Cowbird Parasitism Research Articles

First observations of Brown-headed Cowbird (Molothrus ater) parasitism of Black Phoebe (Sayornis nigricans) in the Sacramento Valley, California

First observations of Brown-headed Cowbird (Molothrus ater) parasitism of Black Phoebe (Sayornis nigricans) in the Sacramento Valley, California

Habitat amount and edge effects, not perch proximity, nest exposure, or vegetation diversity affect cowbird parasitism in agricultural landscapes

Context Prior research documented relationships between brown-headed cowbird (Molothrus ater) brood parasitism and edge effects, proximity of perches, and nest exposure. Those relationships have not been evaluated in agroecosystems containing extremes of fragmentation and vegetation diversity. ObjectivesWe compared three existing hypotheses on how cowbirds locate host nests with two new hypotheses regarding habitat amount and vegetation diversity to determine how the configuration and location of agricultural conservation practices affect grassland bird nest parasitism rates and predicted rates for eight common conservation practices.Methods We assessed cowbird parasitism of grassland bird nests on corn and soybean farms in Iowa, USA, and measured perch proximity, nest exposure, edge effects, habitat amount, and vegetation diversity for each nest. We fit a global generalized linear mixed-effects model and compared importance of model parameters using odds ratios. We predicted parasitism likelihood for every subset model and averaged predictions to explore individual effects.ResultsThe variables that most influenced parasitism rates included main effects for nest initiation day-of-season (OR = 0.71, CI95 = 0.60–0.84) and the landscape variables of distance to nearest crop edge (0.63, 0.51–0.76) and proportion of grass land cover within 660 m (0.75, 0.57–1.00). We found little support that perch proximity, nest exposure, or native vegetation diversity affected parasitism. We also assessed parasitism likelihood by conservation practice and found no significant differences.ConclusionsOur results provide evidence to support the edge effect and habitat amount hypotheses, but not the nest exposure, vegetation diversity, or perch proximity hypotheses.

The Prado Wetlands: 24 years of river diversion treatment wetlands reducing nitrate and phosphate while increasing an endangered bird

Nitrate pollution of surface water from farms and urban runoff is widespread – impairing drinking water supplies, recreation, and wildlife habitat. The scale of the problem in rivers has overwhelmed most watershed-based best management practices. A new paradigm is needed. In this paper we highlight the 119 ha (294 acres) Prado Wetlands in southern California, USA as an example of a gravity-enabled, free surface flow vegetated wetlands to treat a river – the nitrate-rich, effluent-dominated Santa Anna River. The wetland treats ∼1–2 m3/s of the 7.1 m3/s (250 cfs) flow of the river in 28 wetland cells in two parallel trains (residence time 2–4 d, hydraulic loading rate 10–20 cm/d). The cleaner water is returned to the river and infiltrated downstream into a managed groundwater basin supplying 2.4 million customers. Since 1998, ∼1200 t of nitrate-N have been permanently removed from the river and released as N2-gas into the air. Over 24 years, in the warm summers an average of 6.1 mg-N/L entered the wetland and 3.7 mg-N/L flowed out, a removal efficiency of 57%. In the cooler winters, an average of 6.4 mg NO3-N/L entered and 2.6 mg NO3-N/L flowed out, a removal efficiency of 45%. Individual months varied widely in concentration removal efficiency from 7% (winter) to 98% (high summer). Areal summer removal averaged 137 mg NO3-N/m2/d and the apparent “k” was around 46 m/y. All-year rates were 129 mg NO3-N/m2/d or 47 g/m2/y, a reasonable performance considering the size of the system and its configuration for both nitrate removal and duck hunting (i.e., more open water). Annual average nitrate removal efficiency increased substantially over time (45 to 72% for 1998–2004 versus 2016–22) and is attributed to an increase in hydraulic residence time, upgraded internal plumbing, and increased winter temperatures (+3C°). Prado Wetlands also removes phosphorus, though not as well as nitrate; average warm season removal efficiency was 23% (inflow 1.20 mg PO4-P/L, outflow 0.93 mg PO4-P/L). The aerial removal rate was high at 23 mg P/m2/d or 8.5 g/y, probably due to the high inflowing P-concentrations and the hard water. Wildlife was also enhanced. Least Bell's Vireo (Vireo belli pusillus), an endangered bird in the Prado Basin, increased from 19 pairs (1998) to a recent average of 444 pairs due to habitat improvement and reduction of cowbird parasitism. Results from the Prado Wetlands show that cleaning up rivers via diversion through wetlands benefits both water quality and wildlife.

