Cowbird Nestlings Research Articles

Host avian species and environmental conditions influence the microbial ecology of brood parasitic brown-headed cowbird nestlings: What rules the roost?

The role of species interactions, as well as genetic and environmental factors, all likely contribute to the composition and structure of the gut microbiome; however, disentangling these independent factors under field conditions represents a challenge for a functional understanding of gut microbial ecology. Avian brood parasites provide unique opportunities to investigate these questions, as brood parasitism results in parasite and host nestlings being raised in the same nest, by the same parents. Here we utilized obligate brood parasite brown-headed cowbird nestlings (BHCO; Molothrus ater) raised by several different host passerine species to better understand, via 16S rRNA sequencing, the microbial ecology of brood parasitism. First, we compared faecal microbial communities of prothonotary warbler nestlings (PROW; Protonotaria citrea) that were either parasitized or non-parasitized by BHCO and communities among BHCO nestlings from PROW nests. We found that parasitism by BHCO significantly altered both the community membership and community structure of the PROW nestling microbiota, perhaps due to the stressful nest environment generated by brood parasitism. In a second dataset, we compared faecal microbiotas from BHCO nestlings raised by six different host passerine species. Here, we found that the microbiota of BHCO nestlings was significantly influenced by the parental host species and the presence of an inter-specific nestmate. Thus, early rearing environment is important in determining the microbiota of brood parasite nestlings and their companion nestlings. Future work may aim to understand the functional effects of this microbiota variability on nestling performance and fitness.

Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite.

Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird (Molothrus ater), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a 'just right' intermediate brood size) of cowbird survival in nests of prothonotary warbler (Protonotaria citrea) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival.

Preferential Begging Responses of Shiny Cowbirds to the Conspecific Chatter Call

Avian brood parasites lay their eggs into the nests of other species, which incubate them and raise the chicks until their independence. Despite living their early weeks of life surrounded by heterospecifics, young brood parasites have the ability to recognize and associate to conspecifics after independence. It has been proposed that the initial conspecific recognition develops when a young parasite encounters a unique species-specific signal that triggers the learning of other aspects of the producer of the signal. For cowbirds (Molothrus spp.), this species-specific signal is hypothesized to be the chatter call. Young birds also could express auditory biases, which in some cases lead to discrimination in favor of conspecific songs. Therefore, the perceptual selectivity for chatters might be also present in nestlings. Our aim was to assess if nestlings of the shiny cowbird (M. bonariensis) present a preferential begging response to conspecific chatter calls. We evaluated if they respond more to the parasitic vocalization than host chicks and if they respond more to the chatter than to heterospecific nonhost calls. We tested shiny cowbird chicks reared by chalk-browed mockingbirds (Mimus saturninus) or house wrens (Troglodytes aedon) and host chicks, as control species. We randomly presented to 6-day-old chicks the following playback treatments: (1) conspecific chatter calls, (2) host calls, used as positive controls, and (3) nonhost calls, used as negative control. We measured if chicks begged during the playback treatments and the begging intensity. When responding to the playback of chatter calls, shiny cowbird chicks begged at a higher frequency and more intensively than host chicks. Shiny cowbird chicks reared by mockingbirds begged more intensively to playbacks of conspecific chatter calls than to host calls, while those reared by wrens begged with a similar intensity to playbacks of conspecific chatter and host calls. On the contrary, wren nestlings begged more intensively to playbacks of the wren call than to chatter calls. Mockingbird nestlings did not beg during any treatment. None of the three species begged during the playback of nonhost calls. Our results show that the chatter call produced a preferential begging response in cowbird nestlings, which may be the beginning of a process of conspecific recognition.

Brood parasitic nestlings benefit from unusual host defenses against botfly larvae (Philornis spp.)

