Host Nests Research Articles

Wide-Scale Gene Flow, Even in Insects that have Lost their Flight Ability: Presence of Dispersion Due to a Unique Parasitic Ecological Strategy of Piggybacking Hosts.

We focused on Meloe beetles that have lost all flight ability, and conducted molecular phylogeographic analyses based on their mitochondrial DNA COI and nuclear DNA EF1- α regions. Meloe beetles infiltrate bumblebee nests by attaching to bumblebees as they pollinate flowers and thereafter have a unique and specific life history as they complete their life-cycle within the host nest; flight-based dispersal is achieved by piggybacking on bumblebees. In fact, Meloe beetles, which cannot fly, even inhabit remote islands (i.e., "Oceanic Islands"). Regarding four species, i.e., Meloe coarctatus, Meloe proscarabaeus, Meloe violaceus and Meloe corvinus, the conventional morphological classification system based on morphological characteristics was strongly supported by the molecular markers. On the other hand, for two species, Meloe menoko and Meloe auriculatus, it was found that M. menoko may be evaluated as having a paraphyletic relationship with M. auriculatus. Furthermore, two other cryptic, undescribed species were also discovered in this study. One was collected in the Nikko Highland, and inhabited the area sympatrically with M. coarctatus. The other was collected from Hachijo-jima Island. These cryptic species were highly differentiated, independent lineages in terms of mitochondrial and nuclear gene regions. That is to say, a new level of species diversity was revealed among the Meloe beetle species, known for their unique and strange ecological and ethological characteristics.

Ecological and behavioural observations of a nesting aggregation of the endemic Tasmanian Digger Wasp Williamsita tasmanica

The endemic Tasmanian Digger Wasp Williamsita tasmanica is only known from 13 registered museum specimens, with little recorded on its life history. A nesting aggregation of females was observed over a 25-day period, enabling the first detailed observations of host log, nest design, prey and egg deposition, and behavioural patterns to be made. Cells were provisioned with large flies (order Diptera) of nine species from nine genera and four families to observe behaviour. This aggregation used a large eucalypt with an exposed core of friable brown rot for nesting. Our study showed that the day-to-day behaviour of the female is predictable, centring on nest construction, hunting large flies and egg-laying. This information has been lodged on the Atlas of Living Australia and voucher specimens collected for the Queen Victoria Museum and Art Gallery to facilitate further taxonomic study.

Multi-strategy serial cuckoo search algorithm for global optimization

Cuckoo search algorithm (CS) is a simple and effective nature-inspired optimization algorithm, which has been widely applied to solve the complex engineering optimization problem in the real world. However, all cuckoos have similar search behavior because of the unitary search strategy is used in CS, which easily makes the algorithm get trapped in the local optimum and enter the state of premature convergence. Inspired by the three serial behaviors of the cuckoo’s growth process, a multi-strategy serial CS (MSSCS) is proposed to overcome the above problems. In MSSCS, three new learning strategies are proposed to enhance the performance of the algorithm, which observed from the cuckoo’s behavior of seeking host nest, eviction and begging. Based on these three serial behaviors, we propose a multi-strategy serial framework to reduce the complexity of the multi-strategy algorithm and maximize the performance of each learning strategy. In addition, the adaptive parameter is designed to dynamically regulate the serial framework. To verify the effectiveness of MSSCS, extensive experiments are carried out on the CEC2013 test suite. The experimental results illustrate that the proposed algorithm has better performance when considering the quality of the obtained solutions.

A Hybrid Modified Method of the Sine Cosine Algorithm Using Latin Hypercube Sampling with the Cuckoo Search Algorithm for Optimization Problems