Impacts of brood parasitism by shiny cowbird Molothrus bonariensis on the breeding success of a small host, the black-backed water tyrant Fluvicola albiventer.

The shiny cowbird Molothrus bonariensis parasitizes many species with different life-history traits and has a detrimental effect on the survival of the progeny of the hosts. In response, hosts have developed numerous antiparasitic defenses. Here, we examined the effects of brood parasitism by shiny cowbird on the clutch and brood sizes (83 nests) in a small host, the black-backed water tyrant Fluvicola albiventer. We also studied whether the death of parasite nestlings was related to the care of the foster parents and whether the host had any antiparasitic defense against the shiny cowbird. Our results indicate that brood parasitism significantly decreased the host hatching and fledging successes. The majority of nest failures (57%) were caused by brood parasitism. Shiny cowbird parasitism occurred in 52% of nests and the intensity of parasitism was 1.23 ± 0.53 eggs per parasitized nest. Of the total host eggs, 54% were damaged. During the incubation stage, 20 nests (47%) were abandoned because of egg punctures by shiny cowbirds females. Only two parasitic fledglings were recorded, while the remaining nestlings either died from starvation (n = 12) or predation (n = 3). Foster parents abandoned parasitic nestlings between 5 and 10 days old. Our findings demonstrate that the shiny cowbird has very low rates of fledging success when parasitizing black-backed water tyrant. Also, parasitism had a high reproductive cost in the black-backed water tyrant because a very low proportion (7%) of the parasitized nests (n = 43) were successful.

Imprinting in an interspecific brood parasitic bird.

Many animals learn to recognize conspecifics after an early experience with them through sexual imprinting. For brood parasitic birds, it is not possible to develop conspecific recognition using cues provided by their foster parents. One solution is that a unique species-specific signal triggers the learning of additional aspects of the conspecific's phenotype. It has been proposed that for brood parasitic cowbirds, this signal is an innate vocalization, the chatter. This vocalization might act in a cross-modal learning process through which juveniles that listen to the song learn to recognize the visual characteristics of the song's producer. We trained two groups of juvenile shiny cowbirds (Molothrus bonariensis). In one group, individuals listened to the chatter or a heterospecific call while they observed a stuffed model of the corresponding species. In the other group, individuals listened to the call of one species (cowbird or heterospecific) while they observed the stuffed model of the other species. In the preference test, juveniles chose the model associated with the chatter, regardless of whether the model was a cowbird or a heterospecific. These results show how the auditory system through a species-specific signal can lead to cross-modal learning of visual cues allowing conspecific recognition in brood parasitic cowbirds.

Gradual changes in model shape affect egg-directed behaviours by parasitic shiny cowbirds Molothrus bonariensis in captivity.

Eggs are critically important for avian reproduction as all birds are oviparous. Accordingly, the recognition and care of own eggs represent the cornerstones of avian breeding, whereas the elimination of foreign objects, including brood-parasitic eggs and non-egg items from the nest are known to also increase fitness by refocusing incubation effort on the parents' own eggs. But egg recognition also plays a role in some avian obligate brood parasites' reproductive strategy through the pecking of already present eggs in the hosts' clutch to reduce nestmate competition with the parasite's own hatchling. Here we tested egg shape recognition in this parasitic egg-pecking context by exposing two different series of 3D printed models to captive obligate brood-parasitic shiny cowbirds (Molothrus bonariensis) in artificial nests. Natural egg-shaped models were pecked more often compared with increasingly thinner models, but there was no effect of increasing angularity on pecking rates, implying that a natural, rather than an artificial, range of variability elicited adaptive responses from parasitic cowbirds.