Brood parasitic birds lay their eggs into the nests of other birds, abandoning parental care of their nestlings to the unsuspecting hosts. Parasite and host nestlings may themselves be parasitized by botfly larvae (Philornis: Muscidae), which burrow under the nestlings’ skin and can seriously affect growth and survival. Here, we provide the first direct evidence that adult baywings (Agelaioides badius), the primary host of the specialist brood parasitic screaming cowbird (Molothrus rufoaxillaris), regularly remove botfly larvae from their own and parasitic nestlings by pulling them out of the nestlings’ skin. This is the only bird species known to remove botfly larvae. By combining nestling cross-fostering with video recording of baywing nests, we show that due to prompt removal, infection with botfly larvae had negligible effects on nestling growth and survival despite high prevalence. Our results provide the first direct observations for larva removal behavior in botfly hosts. Screaming cowbirds may benefit from using baywings as its main host, as larva removal by adult baywings reduces the costs of botfly parasitism. Infection by botfly larvae of the genus Philornis (Muscidae) causes nestling mortality in many Neotropical birds. Despite the lethal effects, most Philornis hosts studied so far lack specific defenses against these larvae. The grayish baywing (Agelaioides badius), primary host of the brood parasitic screaming cowbird (Molothrus rufoaxillaris), is the only species that, based on indirect evidence, would be able to remove Philornis larvae from infected nestlings. We provide the first direct evidence that adult baywings do indeed remove botfly larvae from their own nestlings as well as from parasitic cowbird nestlings and that this unusual defense may increase the survival of own and screaming cowbird nestlings at infected nests.

Non-mimetic shiny cowbird nestlings escape discrimination by baywings in absence of host nest mates

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.

Innate development of acoustic signals for host parent–offspring recognition in the brood‐parasitic Screaming CowbirdMolothrus rufoaxillaris

Young birds communicate their need to parents through complex begging displays that include visual and acoustic cues. Nestlings of interspecific brood parasites must ‘tune’ into these communication channels to secure parental care from their hosts. Various studies show that parasitic nestlings can effectively manipulate host parental behaviour through their begging calls, but how these manipulative acoustic signals develop in growing parasites remains poorly understood. We investigated the influence of social experience on begging call development in a host‐specialist brood parasite, the Screaming CowbirdMolothrus rufoaxillaris. Screaming Cowbird nestlings look and sound similar to those of the primary host, the Greyish BaywingAgelaioides badius. This resemblance is likely to be adaptive because Baywings discriminate against fledglings unlike their own and provision nests at higher rates in response to Baywing‐like begging calls than to non‐mimetic begging calls. By means of cross‐fostering and playback experiments, we tested whether the acoustic cues that elicit recognition by Baywings develop innately in Screaming Cowbird nestlings or are acquired through social experience with host parents or nest mates. Our results suggest that begging call structure was partially modulated by experience because Baywing‐reared Screaming Cowbird and host nestlings were acoustically more similar as age increased, whereas acoustic similarity between cross‐fostered and Baywing‐reared Screaming Cowbird nestlings decreased from 4–5 to 8–10 days of age. Cross‐fostered Screaming Cowbirds developed begging calls of lower minimum frequency and broader bandwidth than those of Baywing‐reared Screaming Cowbirds by the age of 8–10 days. Despite the observed differences in begging call structure, however, adult Baywings responded similarly to begging calls of 8‐ to 10‐day‐old cross‐fostered and Baywing‐reared Screaming Cowbirds, suggesting that these were functionally equivalent from the host's perspective. These findings support the idea that, although rearing environment can influence certain begging call parameters, the acoustic cues that serve for offspring recognition by Baywings develop in young Screaming Cowbirds independently of social experience.

Resistance is futile: prohibitive costs of egg ejection in an obligate avian brood parasite host

Most hosts of the parasitic brown-headed cowbird, Molothrus ater, demonstrate an astonishing lack of defences against parasitism, typically explained by evolutionary lag. We investigated antiparasite strategies of the dickcissel, Spiza americana, whose apparent acceptance of parasitism is unlikely to be explained by lag because its historic centre of abundance overlaps with that of the cowbird. Cowbirds parasitized almost half of dickcissel nests (343 of 767 nests), and dickcissels suffered significant costs when attempting to eject cowbird eggs. Our predicted responses indicated that acceptance of parasitism would lead to the loss of 0 hosts eggs, attempted ejections would lead to the loss of 1.2 host eggs and successful ejection of cowbird eggs would lead to the loss of 1.6 host eggs. There was no significant cost of raising a single cowbird nestling, but parasitized nests had 1.1 fewer host eggs due to removal by female cowbirds or when the thick-shelled cowbirds eggs struck the host eggs during laying. After accounting for damaged eggs that still hatched, acceptance of parasitism yielded a loss of 1.1 eggs/nestlings, those that attempted to eject the cowbird egg lost 1.8 eggs/nestlings and those that ejected the cowbird egg lost 2.0 eggs/nestlings. The prohibitive costs of egg ejection combined with the relatively low costs of raising a cowbird nestling may explain why most dickcissels (64%) accepted parasitism or stopped trying to eject cowbird eggs. However, some birds persisted in their ejection attempts, so there are likely additional carryover fitness effects on hosts of raising and sharing nests with cowbirds. Because of the difficulty in ejecting cowbird eggs, dickcissels would benefit from a strategy that emphasizes frontline defences to prevent parasitism from occurring in the first place.