The metaheuristic algorithm is a popular research area for solving various optimization problems. In this study, we proposed two approaches based on the Sine Cosine Algorithm (SCA), namely, modification and hybridization. First, we attempted to solve the constraints of the original SCA by developing a modified SCA (MSCA) version with an improved identification capability of a random population using the Latin Hypercube Sampling (LHS) technique. MSCA serves to guide SCA in obtaining a better local optimum in the exploitation phase with fast convergence based on an optimum value of the solution. Second, hybridization of the MSCA (HMSCA) and the Cuckoo Search Algorithm (CSA) led to the development of the Hybrid Modified Sine Cosine Algorithm Cuckoo Search Algorithm (HMSCACSA) optimizer, which could search better optimal host nest locations in the global domain. Moreover, the HMSCACSA optimizer was validated over six classical test functions, the IEEE CEC 2017, and the IEEE CEC 2014 benchmark functions. The effectiveness of HMSCACSA was also compared with other hybrid metaheuristics such as the Particle Swarm Optimization–Grey Wolf Optimization (PSOGWO), Particle Swarm Optimization–Artificial Bee Colony (PSOABC), and Particle Swarm Optimization–Gravitational Search Algorithm (PSOGSA). In summary, the proposed HMSCACSA converged 63.89% faster and achieved a shorter Central Processing Unit (CPU) duration by a maximum of up to 43.6% compared to the other hybrid counterparts.

Development of microsatellite loci and optimization of a multiplex assay for Latibulus argiolus (Hymenoptera: Ichneumonidae), the specialized parasitoid of paper wasps

Microsatellite loci are commonly used markers in population genetic studies. In this study, we present 40 novel and polymorphic microsatellite loci elaborated for the ichneumonid parasitoid Latibulus argiolus (Rossi, 1790). Reaction condition optimisation procedures allowed 14 of these loci to be co-amplified in two PCRs and loaded in two multiplex panels onto a genetic analyser. The assay was tested on 197 individuals of L. argiolus originating from ten natural populations obtained from the host nests of paper wasps. The validated loci were polymorphic with high allele numbers ranging from eight to 27 (average 17.6 alleles per locus). Both observed and expected heterozygosity values were high, ranging between 0.75 and 0.92 for HO (mean 0.83) and from 0.70 to 0.90 for HE (mean 0.85). The optimized assay showed low genotyping error rate and negligible null allele frequency. The designed multiplex panels could be successfully applied in relatedness analyses and genetic variability studies of L. argiolus populations, which would be particularly interesting considering the coevolutionary context of this species with its social host.

Sex differences in the use of spatial cues in two avian brood parasites.

Shiny and screaming cowbirds are avian interspecific brood parasites that locate and prospect host nests in daylight and return from one to several days later to lay an egg during the pre-dawn twilight. Thus, during nest location and prospecting, both location information and visual features are available, but the latter become less salient in the low-light conditions when the nests are visited for laying. This raises the question of how these different sources of information interact, and whether this reflects different behavioural specializations across sexes. Differences are expected, because in shiny cowbirds, females act alone, but in screaming cowbirds, both sexes make exploratory and laying nest visits together. We trained females and males of shiny and screaming cowbird to locate a food source signalled by both colour and position (cues associated), and evaluated performance after displacing the colour cue to make it misleading (cues dissociated). There were no sex or species differences in acquisition performance while the cues were associated. When the colour cue was relocated, individuals of both sexes and species located the food source making fewer visits to non-baited wells than expected by chance, indicating that they all retained the position as an informative cue. In this phase, however, shiny cowbird females, but not screaming, outperformed conspecific males, visiting fewer non-baited wells before finding the food location and making straighter paths in the search. These results are consistent with a greater reliance on spatial memory, as expected from the shiny cowbird female's specialization on nest location behaviour.

Autoclaving Nest-Material Remains Influences the Probability of Ectoparasitism of Nestling Hoopoes (Upupa epops).