Ontogenetic effects of brood parasitism by the Brown-headed Cowbird on host offspring.

Nest-sharer avian brood parasites do not evict or otherwise kill host chicks, but instead inflict a range of negative effects on their nestmates that are mediated by interactions between the parasite and host life history traits. Although many of the negative fitness effects of avian brood parasitism are well documented across diverse host species, there remains a paucity of studies that have examined the impacts of parasitism across the entirety of host ontogeny (i.e., from when an egg is laid until independence). More specifically, few studies have examined the impact of brood parasitism on the pre- and post-fledging development, physiology, behavior, and survival of host offspring. To help fill this knowledge gap, we assessed the effects of brood parasitism by Brown-headed Cowbirds (Molothrus ater) across the ontogeny (incubation, nestling, and post-fledging period) of nine sympatrically breeding host species in central Illinois, USA; due to sample sizes, impacts on the post-fledging period were only examined in two of the nine species. Specifically, we examined the impact of brood parasitism on ontogenetic markers including the embryonic heart rate, hatching rate, nestling period length, nest survival, and offspring growth and development. Additionally, in species in which we found negative impacts of cowbird parasitism on host nestmate ontogeny, we examined whether the difference in adult size between parasites and their hosts and their hatching asynchrony positively predicted variation in host costs across these focal taxa. We found that costs of cowbird parasitism were most severe during early nesting stages (reduction in the host clutch or brood size) and were predicted negatively by host size and positively by incubation length. In contrast, we only found limited costs of cowbird parasitism on other stages of host ontogeny; critically, post-fledging survival did not differ between host offspring that fledged alongside cowbirds and those that did not. Our findings (i) highlight the direct costs of cowbird parasitism on host fitness, (ii) provide evidence for when (the stage) those costs are manifested, and (iii) may help to explain why many anti-cowbird defenses of hosts have evolved for protection from parasitism during the laying and incubation stages.

Effects of nest concealment on nest predation and cowbird parasitism, flight initiation distance and female stress levels in the Wood Thrush

Temperate zone songbirds in North America can experience high levels of nest predation and Brown-headed Cowbird (<em>Molothrus ater</em>) parasitism, which may contribute to population declines, and an important question is whether nesting females can mitigate these threats through greater nest concealment. However, there is little known about the pros and cons of nest concealment to the incubating female, and whether concealment influences female corticosterone levels or nest escape behavior to potentially reduce risk of predation on females. This study investigates whether nest concealment resulted in reduced risk of nest predation and Brown-headed Cowbird parasitism in the Wood Thrush (<em>Hylocichla mustelina</em>), a forest bird that is declining across most of its range. We monitored nests (n = 186) in a highly fragmented forest landscape in southwestern Ontario, Canada and found high nest predation (53.2%) and moderate cowbird parasitism (25.3%). Overall nest concealment, nest height, and forest fragment size were not good predictors of daily nest survival or cowbird parasitism, but greater nest concealment one meter above the nest strongly reduced nest predation risk. We speculate that because most Wood Thrush nests are relatively low (< 2.5 m above ground), above-nest concealment may be effective against detection by avian nest predators. Overall nest concealment was not negatively correlated with flight initiation distance (FID) or positively related to baseline corticosterone level. This suggests that high nest concealment does not increase stress or personal risk to incubating females; however, nest concealment manipulations are needed to further explore whether any such trade-offs exist.

