Cowbird Eggs Research Articles

No evidence of adaptive tolerance of parasitism in a cavity-nesting brood parasite host

Abstract Acceptance of avian brood parasitism by hosts is one of the most enigmatic aspects of brood parasite–host coevolution. The most common explanation for acceptance of parasitism by hosts of the brown-headed cowbird (Molothrus ater) is evolutionary lag, which suggests that hosts have not had enough time to evolve defenses against parasitism. Alternatively, acceptance may be the optimal strategy when the costs of rejecting parasitism exceed the benefits. The lack of nest site hypothesis applies to secondary cavity-nesting birds that cannot excavate their own nests and predicts that hosts accept parasitism instead of deserting a parasitized nest when there are no vacant nest sites available in which to renest. I tested this hypothesis using the prothonotary warbler (Pronotaria citrea), a commonly parasitized, cavity-nesting cowbird host. I used a paired nest box design and predicted that if hosts accept parasitism because of a lack of alternative nest sites, they should desert parasitized nests and renest in the vacant nest box on their territory. I recorded 37 cases where a nest was parasitized and warblers only deserted 2 parasitized nest boxes for a vacant nest box. Both desertions were attributable to factors other than parasitism and the rate of desertion did not differ from controls that only had a single nest box. Moreover, 7 of the warblers initiated clutches in nest boxes that already contained cowbird egg despite having vacant nest boxes available on their territories. These results indicate that warblers do not accept parasitism because of tolerance, but likely due to evolutionary lag.

Conflicting cognitive decisions: Does egg retrieval modify egg rejection in a host of an obligate brood parasite?

AbstractEgg retrieval is an ancient behaviour displayed by birds in which a displaced egg is transferred back into the nest. Egg rejection, in contrast, is a recently evolved adaptation by hosts of brood parasites in which an egg is removed from a nest. These conflicting behaviours are both adaptive because they prevent reproductive losses, but egg retrieval may benefit brood parasites if hosts retrieve parasitic eggs into their nests. We examined these conflicting cognitive processes in the prothonotary warbler (Protonotaria citrea), a cavity‐nesting host of the brown‐headed cowbird (Molothrus ater). Eggs and nestlings are occasionally found on the rims of warbler nest cavities. We determined how this happened using video cameras and tested whether warblers retrieved their own eggs more often than cowbird eggs. We also determined if shape affects retrieval and rejection by placing cylinders inside nest cups or onto nest rims. We found that eggs were displaced by rapidly departing warblers whose nests are constantly prospected by nest competitors and brood parasites. Warblers retrieved their eggs (29%) and cowbird eggs (19%) at similar rates, and never rejected cowbird eggs after retrieval. Warblers removed most cylinders from nest rims and cups (89% and 78%, respectively), and never retrieved them. Warblers have retained egg retrieval at a low level and its expression is costly because they only retrieve a low level of their own eggs and also retrieve cowbird eggs, which benefits the brood parasite.

Brown-headed Cowbird (Molothrus ater) egg and nestling destruction at nests of grassland birds in southwest Wisconsin

Brown-headed Cowbird (Molothrus ater) egg and nestling destruction at nests of grassland birds in southwest Wisconsin

Announcing the 2021 Journal of Experimental Biology Outstanding Paper Prize shortlist and winner.

Announcing the 2021 Journal of Experimental Biology Outstanding Paper Prize shortlist and winner.

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.

Could diffuse coevolution explain the generic eggshell color of the brown-headed cowbird?