Simple SummaryMicroorganisms may have direct negative effects on their animal hosts and cause diseases, but some others provide animals with protection against infections, parasites, and even predators. However, parasites or predators might take their cue from odors produced by bacteria, even those from protecting microorganisms, which could turn net benefits of bacteria that are a priori considered beneficial into a neutral or even negative outcome. This possibility has scarcely been studied in wildlife populations; we manipulated the bacterial community of nest-material of hoopoes and detected a negative effect in terms of the intensity of parasitism by blood-sucking flies that nestlings suffer. We also detected a positive link between the bacterial density of the nest-materials and the intensity of ectoparasitism, which further points at the importance of the bacteria determining the level of parasitism. Blood-sucking ectoparasites are also disease vectors, affecting both humans and livestock, and considering the role of the bacterial environment might help to establish new transmission control protocols.Nest bacterial environment influences avian reproduction directly because it might include pathogenic- or antibiotic-producing bacteria or indirectly because predators or ectoparasites can use volatile compounds from nest bacterial metabolism to detect nests of their avian hosts. Hoopoes (Upupa epops) do not build nests. They rather reuse holes or nest-boxes that contain remains of nest-materials from previous breeding seasons. Interestingly, it has been recently described that the nest’s bacterial environment partly affects the uropygial gland microbiota of hoopoe females and eggshells. Blood-sucking ectoparasites use chemical cues to find host nests, so we experimentally tested the hypothetical effects of microorganisms inhabiting nest-material remains before reproduction regarding the intensity of ectoparasitism suffered by 8-day-old nestling hoopoes. In accordance with the hypothesis, nestlings hatched in nest-boxes with autoclaved nest-material remains from the previous reproductive seasons suffered less from ectoparasites than those hatched in the control nest-boxes with nonautoclaved nest-material. Moreover, we found a positive association between the bacterial density of nest-material during the nestling phase and ectoparasitism intensity that was only apparent in nest-boxes with autoclaved nest-material. However, contrary to our expectations, nest bacterial load was positively associated with fledgling success. These results suggest a link between the community of microorganisms of nest-material remains and the intensity of ectoparasitism, and, on the other hand, that the nest bacterial environment during reproduction is related to fledging success. Here, we discuss possible mechanisms explaining the experimental and correlative results, including the possibility that the experimental autoclaving of nest material affected the microbiota of females and nestlings’ secretion and/or nest volatiles that attracted ectoparasites, therefore indirectly affecting both the nest bacterial environment at the nestling stage and fledging success.

Melittobia parasitoid wasps (Hymenoptera: Eulophidae) in the Iberian Peninsula: new data on distribution, biology and morphology

Summary Most data about parasitoids of the genus Melittobia (Hymenoptera: Eulophidae) in Europe, where only the native M. acasta (Walker, 1839) and the alien M. australica Girault, 1912 occur, come from studies carried out in the central-northern part of the continent. In the present study, we provide new information on both species in the Iberian Peninsula. We obtained M. acasta from mud nests of Sceliphron destillatorium (Illiger, 1807) (Hymenoptera: Sphecidae) and both M. australica and M. acasta from nests of Sceliphron spirifex (Linnaeus, 1758). Melittobia australica records are the first for this allochthonous species in the Iberian Peninsula, which is the second record in Europe. Rate of parasitism (number of parasitized nests/total number of collected nests) reached almost 50% of nests for M. australica and was much lower for M. acasta (up to 6%). The probability of M. australica to be found in a host nest did not depend on the number of brood cells in the nest, suggesting that nest size does not affect parasitism. Scanning electron microscope (SEM) images provided new information about the morphology of the antennal sensory system of both species. Energy-dispersive X-ray spectroscopy (EDS) revealed the presence of Zn in the mandibles of both species and sexes, suggesting an adaptation to egress from host nests.

Host Ant Change of a Socially Parasitic Butterfly (Phengaris alcon) through Host Nest Take-Over.