Brood parasitism of Hooded Warblers by Brown-headed Cowbirds: Severe impact on individual nests but modest consequences for seasonal fecundity and conservation

Abstract Brood parasitism by Brown-headed Cowbirds (Molothrus ater) often has pronounced negative effects on host nests. However, the extent to which parasitism reduces annual reproduction and presents conservation challenges for host species is unclear. We address this issue with data from a color-banded population of Hooded Warblers (Setophaga citrina) in Pennsylvania, where Hooded Warblers have increased dramatically despite frequent nest parasitism. Our analysis is based on both an extensive dataset (8 years, 847 nests) on the per-nest impacts of cowbird parasitism and female-based stochastic simulations that accurately reflect the reproductive biology and parasitism rate (30%) of our study population. Cowbird parasitism has multiple negative consequences for Hooded Warbler nests, including (1) reduced host clutch size; (2) increased nest abandonment; (3) increased risk of complete failure due to predation; and (4) in surviving nests increased egg loss, hatching failure, and nestling mortality. We estimate that parasitism reduces success of Hooded Warbler nests 68%, from 1.29 to 0.41 fledglings per nest. For females and populations, however, the consequences of nest parasitism are considerably less extreme; female annual fecundity decreases 25% for each nesting attempt parasitized, and population-level fecundity drops 5.6% for each 10% increase in the frequency of parasitism. These more modest impacts are attributable to (1) steep declines in rates of cowbird parasitism as the nesting season progresses, (2) rapid renesting following abandonment or failure of parasitized nests, and (3) regular double brooding, with second broods initiated in late June and July when the incidence of cowbird parasitism is low. Our results help resolve the paradox of how cowbird parasitism can have both severe consequences for individual host nests but more modest and sustainable conservation impacts on the seasonal fecundity of females and populations. They further underscore the importance of determining population-level effects of brood parasites before investing in costly management efforts.

Developing an adaptive management approach to cowbird control on the Santa Clara River, California

AbstractBrood parasitism by the brown‐headed cowbird (Molothrus ater) has caused population declines for conservation‐reliant songbird species such as the federally and state endangered subspecies least Bell's vireo (Vireo bellii pusillus). Although cowbird trapping has increased vireo populations, it is costly, and may prevent the development of parasitism‐avoidance behaviors and result in high rates of non‐target species captures. Therefore, wildlife managers have a compelling interest in evaluating multiple means of cowbird control. We conducted a study between 2016 and 2018 on the Santa Clara River (SCR) in Ventura County, California, USA, to test how the removal of cowbird traps would affect cowbird parasitism of the vireo. At our control site, cowbird trapping occurred in spring using modified Australian crow (Corvus spp.) traps. At our treatment site, we stopped this method of trapping and instead used target‐netting of individual cowbirds and a very short duration trap to capture specific individual cowbirds (i.e., contingency trapping). We also tested the impact of shortening the cowbird trapping period on nest parasitism rates, and used point counts to compare cowbird densities between sites. Additionally, we compared the costs of these techniques. A shorter trapping period still resulted in a large number of cowbird captures on the control site (n = 452 captures) with no observed brood parasitism (n = 67 nests). On the treatment site, cowbird parasitism rates varied from 0% to 14% (n = 75 broods). The target‐netting method was not effective for capturing female cowbirds, probably because of their low densities. The short‐term contingency trapping method effectively removed female cowbirds in the vicinity of vulnerable vireo nests. Cowbird traps captured &gt;300 individuals of 9 different non‐target species each year on the control site, whereas target‐netting and contingency trapping resulted in only 8 captures of 4 different non‐target species. Costs of continuous trapping for 2 months versus point counts, nest monitoring, and contingency trapping were $21,100.00 versus $16,996.00, respectively. Based on these findings, we recommend that an adaptive cowbird management program to benefit songbird populations should consider using regular point counts and nest monitoring to provide data on cowbird impacts, in lieu of continued intensive trapping without monitoring. Thresholds for the density of cowbird females and the allowable level of parasitism for host species should also be established to guide an adaptive cowbird management program. Short‐term contingency trapping based on the thresholds should be considered, but restoration of native vegetation, especially in southwestern riparian areas, is also crucial and effective at limiting parasitism, and so should strongly be considered. Lastly, large numbers of non‐target species captures in cowbird traps are probably having consequential impacts to these species, and need to be monitored and minimized.