The brown-headed cowbird (hereafter cowbird) is an avian brood parasite that produces an egg dissimilar to those produced by the majority of its diverse host community. The cowbird’s generic egg may result from a Jack-of-all-trades strategy; however, the evolutionary mechanisms that select for their generic eggs are unclear. Here we propose that the cowbird’s eggshell phenotypes have evolved via diffuse coevolution, which results from community-level selective pressures, rather than via pairwise coevolution that occurs between a particular host species and its brood parasite. Under diffuse coevolution the cowbird’s host community, with varying eggshell phenotypes and recognition abilities, would select for a cowbird eggshell phenotype intermediate to those of its host community. This selection is exerted by hosts that reject cowbird eggs, rather than those that accept them; therefore, we expect cowbird eggshell colors can be approximated by both the phenotypes and rejection abilities of their host community. Here we use eggshell reflectance data from 43 host species to demonstrate that the cowbird eggshell phenotypes are reasonably predicted (within 2 just noticeable differences) by the eggshell phenotypes and rejection rates of their hosts. These findings suggest that cowbird eggshell phenotypes, and potentially those of other some generalist parasites, may evolve via diffuse coevolution. Importantly, this research provides insight into the underlying evolutionary processes that explain observed phenotypic variation and provides a framework for studying selection on both specialist and generalist parasites’ traits.

Imposter eggs of pecking cowbirds have stronger shells for protection

When Alfred, Lord Tennyson, described, ‘Nature, red in tooth and claw’, he should probably have thrown in beaks too. Cowbirds that inveigle their eggs into the nests of others often pierce the resident eggs to give their impostors the best chance of a future. But there is nothing to prevent subsequent intruders from meting out the same fate to the eggs of previous interlopers. Analía López from the Universidad de Buenos Aires, Argentina, explains that birds like cuckoos, which lay their eggs in others’ nests, tend to have thicker eggshells. But she and her colleagues from Argentina and the USA, knew that other aspects of the eggshell's construction also contribute to its strength. They decided to take a closer look at the eggs from screaming cowbirds and shiny cowbirds – which puncture resident eggs to rid their young of competition – and compared them with the eggs of brown-headed cowbirds – which toss out the eggs of the species upon which they prey – to find out how the arms race between the eggshells of hard-working foster parents and their lackadaisical imposters has played out.‘We spent countless hours in the field looking for hidden bird nests to collect a few eggs’, says López, who sometimes used an extendable ladder – and even free climbed a few trees – to locate the eggs. Then, she and Lía Gerschenson (Universidad de Buenos Aires) tested how strong the shells were by recording the force required to puncture the eggs at the widest point, which is usually targeted by the pecking parasites. Sure enough, the shells of the egg puncturing screaming cowbirds and shiny cowbirds were up to 3.2 times stronger than the shells of their unwitting hosts; they were also 2.2 times stronger than brown-headed cowbirds’ shells. And when López checked how much energy it took to form the cracks that shatter the shells, the screaming and shiny cowbird's eggs were 1.5 times tougher.Next López teamed up with Raúl Bolmaro (Instituto Física Rosario, Argentina) and colleagues to compare the microscopic structures of the eggshells, and discovered that the external layer of the shell – known as the palisade layer, which provides the shell's strength – is thicker in screaming and shiny cowbirds than in brown-headed cowbirds. Also, the mammillary cones, which provide calcium for shell development, are packed more densely into the lower layer of the eggshell. And when López compared how calcium carbonate crystals are arranged in the shell, she found that the packing is far more complex in screaming and shiny cowbird eggs than in the shells of the other species, making them more difficult to crack open.Screaming and shiny cowbirds have evolved eggshells that are far more resistant to pernicious pecks than the eggs they usurp, providing their developing chicks with the best protection for a safe start in life. But López, Mark Hauber (University of Illinois at Urbana-Champagne, USA) and Juan Reboreda (Universidad de Buenos Aires) also point out that the eggshells can't be so tough that the tiny inhabitants within are unable to liberate themselves. Fortunately, it seems that toward the end of incubation the shells are less resistant to the chick's frail pecks from within. ‘As the chick tries to break the shell it does so by imposing expansion forces through internal shell pressures, which tend to open the small cracks produced by the beak's pressure points’, says López, explaining that this is how the imposter can break free of a shell that likely withstood the pounding of an adult cowbird's beak intent upon its destruction.