Simple SummaryThe endangered Alcon blue butterfly (Phengaris alcon) starts its larval stage by feeding on the seeds of gentian plants, after which it completes development in the nests of suitable Myrmica ant species. Any particular population often uses more than one host ant species, and some host switching is likely. To test switching in the lab we introduced relatively strong colonies of alien Myrmica species to the arenas of weaker colonies, and to orphaned caterpillars. Most of the caterpillars were successfully readopted by alien ants, and survived well. Our results suggest higher ecological plasticity in host ant usage of this butterfly than generally thought. The Alcon blue is an iconic species, e.g., its special life cycle has featured in several high profile television and streaming media wildlife series, and the more we know about its unusual life the more we can do for its protected sites.The socially parasitic Alcon blue butterfly (Phengaris alcon) starts its larval stage by feeding on the seeds of gentians, after which it completes development in the nests of suitable Myrmica ant species. The host plant and host ant species can differ at the population level within a region, and local adaptation is common, but some host switches are observed. It has been suggested that one mechanism of change is through the re-adoption of caterpillars by different ant species, either through occupation of abandoned nests or take-over of established nests by competitively superior colonies. To test this question in the lab we introduced relatively strong colonies (50 workers) of alien Myrmica species to the arenas of weaker colonies (two caterpillars with six workers), and to orphaned caterpillars (two caterpillars without ants). We used caterpillars from a xerophylic population of P. alcon, and both local hosts, M. sabuleti and M. scabrinodis, testing the possibility of host switch between these two host ant species during larval development. Most of the caterpillars were successfully readopted by alien ants, and survived well. Our results suggest higher ecological plasticity in host ant usage of this butterfly than generally thought.

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.

Falling out of the host nest: an overlooked factor decreasing survival of brood parasite chicks

Animals rely on an array of environmental triggers or cues to make their behavioural and life‐history decisions. Brood–parasitic females should prefer the best host to maximize their reproductive success. Despite the fact that Eurasian reed warbler Acrocephalus scirpaceus is one of the most frequent host of the common cuckoo Cuculus canorus, parasitic chicks often prematurely fall out of the nests and drown. To establish whether these events are influenced by the size of the nest, we experimentally replaced original Eurasian reed warbler nests for much bigger great reed warbler Acrocephalus arundinaceus ones and transferred cuckoo chicks into them to record their fate. The chicks in bigger nests showed significantly lower falling‐out rate (4 of 29) compared with those remained in smaller nests (12 of 32). These findings suggest that host nest size plays an important role in the suitability of host species and that smaller host nests may pose a high mortality risk to the relatively big cuckoo chicks. Moreover, cuckoo chicks that fell out of the nests were significantly less fed by foster parents before falling‐out event than those stayed in the nests. This indicates that insufficient parental care may also facilitate falling‐out of cuckoo chicks, probably because hungry chicks are more restless in host nests. The relatively frequent falling‐out events represent a substantial and underestimated cost of brood parasites. Without video surveillance it can incorrectly be confused with predation. We therefore strongly recommend using video‐recording for determining predation costs in birds.

Microbiomes and Specific Symbionts of Social Spiders: Compositional Patterns in Host Species, Populations, and Nests.

Social spiders have remarkably low species-wide genetic diversities, potentially increasing the relative importance of microbial symbionts for host fitness. Here we explore the bacterial microbiomes of three species of social Stegodyphus (S. dumicola, S. mimosarum, and S. sarasinorum), within and between populations, using 16S rRNA gene amplicon sequencing. The microbiomes of the three spider species were distinct but shared similarities in membership and structure. This included low overall diversity (Shannon index 0.5–1.7), strong dominance of single symbionts in individual spiders (McNaughton’s dominance index 0.68–0.93), and a core microbiome (>50% prevalence) consisting of 5–7 specific symbionts. The most abundant and prevalent symbionts were classified as Chlamydiales, Borrelia, and Mycoplasma, all representing novel, presumably Stegodyphus-specific lineages. Borrelia- and Mycoplasma-like symbionts were localized by fluorescence in situ hybridization (FISH) in the spider midgut. The microbiomes of individual spiders were highly similar within nests but often very different between nests from the same population, with only the microbiome of S. sarasinorum consistently reflecting host population structure. The weak population pattern in microbiome composition renders microbiome-facilitated local adaptation unlikely. However, the retention of specific symbionts across populations and species may indicate a recurrent acquisition from environmental vectors or an essential symbiotic contribution to spider phenotype.