Host provisioning behavior favors mimetic begging calls in a brood-parasitic cowbird

Brood-parasitic screaming cowbird nestlings manipulate host parent care via a begging call structure that matches that of the host’s own young. Using playback of calls at nests, we investigated the role of begging call structure in stimulating parental provisioning by a host (baywing) to 2 brood parasites. Baywings provisioned more in response to screaming cowbird calls, which closely match those of its own young, than to shiny cowbird calls, which do not.

A Test of the Nestling Discrimination Hypothesis for Parasitism of Red-Winged Blackbirds (Agelaius phoeniceus) by Brown-headed Cowbirds (Molothrus ater)

ABSTRACT We tested the nestling discrimination hypothesis by observing responses of female Red-winged Blackbirds (Agelaius phoeniceus) to playback of begging calls by conspecifics or Brown-headed Cowbirds (Molothrus ater). Female redwings departed the nest to forage and returned with food significantly faster in response to redwing playback than to cowbird playback. We also observed feeding rates at redwing nests before and after we removed either a redwing or cowbird nestling. Female redwing feeding rates changed significantly with these manipulations, but there was no difference in response to redwing versus cowbird nestling removal. Although the removal experiment shows that parasitized and unparasitized broods receive equal care from their hosts, the playback results are consistent with the hypothesis that female Red-winged Blackbirds detect a difference between begging calls of conspecifics and parasites.

Mismatched begging displays between foreign and host offspring reduce brood parasite fitness

Obligate brood parasites must obtain food from unrelated host parents during the developmental period, and this may be especially difficult when parasitic nestlings are raised alone. Unlike specialist brood parasites, generalist brood parasites use a single begging display for a range of host species. In this study, we 1) tested whether the begging display of the generalist brown-headed cowbird (Molothrus ater) was more exaggerated than the closely related, nonparasitic red-winged blackbird (Agelaius phoeniceus) and 2) examined how begging displays were linked to food provisioning when both foreign species were raised by the Bell’s vireo (Vireo bellii), a small cowbird host. We also quantified parental provisioning to unparasitized broods containing 1–4 vireo nestlings to provide a reference provisioning curve to provide context for assessing the feeding rate to foreign offspring. We found blackbirds begged as intensely as (or more intensely than) cowbird nestlings, vireo parents provisioned both species similarly, and the amount of food provisioned to unparasitized broods increased with brood size and maintained per-capita vireo growth. However, feeding of a single foreign nestling was significantly less than that of a modal brood of 4 vireos, indicating parents fed foreign nestlings at submaximal rates. Vireos did not respond to begging of foreign nestlings in the same manner as their own offspring, and this led to significant fitness costs for foreign nestlings. We conclude that a single foreign nestling provided an inadequate stimulus for vireos, probably because of a mismatch between the begging displays of foreign and host offspring.

Sampling locality is more detectable than taxonomy or ecology in the gut microbiota of the brood-parasitic Brown-headed Cowbird (Molothrus ater)

Brown-headed Cowbirds (Molothrus ater) are the most widespread avian brood parasite in North America, laying their eggs in the nests of approximately 250 host species that raise the cowbird nestlings as their own. It is currently unknown how these heterospecific hosts influence the cowbird gut microbiota relative to other factors, such as the local environment and genetics. We test a Nature Hypothesis (positing the importance of cowbird genetics) and a Nurture Hypothesis (where the host parents are most influential to cowbird gut microbiota) using the V6 region of 16S rRNA as a microbial fingerprint of the gut from 32 cowbird samples and 16 potential hosts from nine species. We test additional hypotheses regarding the influence of the local environment and age of the birds. We found no evidence for the Nature Hypothesis and little support for the Nurture Hypothesis. Cowbird gut microbiota did not form a clade, but neither did members of the host species. Rather, the physical location, diet and age of the bird, whether cowbird or host, were the most significant categorical variables. Thus, passerine gut microbiota may be most strongly influenced by environmental factors. To put this variation in a broader context, we compared the bird data to a fecal microbiota dataset of 38 mammal species and 22 insect species. Insects were always the most variable; on some axes, we found more variation within cowbirds than across all mammals. Taken together, passerine gut microbiota may be more variable and environmentally determined than other taxonomic groups examined to date.

Does Brood Parasitism Induce Paternal Care in a Polygynous Host?