High egg rejection and low aggressive responses of Mountain Bluebirds (Sialia currucoides) toward Brown-headed Cowbirds (Molothrus ater)

Hosts of brood parasitic Brown-headed Cowbirds (Molothrus ater) have acquired a suite of behavioral responses against them, including egg rejection and nest defense. Several cavity-nesting species have been shown to exhibit egg rejection, including the Mountain Bluebird (Sialia currucoides). Previous work reported egg rejection rates as low as 20%, under the assumption that they may instead be relying heavily on aggressive interactions to defend their nests from parasitism. In the present study, we evaluated the egg rejection and aggression behaviors of a population of Mountain Bluebirds in western Montana. Results indicate a higher rate of rejection than previously documented, with model cowbird eggs rejected in over 40% of trials (n = 37), compared with 5% rejection of mimetic eggs. In contrast, we found little aggressive response by Mountain Bluebirds toward both an adult cowbird model and a nonthreatening American Goldfinch (Carduelis tristis) model. Aggression patterns were consistent between sexes, with the most common behaviors including sitting and watching from within 5 m and flying past the models (n = 21). Hovering above the model was observed on a few occasions, and physical contact by bluebirds happened on only 3 occasions toward the goldfinch model. In summary, Mountain Bluebirds in western Montana showed relatively high rates of cowbird egg rejection but little aggressive response toward adult heterospecific nest intruders.

The blunt pole is not a source of more salient recognition cues than the sharp pole for the rejection of model eggs by American robins (Turdus migratorius)

Hosts of obligate avian brood parasites can reduce the costs of raising parasitic offspring by rejecting foreign eggs from their nests. Rejecter hosts use various visual and tactile cues to discriminate between own and foreign eggs. The blunt pole hypothesis specifically states that avian-perceivable visual information at and around the broader pole of the eggshell contains more salient recognition cues than does the sharp pole of the same egg. The directional prediction is, therefore, that eggs painted non-mimetically on their blunt pole should more likely be rejected relative to those similarly painted on their sharp pole. This hypothesis had been experimentally tested and its predictions supported solely in mimetic avian host-parasite systems, with hosts producing denser and more variable eggshell maculation patterns at the blunt pole, and in one species with immaculate eggs but still with distinctly discernible blunt-pole specific colouration. Here we aimed to expand upon these previous works and assessed whether the blunt pole of model eggs contains more salient egg rejection cues, relative to the sharp pole, for the American robin (Turdus migratorius), a robust rejecter of non-mimetic brown-headed cowbird (Molothrus ater) eggs. In this system host eggs are uniformly immaculate whereas the brood parasitic shell is maculated. We painted model cowbird-sized eggs on either the blunt or the sharp half to mimic the immaculate robin egg colours and the other half to resemble non-mimetic egg colours and patterns. There was no statistical support for the predicted outcomes of the blunt pole hypothesis in our trials as rejection rates were similar regardless of whether eggs were painted with non-mimetic colours on the blunt or sharp poles. Future work should test the role of asymmetrical signalling content for anti-parasitic rejection of eggs in additional host species, especially those with both immaculate own and mimetic parasitic eggs.

Best of a bad job or masters of illusion: Do nest light conditions make the eggs of brood parasitic brown‐headed cowbirds (Molothrus ater) more similar to the eggs of their hosts?