Spotless starlings prefer spotless eggs: conspecific brood parasites cue on eggshell spottiness to avoid ectoparasites

Avian brood parasites are expected to select host nests according to characteristics that maximize offspring fitness, such as reduced probability of ectoparasitism. Spotless starlings, Sturnus unicolor, lay immaculate blue eggs that sometimes become brownish-spotted due to the activity of the ectoparasitic fly Carnus hemapterus on incubating birds. Therefore, conspecific parasitic females should adaptively avoid parasitizing nests with spotted eggshells. Here, we manipulated perceived risk of ectoparasitism by painting the eggs with either brown spots (similar to those due to ectoparasite activity) or blue spots. A third group of nests was maintained with immaculate eggshells. Nests with nonspotted eggshells showed the highest rate of brood parasitism, while nests with brownish or bluish spots on the eggshells were parasitized at a similar lower rate. These results suggest that brood-parasitic females use the presence of spots on the eggshells in their selection of host nests. This study adds to the scarce evidence showing that brood-parasitic birds select host nests with a low risk of ectoparasitism and demonstrates that colour patterns of the eggshell (i.e. spottiness) of their potential hosts represent a major cue employed to appraise the risk of ectoparasitism.

Smaller distance between nest contents and cavity entrance increases risk of ectoparasitism in cavity‐nesting birds

Determining costs associated with nest‐building behaviour and nest characteristics is of prime importance to understand the evolution of such extended phenotypic traits. We explored a cost of nest building experimentally to determine whether large nests attracted more ectoparasites. This cost may be especially important to cavity‐nesting species where larger nests likely reduce the distance between nest contents and the cavity entrance, making the host nest more vulnerable to ectoparasite detection. We manipulated the amount of nest material and the distance between nest and cavity entrance and estimated the abundance of ectoparasites (Carnus hemapterus flies) before and after manipulation in nests of spotless starlings Sturnus unicolor breeding in nest boxes. Nest size was enlarged and elevated in a first group of nests by adding nest material to the base of the nest (enlarged nests); a second group of nests was elevated with a piece of plastic but without adding nest material (elevated nests); a third group of nests was not manipulated (control nests). Number of flies per nest increased significantly in enlarged and elevated nests compared to control nests. Moreover, distance to cavity entrance after manipulation was negatively associated with the increase in the number of flies per nest (i.e. nests closer to the entrance had more parasites). Thus, nest size, through its effect on distance between nest and cavity entrance, affected fly abundance. Cues produced by nest contents located closer to the cavity entrance may be detected more readily by ectoparasitic flies searching for hosts. These results suggest a novel constraint for the evolution of nest size in cavity‐nesting birds.

Avian Egg Timers: Female Cowbirds Judge Past, Present, and Future When Making Nest Parasitism Decisions

The cognitive demands associated with brood parasitism are substantial. Not only must female parasites locate nests and assess their suitability for parasitism, they must also time parasitism to correspond with the breeding behavior of the host. Keeping track of the reproductive state of hosts for a variety of nests allows the parasite to select a nest where their egg can be incubated successfully. Thus, nest selection decisions are integral to obligate brood parasites’ reproductive success. In captive breeding flocks of brown-headed cowbirds (Molothrus ater), I studied (1) females’ abilities to time a host nest’s readiness for parasitism and (2) the timing of females’ nest selection decisions. I found that cowbirds can attend to the amount of time that elapsed since a host egg was added to a nest and can use that information to choose a nest for parasitism. Females made their choice of nest during prospecting the day before they laid, and, once decided, they did not update their decisions on the day of laying. Taken together, the results reveal that female cowbirds process substantial amounts of information about location, time, number, and rate in order to plan for future parasitism. This research program reveals that species-typical decisions integrate a variety of general and specialized cognitive abilities to allow females to behave adaptively and maximize reproductive success.

Female-female aggression and male responses to the two colour morphs of female common cuckoos

Female-only colour polymorphism is rare in birds, but occurs in brood parasitic cuckoos (Cuculidae). Obligate brood parasites leave incubation and parental care to other species (hosts), so female-female interactions can play a role in how parasites guard critical resources (host nests) within their laying areas. The plumage of adult female common cuckoos (Cuculus canorus) is either rufous (typically rare) or grey (common), whereas adult male conspecifics are monochromatic (grey). In previous studies, hosts and conspecific males responded with less intensity toward the rare female morph in support of a negative frequency-dependent benefit of female plumage polychromatism. Here, we assessed responses of both conspecific females and males to vocal playbacks of female calls, coupled with one of two 3D models of the different morphs of female cuckoos. At our study population, the rufous female morph was as common as the grey morph; therefore, we predicted similarly high rates of conspecific responses in both treatments. Both female and male cuckoos responded to playbacks acoustically, which demonstrated the primary role of acoustic communication in social interactions amongst cuckoos. Following this, some cuckoos flew closer to the models to inspect them visually. As predicted, no significant differences were detected between the live cuckoos’ responses toward the two colour morphs in this population. We conclude that dichromatism in female cuckoos evolved to serve one or more functions other than conspecific signalling.