AbstractRed‐winged blackbirds (Agelaius phoeniceus) are a polygynous songbird with facultative biparental care, and a common host for brown‐headed cowbirds (Molothrus ater), an obligate brood parasite. We examined brood parasitism and paternal care in a long‐term study of parental care in red‐winged blackbirds. The presence of a cowbird nestling was associated with a higher likelihood of paternal care by the host male redwing in both naturally and experimentally parasitized nests. This result indicates that it was the presence of the brood parasite that was important and not simply that brood parasites chose hosts where paternal care was more likely. Both male and particularly female redwings increased provisioning to parasitized broods. Our work suggests that brood parasites raise the cost of parental care and push a polygynous host species toward monogamy.

Brood parasitism causes female-biased host nestling mortality regardless of parasite species

Inequality in male and female numbers may affect population dynamics and extinction probabilities and so has significant conservation implications. We previously demonstrated that Brown-headed Cowbird Molothrus ater brood parasitism of Song Sparrows Melospiza melodia results in a 50% reduction in the proportion of female host offspring by day 6 post-hatch and at fledging, which modelling demonstrated is as significant as nest predation in affecting demography. Many avian brood parasites possess special adaptations to parasitize specific hosts so this sex-ratio effect could be specific to the interaction between these two species. Alternatively, perturbations associated with brood parasitism per se (e.g. the addition of an extra, larger, unrelated nestling), rather than a Cowbird nestling specifically, may be responsible. We experimentally eliminated the effects of Cowbird-specific traits by parasitizing nests with a conspecific nestling rather than a Cowbird, while otherwise emulating the circumstances of Cowbird parasitism by adding an extra, larger (2-day-older), unrelated Song Sparrow nestling to Song Sparrow nests. Our parasitism treatment led to few host offspring deaths and no evidence of male-biased sex ratios by day 6 post-hatch. However, after day 6, female nestling mortality rates increased significantly in experimentally parasitized nests, resulting in a 60% reduction in the proportion of females fledging. Cowbird-specific traits are thus not necessary to cause female-biased host nestling mortality and far more general features associated with Cowbird parasitism instead appear responsible. Our results suggest, however, that Cowbird-specific traits may help accelerate the pace of female host deaths. The conservation implications of our results could be wide reaching. Cowbirds are unrelated to all their hosts, are larger than the great majority, and a Cowbird nestling's presence can mean there is an extra mouth to feed. Thus, sex-biased mortality in parasitized nests could be occurring across a range of host species.

Flange Color Differences of Brood Parasitic Brown-Headed Cowbirds from Nests of Two Host Species

Abstract We compared the red, green, and blue color values from digital photographs of the rictal flanges of nestling Brown-headed Cowbirds (Molothrus ater), a generalist obligate brood parasite, in sympatric Yellow Warbler (Setophaga petechia) and Song Sparrow (Melospiza melodia) nests at Mono Lake, California, USA. We detected significant differences in all three color components across nestlings of different species (R: P < 0.0001; G: P < 0.0001; B: P < 0.0001), but differences among cowbird nestlings from the nests of these two hosts were not significant (R: P = 0.543; G: P = 0.737; B: P = 0.319). Principal components results were mixed: Principal Component I described brightness and accounted for 84% of the variance. It did not differ among cowbird nestlings from nests of different hosts (P = 0.319). Principal Component II described chromaticity and accounted for 14% of the variance, which differed significantly among cowbird nestlings from the two different hosts' nests (P = 0.026). Color ...

Rejection of brood-parasitic shiny cowbird Molothrus bonariensis nestlings by the firewood-gatherer Anumbius annumbi?

Costs imposed by brood parasitic birds exert strong selection on their hosts to avoid parasitism. While egg rejection is a common defence, nestling rejection is rarer and less well understood. Theoretical models suggest that among non-evicting parasites such as cowbirds nestling rejection can only evolve when levels of parasitism are high. Here we describe a possible case of early rejection of cowbird nestlings, by an infrequently parasitised host, the firewood-gatherer Anumbius annumbi. Firewood-gatherers accepted most shiny cowbird Molothrus bonariensis eggs despite clear differences in coloration. Cowbird eggs usually hatched 4–5 d before host eggs. All parasitic nestlings died within 48 h, and hosts continued their breeding attempts. Nestling death was most likely due to neglect since little food was found in the stomach of dead nestlings. Feeding neglect could be due to differences in visual or acoustic appearance between host and parasite hatchlings. Alternatively, hosts may refrain from feeding nestlings that hatch too early compared to their normal incubation time. At the moment our data do not allow distinction between active nestling recognition or cowbird nestling failure due to the unsuitability of the firewood-gatherer as a host (i.e. too long incubation). Experiments are needed to tease these alternatives apart.