AbstractBrown‐headed cowbirds (Molothrus ater) are generalist obligate brood parasites, laying in the nest of nearly 300 avian species, and successfully parasitizing well over 100 host species. Cowbird eggs are generally considered non‐mimetic, although some have suggested that cowbird eggs resemble several of their host species’ eggs. To date, no investigation has examined the level of avian‐perceived similarity between cowbird and diverse host eggs in the contexts of light characteristics at the nest and the visual system of the relevant viewer. Because the cowbird exploits a wide range of species that lay in a variety of nest types, hosts view these eggs under an array of light conditions which could facilitate or hinder egg discrimination. When considering the visual system of the relevant viewers and the light conditions at their nest, we found that the coloration of cowbird eggs was more similar to host than non‐host species’ eggs. Host responses (whether they accept or reject cowbird eggs) were not statistically different when hosts perceived a large chromatic difference between their own eggs and the cowbird's eggs. Instead, we found that host responses were predicted by the degree to which nesting light conditions facilitated color similarity between host and cowbird eggs, such that hosts typically nesting under light conditions where this color discrimination task was more challenging were more likely to reject cowbird eggs. This suggests that the nesting light environment may have selected for increased coevolved egg recognition abilities in a suite of cowbird host species, even in the absence of parasitic egg color mimicry.

Natural and artificial scents do not increase egg rejection rates of model brood parasitic eggs by American robins (Turdus migratorius)

Hosts of obligate avian brood parasites can diminish or eliminate the costs of parasitism by rejecting foreign eggs from the nests. A vast literature demonstrates that visual and/or tactile cues can be used to recognize and reject natural or model eggs from the nests of diverse host species. However, data on olfaction-based potential egg recognition cues are both sparse and equivocal: experimentally-applied, naturally-relevant (heterospecific, including parasitic) scents do not appear to increase egg rejection rates in two host species, whereas unnatural scents (human and tobacco scents) do so in one host species. Here I assessed the predictions that (i) human handling of mimetically-painted model eggs would increase rejection rates, and (ii) applying unnatural or natural scents to mimetically or non-mimetically painted model eggs alters these eggs’ respective rejection rates relative to controls. I studied wild American Robins (Turdus migratorius), a robust rejecter species of the eggs of obligate brood parasitic Brown-headed Cowbirds (Molothrus ater). There was no statistical evidence to support either prediction, whereas poorer color-mimicry was still a predicted cause of greater egg rejection in this data set. Nonetheless, future studies could focus on this and other host species and using these and different methods to apply and maintain the scenting of model eggs to more directly test hosts’ use of potential olfactory cues in the foreign-egg rejection process.

Genetic patterns of repeat and multiple parasitism by screaming cowbirds, a specialist brood parasite

Avian brood parasites lay their eggs in the nests of other species, leaving the hosts to care for the parasitic offspring. The bookkeeping hypothesis predicts that, in order to reduce competition between parasitic nestlings, female parasites should keep a mental inventory of host nests that they have already parasitized and avoid laying multiple eggs in the same host nest. However, selection against repeat parasitism should be weaker when host nests are limited, or when hosts are able to rear more than one parasitic nestling. Here we use microsatellite genotyping of parasitic eggs to test whether female screaming cowbirds, Molothrus rufoaxillaris, avoid repeatedly parasitizing nests of their primary host, the greyish baywing, Agelaioides badius, in Argentina. Parasitism rates were extremely high (96.5% of 57 host clutches were parasitized with an average of 5.7 cowbird eggs each), indicating that host nests are limited. Although eggs laid by the same female showed moderate spatiotemporal clustering, individual females rarely laid more than one egg in the same host clutch (2 of 57 clutches, 26 of which contained multiple genotyped cowbird eggs). Females were much more likely to lay subsequent eggs in different host nests than to return to the same host nest. We found no evidence for kin structure among female cowbirds parasitizing the same host nest, which were no more closely related than chance would predict. These results suggest that female screaming cowbirds frequently lay eggs in host nests that have already been parasitized by unrelated females. However, they typically lay just one egg per host clutch, even though greyish baywings are capable of rearing several nestlings. Since screaming cowbird laying is often poorly synchronized with that of their host, avoidance of repeat parasitism may be adaptive if it allows females to spread the risk of failure among multiple host nests.