Cuckoos use host egg number to choose host nests for parasitism.

To maximize their offspring success common cuckoo (Cuculus canorus) females should lay their eggs into host nests before incubation has begun. This ensures that the parasite chick hatches before all host chicks and can evict its foster siblings to monopolize host parental care. Many studies have demonstrated that most cuckoo eggs are indeed laid before the onset of host incubation. But cues used by female cuckoos to choose the right nest at the right time remain unclear. Here, we combine field observations with a field experiment to test whether female cuckoos use the number of eggs in the nests of their Oriental reed warbler (Acrocephalus orientalis) hosts to direct their choice. Over 8 years of field observations and 5 years of experiments, cuckoo females placed the majority of their eggs in nests with fewer than three host eggs, i.e. early in the laying sequence. For natural nests, the cuckoos may use information gleaned from the activity and behaviour of the host parents to make their choice. In our sets of experimental nests containing different numbers of model eggs, the vast majority of parasitism events occurred in nests containing a single egg. To our knowledge, this is the first field experiment showing that cuckoos choose host nests for parasitism based on the number of host eggs they contain. It appears that cuckoo females use the egg number to estimate the appropriate host nest stage for timely parasitism.

Diverse reproductive patterns of Bluehead Chub (Nocomis leptocephalus) and their relationships with nest size and interactions with an associate, Yellowfin Shiner (Notropis lutipinnis)

Nest construction by males is a common reproductive strategy in fishes, but little is known about intra-specific variation in nesting behavior and reproductive interactions with other species. In the southeastern USA, reproductive interactions between bluehead chub (Nocomis leptocephalus) and their nest associate, such as yellowfin shiner (Notropis lutipinnis), are mutualistic because the shiner requires chub nests to spawn but the chub also benefits from additional shiner eggs by reducing predation risk on their own eggs (i.e., dilution effect). As proxies for reproductive success of bluehead chub, we studied factors affecting their nest size variation and utilization by yellowfin shiner in three streams in the upper Piedmont region of South Carolina, USA. A total of 92 nests (71 spawned and 21 abandoned) were located, and we recorded spawning activity, nest size, number and size of male bluehead chub, number of yellowfin shiner, and environmental variables for 89 consecutive days in 2016. Path analysis showed that larger males spawned earlier in the season than smaller males, and instances of pebble re-use for nest construction increased through the spawning season, which extended from April to June. Larger males more likely shared the same nest with each other than smaller males, and larger or more males constructed larger nests and attracted more yellowfin shiner. Nest size also increased when males re-used substrate materials from old nests, and nests built in this manner (36 nests) were equally prevalent as newly constructed nests (35 nests). This study suggests that complex mechanisms mediate host nest size and interactions with nest associates, and individual variation likely shapes dynamics of the mutualistic reproductive interaction.

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.

Caught on camera: circumstantial evidence for fatal mobbing of an avian brood parasite by a host

Hosts have evolved a multiplicity of defensive responses against avian brood parasites. One of them is mobbing behaviour which often includes direct contact attacks. These aggressive strikes may not only distract the parasites but may also be fatal to them, as documented by cases of dead brood parasite females found near host nests. Here, we present the first video-recording of a great reed warbler (Acrocephalus arundinaceus) host whose vigorous nest defence appears to directly lead to the death of a female common cuckoo (Cuculus canorus). We suggest that the chance of parasite death probably rises with the presence of unfavourable factors, such as water below the nest. Our observation supports previous suggestions that hosts may pose a lethal danger to their parasites.