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.

Giant Cowbird (Molothrus oryzivorus) Parasitism of Red-rumped Caciques (Cacicus haemorrhous) in the Atlantic Forest, Northeastern Argentina

Abstract Host-parasite interactions between Red-rumped Caciques (Cacicus haemorrhous) and Giant Cowbirds (Molothrus oryzivorus) were studied at 52 nests in the Atlantic Forest, Misiones, Argentina. Cowbird eggs (1–6) occurred in 27 of 38 cacique nests (71%) found during the egg stage. Giant Cowbird eggs were white and unmarked (85%), or marked and spotted over pale buff (15%); the marked eggs somewhat resembled host eggs, but were twice as large. Four host and three parasite eggs were found punctured and broken. Only three cowbird nestlings were observed, and resembled oropendola (Psarocolius spp.) nestlings more than those of Red-rumped Caciques. Botflies (Philornis spp.) infested cowbird and cacique nestlings, but there was no evidence of a cowbird-cacique preening mutualism.

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 ...

Patterns of egg yolk antioxidant co-variation in an avian brood parasite–host system

Despite their importance to offspring fitness outcomes, there has been little previous work on egg-mediated maternal effects in avian brood parasites which lay their eggs in the nests of other species. Here, we examine patterns of egg yolk antioxidant deposition in an avian host (red-winged blackbird, Agelaius phoeniceus) and their brood parasite (brown-headed cowbird, Molothrus ater). Cowbird nestlings compete directly for food with host blackbird chicks and succeed, despite hatching from a smaller egg, by begging more intensely and growing as or more rapidly than their hosts. Dietary-derived antioxidants, such as carotenoids and vitamins E and A, deposited in egg yolk, may enhance growth rate and reduce oxidative stress, and therefore provide a potential mechanism by which female cowbirds could enhance the competitiveness of their young in host nests. However, provision of antioxidants to eggs is costly so we predicted that female cowbirds should adjust the amount of antioxidants in response to variation in host quality. We found that whilst red-winged blackbird clutches parasitised by cowbirds did not differ in the pattern of antioxidant deposition compared to non-parasitised clutches, levels of vitamin E deposited in cowbird eggs were closely matched to that of the individual host clutch in which cowbirds laid their egg. This supports the prediction that female cowbirds adjust the antioxidant concentration of their eggs to increase the likely competitiveness of their offspring in the host nest into which they are laid and may help explain the success of cowbirds in competing with larger host young.

Food supplementation leads to bottom‐up and top‐down food–host–parasite interactions

1. Food-prey-predator interactions may involve both 'bottom-up' and 'top-down' processes. Conventionally, food-host-parasite interactions have been seen as governed solely from the 'bottom-up', i.e. well-fed hosts can better resist parasites and so suffer less parasitism. Recent studies on diverse endo- and ecto-parasites increasingly highlight that well-fed hosts provide parasites with a better resource base, and so may be more likely to be parasitized. 2. Brood parasites exploit host parental behaviour by laying their eggs in others' nests. The brown-headed cowbird (Molothrus ater) is a North American brood parasite that exploits over 100 host species. 3. We conducted a food addition experiment on song sparrows (Melospiza melodia), a frequently parasitized cowbird host, near Victoria, BC, Canada. We expected results consistent with conventional 'bottom-up' effects because we previously found that food supplemented sparrows better eluded nest predation, and we thus also expected them to be better at eluding cowbird parasitism. 4. Here, we report results to the contrary. Food supplemented sparrows were parasitized as often as non-food supplemented sparrows, were multiply parasitized significantly more often, and suffered significantly more parasitism-induced egg loss. Our results suggest cowbirds preferentially parasitized better fed hosts and cowbirds benefited from doing so as food supplemented sparrows fledged significantly more cowbird young per multiply parasitized nest. The pattern of egg loss also accorded with recent evidence indicating that cowbirds may remove just the right number of host eggs to maximize provisioning of the cowbird nestling. 5. Our work suggests that brood parasitism in vertebrates involves both 'top-down' and 'bottom-up' processes consistent with the growing number of studies showing that food-host-parasite interactions are more complex than previously thought. One of the conservation implications of our results is that greater food availability may not provide hosts a respite from brood parasitism, but is, nonetheless, beneficial overall.