Endangered White-breasted Thrashers (Ramphocinclus brachyurus) reject mimetic Shiny Cowbird (Molothrus bonariensis) eggs

Brood parasitism can decrease the reproductive success of host species and contribute to population declines, especially in small populations with naturally low fecundity. The ability of host species to recognize foreign eggs varies, as do the behavioral responses of hosts to brood parasitism. We experimentally tested the ability of a putative host, the White-breasted Thrasher (Ramphocinclus brachyurus), to recognize and reject the eggs of a recently established generalist brood parasite. The White-breasted Thrasher is an endangered songbird restricted to 2 small Caribbean islands that are not known to ever have supported obligate brood parasites. We experimentally parasitized active nests within the stronghold of the species' distribution in Saint Lucia and found that White-breasted Thrashers rejected 83% of mimetic Shiny Cowbird (Molothrus bonariensis) eggs (i.e., wooden eggs painted to look like Shiny Cowbird eggs) within 5 d of being parasitized. Rejection of mimetic eggs was not associated with nest initiation date, distance to habitat edge or agricultural area, nest-attempt number, or presence of helpers or dependent young. We conclude that White-breasted Thrashers can detect foreign eggs and possess behavioral adaptations to counter brood parasitism, and propose that these anti-parasitism traits are likely retained from ancestors that evolved in the presence of brood parasites. Our results suggest that brood parasitism by Shiny Cowbirds is unlikely to become a major threat to the White-breasted Thrasher in Saint Lucia. However, ongoing anthropogenic land alteration is likely to increase contact between the 2 species, warranting continued monitoring of their nascent relationship.

Female and male rufous horneros eject shiny cowbird eggs using a mental template of the size of their own eggs

Hosts of interspecific brood parasites often evolve antiparasitic defences, like the recognition and rejection of parasite's eggs. Most hosts use differences in coloration and maculation to discriminate between their own and parasitic eggs, but there are a few cases of hosts using the size of eggs as a cue. To recognize parasite eggs, hosts may require the presence of their own eggs and use a discordancy rule or may use a mental template of their own eggs. Females are responsible for egg rejection in hosts in which they incubate alone, but if incubation is shared, males can also reject parasitic eggs. The rufous hornero, Furnarius rufus, a host of the shiny cowbird Molothrus bonariensis, ejects parasite eggs using egg size as a cue. We studied the cognitive mechanism underlying the recognition and ejection of parasitic eggs by this host. We experimentally parasitized hornero nests with eggs of different size, with and without the presence of host eggs and determined which sex was responsible for the ejection. We found that horneros ejected parasitic eggs using the size of the egg as a cue and did not need to compare parasitic eggs with their own eggs, which is consistent with the hypothesis of a mental template. Females and males ejected eggs at similar frequencies. We also found that cowbird eggs laid in hornero nests were longer and wider than those laid in nests of other host in the same area, which is consistent with the hypothesis of host-specific female cowbird lineages evolving larger eggs to deceit horneros from recognizing and ejecting their eggs.

Egg rejection behavior does not explain the lack of cowbird parasitism on an eastern North American population of Red-winged Blackbirds

BackgroundRed-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Although hosts of Old World cuckoos (Cuculidae) often show geographic variation in egg rejection behavior, cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs. Thus, geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. In this study, we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population, which may provide insight into low parasitism levels across eastern North America.MethodsWe parasitized red-wing nests with model cowbird eggs to determine their response to parasitism. Nests were tested across three nest stages and compared to control nests with no manipulations. Because rejection differed significantly by stage, we compared responses separately for each nest stage. We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.ResultsRed-wings showed significantly more rejections during the building stage, but not for the laying and incubation stages. Rejections during nest building involved mostly egg burials, which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg. Excluding these responses, red-wings rejected 15% of cowbird eggs, which is similar to rejection levels from other studies and populations. The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.ConclusionsEgg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population. Alternatively, we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism. Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.