Increased abundance and productivity of a grassland bird after experimental control of invasive tall fescue

Tall fescue (Schedonorus arundinaceus) is a widespread invasive grass in the United States that degrades habitat quality for biodiversity. Herbicide followed by seeding of native plants reduces tall fescue and is predicted to restore habitat quality over time, but little is known about short‐term (1–2 year) impacts on native species. We conducted a landscape‐scale controlled experiment to assess the short‐term effects of herbicide and seeding on the reproduction of an obligate grassland bird, the dickcissel (Spiza americana). In 2014, four sites in southern Iowa were each subdivided into three patches (mean 7 ha). One patch in each site was treated with glyphosate herbicide (spray‐only), one with herbicide and native seeding (spray‐and‐seed), with the third serving as a control. Two sites were grazed by cattle from April to July and two sites were ungrazed. We monitored dickcissel reproduction in 2016, finding that they were more abundant, built more nests, experienced less cowbird parasitism, had increased nest survival, and produced more fledglings on spray‐only and spray‐and‐seed treatments compared to control patches. Dickcissels nested infrequently on grazed sites—especially grazed control patches. We did not detect any impacts on clutch size, provisioning rates, or nestling mass, but Araneae (spiders) and Lepidoptera larvae (caterpillars) may have been smaller on sprayed patches. Positive responses by dickcissels were likely due to successful reduction of tall fescue and improved access to suitable nest sites through increased vegetation heterogeneity. Our results indicate that using herbicide and seeding to restore tall fescue‐dominated sites improves habitat quality for this grassland bird, shortly after restoration.

The location of thrush nests on buildings affects the chance of cowbird parasitism

Nest site placement is a critical choice among passerines, being an important factor that affects their breeding success. In urban areas, human buildings offer suitable nesting sites usually less exposed to predators and brood parasitic birds and readily available to be reused for several breeding seasons. However, the extent to which the features of nest placement sites in buildings contribute to reduce nest detectability by predators and brood parasites is still unknown. Here, we tested whether the features of the nesting site (i.e., lateral concealment, distance to the building ceiling, and height above ground) affect the chance of brood parasitism by Shiny Cowbirds (Molothrus bonariensis) in Pale-breasted Thrushes (Turdus leucomelas) nests placed on buildings in a Brazilian suburban area from 2013 to 2019. Cowbird parasitism increased throughout the study years, and nests closer to ceilings, supposedly better concealed, were more likely to be parasitized. Laying date, height above ground, and lateral concealment were not related to the probability of cowbird parasitism. We suggest that less concealed nests enhance vicinity monitoring by parents, allowing a faster agonistic response to the presence of cowbirds near the nest. Our results indicate that nest site location in cities may have consequences for the breeding success of cowbird hosts.

Host parent responses to heterospecific parasite nestling alarm calls are independent of past and current experience with experimental brood parasitism

Communication between parents and dependent offspring is critical not only during provisioning, but also in antipredator contexts. In altricial birds, a top cause of reproductive failure is nest predation, and alarm calls both by parents and chicks can serve to alert others and increase the likelihood of offspring escaping predation. Understanding the factors that determine the strength of parental antipredator responses to different nestling alarm calls can provide insight into parent-offspring recognition. The prothonotary warbler (Protonotaria citrea), a host of the obligate brood parasite, the brown-headed cowbird (Molothrus ater), never rejects cowbird young and raises the parasite together with its own offspring. To determine whether warbler parents learn cowbird nestling alarm calls, we presented experimentally parasitized or non-parasitized parents with playbacks of conspecific warbler, parasitic cowbird, and a harmless heterospecific control, eastern bluebird (Sialis sialis), nestling alarm calls. We recorded the latency to respond and the number of chips given by members of the resident warbler pair. We found that parents were most likely to respond to warbler nestling alarm calls, least likely to respond to bluebird calls, with a statistically intermediate likelihood of responding to cowbird calls. Critically, current and past parasitism status did not affect the likelihood of response to any playback or the number of chips given, however, currently parasitized parents had greater response latencies to playbacks than non-parasitized parents. These results suggest that warbler parents do not learn cowbird alarm calls from breeding experiences and, in turn, that cowbirds may employ a generalized, bet-hedging alarm call.

Do Attributes of Gray Vireo Nest Sites Influence Brown-headed Cowbird Brood Parasitism?

