The hatching success of warbler and cowbird eggs decreased with increased cowbird eggs in nests, but was always higher for cowbird eggs than warbler eggs. The survival of warbler nestlings, but not cowbird nestlings, decreased with increased cowbird nestlings in the brood. An increase in the number of cowbird nestlings in broods resulted in a reduction in the average mass of warbler nestlings but not cowbird nestlings. The number of cowbird eggs or nestlings present did not affect nest predation, and the fledging of cowbirds did not influence the renesting interval of female warblers. In addition, the between-year returns of adult warblers were not negatively affected by brood parasitism. Decreased hatching success and nestling survival reduced the reproductive output of the warblers the most. These effects were substantial and appear to favour the evolution of behavioural responses that reduce the effects of brood parasitism on prothonotary warblers. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour      

Predicting the responses of native birds to transoceanic invasions by avian brood parasites

Three species of brood parasites are increasingly being recorded as transoceanic vagrants in the Northern Hemisphere, including two Cuculus cuckoos from Asia to North America and a Molothrus cowbird from North America to Eurasia. Vagrancy patterns suggest that their establishment on new continents is feasible, possibly as a consequence of recent range increases in response to a warming climate. The impacts of invasive brood parasites are predicted to differ between continents because many host species of cowbirds in North America lack egg rejection defenses against native and presumably also against invasive parasites, whereas many hosts of Eurasian cuckoos frequently reject non‐mimetic, and even some mimetic, parasitic eggs from their nests. During the 2014 breeding season, we tested the responses of native egg‐rejecter songbirds to model eggs matching in size and color the eggs of two potentially invasive brood parasites. American Robins (Turdus migratorius) are among the few rejecters of the eggs of Brown‐headed Cowbirds (M. ater), sympatric brood parasites. In our experiments, robins rejected one type of model eggs of a Common Cuckoo (C. canorus) host‐race, but accepted model eggs of a second cuckoo host‐race as well as robin‐mimetic control eggs. Common Redstarts (Phoenicurus phoenicurus), frequent hosts of Common Cuckoos in Eurasia, rejected ～50% of model Brown‐headed Cowbird eggs and accepted most redstart‐mimetic control eggs. Our results suggest that even though some hosts have evolved egg‐rejection defenses against native brood parasites, the invasion of brood parasites into new continents may negatively impact both naïve accepter and coevolved rejecter songbirds in the Northern Hemisphere.     

UV reflectance of eggs of brown-headed cowbirds (Molothrus ater) and accepter and rejecter hosts

Many hosts of obligate brood parasites accept parasitic eggs despite the high costs of parasitism. Acceptance is particularly perplexing in brown-headed cowbird (Molothrus ater) (hereafter “cowbird”) hosts because the eggs of cowbirds and most hosts do not appear to match closely in visual characteristics detectable by humans. However, recent evidence suggests that parasite and host eggs may match in their ultraviolet (UV) reflectance, undetectable by humans, and that birds may use UV signals for egg discrimination. We determined whether egg colour matching in UV reflectance separates accepters and rejecters of cowbird parasitism by comparing the total UV (300–400 nm) reflectance of the eggs of 11 host species to cowbird eggs. Eggs of three of five accepter species and five of five rejecter species differed significantly from cowbird eggs in UV reflectance. We found no significant difference in the UV reflectance of the eggs of three closely related pairs of accepter and rejecter species. There also was no significant difference in the UV reflectance of cowbird eggs laid in nests of five host species, and the UV reflectance of cowbird eggs was not significantly correlated with that of host clutches. Thus, we found no support for the UV-matching hypothesis in brown-headed cowbirds and UV reflectance does not appear to separate accepters and rejecters of parasitism. Differences in UV reflectance between cowbird and host eggs, however, provide potential cues for use in egg discrimination. Experimental testing is needed to determine the relative importance of UV reflectance compared to other visual cues.     

Function of egg punctures by Shiny Cowbirds in parasitized and nonparasitized Creamy‐bellied Thrush nests

ABSTRACT Avian brood parasites usually remove or puncture host eggs. Several hypotheses have been proposed to explain the function of these behaviors. Removing or puncturing host eggs may enhance the efficiency of incubation of cowbird eggs (incubation‐efficiency hypothesis) or reduce competition for food between cowbird and host chicks in parasitized nests (competition‐reduction hypothesis) and, in nonparasitized nests, may force hosts to renest and provide cowbirds with new opportunities for parasitism when nests are too advanced to be parasitized (nest‐predation hypothesis). Puncturing eggs may also allow cowbirds to assess the development of host eggs and use this information to decide whether to parasitize a nest (test‐incubation hypothesis). From 1999 to 2002, we tested these hypotheses using a population of Creamy‐bellied Thrushes (Turdus amaurochalinus) in Argentina that was heavily parasitized by Shiny Cowbirds (Molothrus bonariensis). We found that 56 of 94 Creamy‐bellied Thrush nests (60%) found during nest building or egg laying were parasitized by Shiny Cowbirds, and the mean number of cowbird eggs per parasitized nest was 1.6 ± 0.1 (N= 54 nests). At least one thrush egg was punctured in 71% (40/56) of parasitized nests, and 42% (16/38) of nonparasitized nests. We found that cowbird hatching success did not differ among nests where zero, one, or two thrush eggs were punctured and that the proportion of egg punctures associated with parasitism decreased as incubation progressed. Thus, our results do not support the incubation‐efficiency, nest‐predation, or test‐incubation hypotheses. However, the survival of cowbird chicks in our study was negatively associated with the number of thrush chicks. Thus, our results support the competition‐reduction hypothesis, with Shiny Cowbirds reducing competition between their young and host chicks by puncturing host eggs in parasitized nests.     

Phylogenetic analysis of life-history adaptations in parasitic cowbirds

Parasitic cowbirds lay eggs in the nests of other species anddupe them into caring for their young. Unlike other brood parasites,cowbirds have not developed egg mimicry or bizarre chick morphology.However, most of them parasitize a large number of hosts. Severalfeatures of cowbirds have been proposed as more general adaptationsto brood parasitism. In this study, we used a recent molecularphylogeny as a historical framework to test the possible adaptationsof the parasitic cowbird, including egg size, eggshell thicknessand energy content of the eggs, length of the incubation period,and growth pattern of cowbird nestlings. We used a recentlydeveloped extension of independent contrasts to test whetherthe five cowbird species deviate from general allometric equations.We generated prediction intervals for a nonparasite that evolvedin the place of the cowbirds. By using these prediction intervals,we found that parasitic cowbirds had not reduced weight or energycontent of their eggs, nor their incubation period over evolutionarytime. Cowbird chicks and those of nonparasitic relatives hadsimilar growth pattern. The only characteristic that separatedparasitic cowbirds from their nonparasitic relatives was anincrease in eggshell thickness. All these findings were robustand resisted the use of three models of character evolution.The fact that most traits exhibited by cowbirds were inheritedfrom a nonparasitic ancestor does not rule out that they areadvantageous for parasitism. Future research should focus onsuch traits of cowbird relatives and on how these traits preadapteda particular lineage to become parasites.