Video recordings of Brown-headed (Molothrus ater) and Shiny (M. bonariensis) cowbirds reveal oviposition from an elevated position: Implications for host–parasite coevolution

Molothrine cowbirds are obligate brood parasites that lay eggs within 5–10 s in the minutes before sunrise. Robust analysis of their position at laying has therefore only been made possible with the development of high-resolution and low-light video recording technologies. From video recordings, we observed the moment of oviposition clearly in 8 and 31 instances of egg laying by Brown-headed (Molothrus ater) and Shiny (M. bonariensis) cowbirds, respectively. Both species laid from an elevated position such that their eggs dropped into nests from an estimated height of 4–8 cm. We propose this elevated laying position has selected for high eggshell strength in cowbirds, because stronger eggshells would be less likely to be damaged during egg laying. Stronger eggshells might also damage other eggs when dropped into the nest, thereby further reducing nestling competition from nest mates and drive selection for cowbird eggshell strength. A role for laying position in the evolution of cowbird egg strength is not mutually exclusive of the view that thickly shelled eggs have evolved as a defense to egg rejection by hosts and/or egg puncture behavior of cowbirds.

Cognition and the brain of brood parasitic cowbirds.

Cowbirds are brood parasites. Females lay their eggs in the nests of other species, which then incubate the cowbird eggs and raise the young cowbirds. Finding and returning to heterospecific nests presents cowbirds with several cognitive challenges. In some species, such as brown-headed cowbirds (Molothrus ater), females but not males search for and remember the locations of potential host nests. We describe recent research on sex differences in cognition and the hippocampus associated with this sex difference in search for host nests. Female brown-headed cowbirds perform better than males on some, but not all, tests of spatial memory and females show a pattern of adult hippocampal neurogenesis not found in males or in closely related non-parasitic birds. Because of the apparent specialization of the hippocampus, brown-headed cowbirds may be a good model in which to examine spatial information processing in the avian hippocampus and we also describe recent research on the spatial response properties of brown-headed cowbird hippocampal neurons.

Increased egg–nest visual contrast does not induce egg ejection in the eastern phoebe (Sayornis phoebe), an accepter host of the brood parasitic brown-headed cowbird (Molothrus ater).

Hosts of obligate brood parasitic birds can offset the costs of parasitism by rejecting foreign eggs. Like the vast majority of brown-headed cowbird (Molothrus ater) hosts, the eastern phoebe (Sayornis phoebe) paradoxically incurs substantial fitness costs by accepting cowbird parasitism. We investigated whether acceptance of brown-headed cowbird eggs may be mediated via lack of recognition due to crypsis in eastern phoebe nests, and whether egg-rejection behavior could be induced by manipulating egg-nest visual contrast as a means of facilitating parasitic egg detection in an artificial parasitism experiment. We found that brown-headed cowbird eggs have significantly lower achromatic, but not chromatic, visual contrast against the natural nest lining relative to phoebe eggs, suggesting that cowbird eggs may be cryptic in the low ambient light environments of natural or artificial covered ledges where eastern phoebes tend to nest. We also found that despite successfully increasing egg-nest visual contrast in our treatment, eastern phoebes did not reject model parasitic eggs. Our findings are consistent with previous work showing that increased egg-nest visual contrasts do not increase parasitic egg discriminability and rejection in hosts of avian brood parasites. Because sensory modeling showed that eastern phoebes were predicted to be able to visually perceive natural and experimental parasitism events, these hosts are suggested to lack the cognitive flexibility necessary to reject dissimilar eggs in their nests. Future work should determine whether this and other accepter species can be made to reject parasitism by using a more diverse array of artificial egg stimuli. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

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.

Thick eggshells of brood parasitic cowbirds protect their eggs and damage host eggs during laying

Avian brood parasites have thick eggshells and numerous hypotheses have been proposed to account for the significance of this trait. We examined whether thick eggshells protect the eggs of the shiny and brown-headed cowbirds during laying events. We found that cowbird eggs were significantly less likely to be damaged compared to host eggs during laying events and, at the same time, these eggs also caused significantly more damage to host eggs.