Brood parasitism can have major detrimental effects on the fitness of reproducing songbirds and often contributes to nest failure. As the Gray Vireo (Vireo vicinior) suffers high rates of brood parasitism by the Brown-headed Cowbird (Molothrus ater), we hypothesized that vireos might choose nest sites selectively to thwart cowbird parasitism. To investigate this question, we measured and compared vegetative and spatial attributes of nest sites in a known breeding population of the Gray Vireo on the Sevilleta National Wildlife Refuge, New Mexico, in 2017 and 2018. Specifically, we compared parasitized and unparasitized nests with respect to the nest’s height and the average height, diameter, and foliage density of the nest tree. We also compared the spatial attributes of the nearest neighboring nest, nearest parasitized nest, and the density of surrounding juniper trees within 50 m of a nest. Interestingly, among none of the variables measured did we find an association with the incidence of brood parasitism, suggesting that nest parasitism might be more strongly linked to other larger-scale ecological or behavioral variables.

Twenty-two years of brood parasitism data do not support the mafia hypothesis in an accepter host of the Brown-headed Cowbird (Molothrus ater)

Most host species of the obligate brood parasitic Brown-headed Cowbird (<em>Molothrus ater</em>) accept foreign eggs and offspring despite the associated reproductive costs. One theoretical explanation for this host tolerance is the potential for mafia-like behavior by the brood parasite, which involves the retaliatory destruction of a host’s eggs and/or nestlings in response to that host’s rejection of the parasitic progeny. Here, we use a long-term dataset of an accepter cowbird host, the Dark-eyed Junco (<em>Junco hyemalis carolinensis</em>), where researchers opportunistically removed parasitic cowbird eggs from the clutch, to test critical assumptions and predictions of the mafia hypothesis in this cowbird-host system. For the mafia hypothesis to be supported, at least some junco nestlings should survive in cowbird parasitized nests. In turn, if cowbirds parasitizing juncos engage in mafia behavior, then nests where the parasitic egg was removed should have lower nest success than nests where the cowbird egg remained. We found no support for the mafia hypothesis, because fledgling success was higher in nests where the cowbird egg was removed compared to nests where the cowbird egg remained. If mafia behavior is present in this cowbird population, as suggested by anecdotal observations, it is likely inconsistent over time and space or localized to a small number of individual female parasites rather than a population-wide phenomenon. Further studies are needed to assess how widespread cowbird mafia behavior is across host species and populations and what its implications and impacts may be on host fitness.

Cattle grazing in CRP grasslands during the nesting season: effects on avian reproduction

AbstractBird populations in grasslands have experienced declines coinciding with loss and fragmentation of prairies. The United States Department of Agriculture (USDA)‐administered Conservation Reserve Program (CRP) is the most extensive grassland restoration program in North America and it has especially benefitted grassland birds. Grazing by domestic cattle has been restricted in CRP during avian nesting seasons despite the potential improvements in structuring habitat for a greater diversity of grassland bird species. Potential negative consequences of grazing in CRP grasslands include trampling of nests by cattle, reductions in nest concealment from predators, and attraction of brood‐parasitic brown‐headed cowbirds (Molothrus ater). We designed an experiment to test for effects of cattle grazing in CRP fields during the nesting season on nest survival and brood parasitism of 5 bird species that commonly nest in CRP grasslands: mourning dove (Zenaida macroura), grasshopper sparrow (Ammodramus savannarum), dickcissel (Spiza americana), and eastern (Sturnella magna) and western (S. neglecta) meadowlarks. Grazing was implemented during summers 2017 and 2018 on 17 of 36 fields followed by a year of rest on all fields in 2019. Of the 879 nests on grazed fields, only 4 were likely trampled by cattle (vs. 54% of all nests estimated as failing because of depredation). Experimental grazing (grazed vs. ungrazed fields) had variable effects on nest survival and cowbird parasitism among the bird species analyzed. Negative effects of grazing on daily nest survival of dickcissel and meadowlarks were apparent, at least in some years. We found no direct effects of grazing on nest survival of mourning dove or grasshopper sparrow. Probability and intensity (cowbird offspring/nest) of cowbird parasitism in dickcissel nests was higher on grazed versus ungrazed sites but only in conservation practice (CP) CP2 (vs. CP25 fields). Parasitism probability of grasshopper sparrow nests by cowbirds was higher on grazed fields in the 2 years after introduction of cattle in 2017. Greater vegetative concealment around nest sites was associated with reduced cowbird parasitism of meadowlark and grasshopper sparrow nests and higher nest survival for grasshopper sparrows. Reductions in vegetative height caused by longer‐term or high‐intensity grazing might therefore have negative consequences for some grassland birds by increasing nest site visibility and exposure to cowbird parasitism. Our results indicate that cattle grazing in CRP fields during the nesting season might have some negative effects on reproductive success of some grassland bird species, at least in the short term; however, the potential improvements of structuring habitat to accommodate more grassland bird species and increasing landowner participation in the CRP are considerable.

How to build a puncture- and breakage-resistant eggshell? Mechanical and structural analyses of avian brood parasites and their hosts.

Evolved eggshell strength is greater in several lineages of obligate avian brood parasites (birds that lay their eggs in other species' nests) than in their hosts. Greater strength is typically indirectly implied by eggshell thickness comparisons between parasites and hosts. Nevertheless, there is strong evidence that the eggshell structural organization differentially influences its mechanical properties. Using instrumental puncture tests and SEM/EBSD and XRD techniques, we studied the most relevant eggshell mechanical, textural, ultrastructural and microstructural features between several host species and their parasitic cowbirds (Molothrus spp.). These parasitic species display different egg-destructive behaviors, reducing host reproductive fitness, including the more frequently host-egg puncturing M. rufoaxillaris and M. bonariensis, and the host egg-removing M. ater. The results, analyzed using a phylogenetic comparative approach, showed interspecific patterns in the mechanical and structural features. Overall, the eggshells of the two egg-puncturing parasites (but not of M. ater) were stronger, stiffer and required greater stress to produce a fracture than the respective hosts' eggs. These features were affected by eggshell microstructure and ultrastructure, related to the increase in the intercrystalline boundary network acting in cooperation with the increase in palisade layer thickness. Both structural features generate more options and greater lengths of intercrystalline paths, increasing the energy consumed in crack or fissure propagation. The reported patterns of all these diverse eggshell features support a new set of interpretations, confirming several hypotheses regarding the impact of the two reproductive strategies (parasitic versus parental) and parasitic egg destruction behaviors (more versus less frequently puncturing).

Parasitic cowbird development up to fledging and subsequent post-fledging survival reflect life history variation found across host species

Parasitic cowbird development up to fledging and subsequent post-fledging survival reflect life history variation found across host species

A comparative study of the structural and mechanical properties of avian eggshells among hosts of obligate brood parasitic cowbirds (genusMolothrus)

AbstractObligate avian brood parasites depend on hosts for parental care, which in turn suffer fitness losses as a result of parasitism. Mechanisms by which brood parasitic cowbirds (Molothrus spp.) reduce host breeding success include the puncture (M. rufoaxillaris and M. bonariensis) or removal (M. ater) of the eggs of the host. Our working hypothesis is that the host eggs’ mechanical strength and their size and shape in species with higher frequency of parasitism covaries with the cowbird’s strategy to reduce host clutch size. Our results, obtained through phylogenetic analyses based on egg 2D geometric morphometry and eggshell mechanical and ultrastructural measurements, suggest that egg-puncturer behaviour has led to an increase in the strength of the host’s eggshell, which might make them more difficult to be pierced. We also characterized larger, more rounded and asymmetrical eggs in frequent hosts of M. ater, which might be more difficult to be removed. These interspecific host egg and shell traits were also positively affected by the frequency of parasitism, indicating that species-specific patterns of parasitic costs select for respective anti-parasitic defences in hosts.

Nest reuse by Pale‐breasted Thrushes reduces the chance of cowbird parasitism and allows earlier initiation of breeding

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