Great spotted cuckoos improve their reproductive success by damaging magpie host eggs

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.

No evidence of host-specific egg mimicry in Asian koels

Avian brood parasitism is costly for the host, in many cases leading to the evolution of defenses like discrimination of parasitic eggs. The parasite, in turn, may evolve mimetic eggs as a counter-adaptation to host egg rejection. Some generalist parasites have evolved host-specific races (gentes) that may mimic the eggs of their main hosts, while others have evolved ‘jack-of-all-trades’ egg phenotypes that mimic key features of the eggs of several different host species. The Asian koel (Eudynamys scolopaceus) is a widely distributed generalist brood parasite that exploits a wide range of host species. Based on human vision, previous studies have described Asian koel eggs as resembling those of its main host, the house crow (Corvus splendens). Using measurements of egg length and breadth, digital image analysis, reflectance spectrophotometry and avian visual modelling, we examined Asian koel egg variation and potential mimicry in egg size and shape, and eggshell pattern and color in three sympatrically occurring host species in Bangladesh: the common myna (Acridotheres tristis), house crow, and long-tailed shrike (Lanius schach). We found some differences among Asian koel eggs laid in different host nests: a) Asian koel eggs in long-tailed shrike nests were larger than those laid in common myna and house crow nests, and b) Asian koel eggs in house crow nests were less elongated than those in common myna nests. However, these changes in Asian koel egg volume and shape were in the opposite direction with respect to their corresponding host egg characteristics. Thus, our study found no evidence for Asian koel host-specific egg mimicry in three sympatrically occurring host species.

Age-Related Brood Parasitism and Egg Rejection in Magpie Hosts

When the strength or nature of a host-parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts' breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpie (Pica pica) hosts is associated with parasite attack or their ability to reject foreign eggs. We recorded parasitism and model egg rejection of known-age individuals over their lifetime and established whether the likelihood of parasitism or egg rejection changed with age or longevity. Parasitism probability did not change with female age, and there was a trend toward longer-lived females being less likely to be parasitized. However, model egg rejection probability increased with age for each individual female, and longer-lived females were more prone to reject model eggs. Most females in the population were young, and the majority of them accepted model eggs, suggesting that brood parasites exploiting younger host individuals are benefitting from a lower defense level of their hosts. Our results stress that the intensity of selection by brood parasites may be mediated by the age structure of host populations, a to-date neglected aspect in brood parasite-host research.

Great spotted cuckoos show dynamic patterns of host selection during the breeding season. The importance of laying stage and parasitism status of magpie nests

Avian brood parasites depend entirely on their hosts to raise their nestlings until independence. Thus, parasite females should select suitable host nests for egg laying according to traits that enhance offspring survival. The availability of nests of certain characteristics influencing the survival of parasitic offspring is, however, temporally dynamic and, thus, patterns of host selection should be evaluated considering characteristics of available host nests the day of parasitism. This allows detecting possible seasonal changes and, therefore, a more realistic picture of host selection by brood parasites. In this paper, we adopt such a new approach and consider daily availability of magpie (Pica pica) host nests at different breeding stage that were or were not parasitized by the great spotted cuckoo (Clamator glandarius). Theory predicts that cuckoos should select host nests at the laying stage. Accordingly, we detected that cuckoos preferred to parasitize magpie nests at the laying stage but, mainly, those that already harbored one or two cuckoo eggs, which may seem counterintuitive. We also showed that patterns of host selection by cuckoos varied during the breeding season, which implies that brood parasite–host interaction is dynamic depending on phenology. These patterns are hidden when not considering the temporally dynamic nature of the availability of host nests of characteristics of interest. We discuss the importance of such patterns and considering diary hosts nests availability for detecting them.

Common cuckoos (Cuculus canorus) affect the bacterial diversity of the eggshells of their great reed warbler (Acrocephalus arundinaceus) hosts

The common cuckoo (Cuculus canorus) is an avian brood parasite, laying its eggs in the nests of other bird species, where these hosts incubate the parasitic eggs, feed and rear the nestlings. The appearance of a cuckoo egg in a host nest may change the bacterial community in the nest. This may have consequences on the hatchability of host eggs, even when hosts reject the parasitic egg, typically within six days after parasitism. The present study revealed the bacterial community of cuckoo eggshells and those of the great reed warbler (Acrocephalus arundinaceus), one of the main hosts of cuckoos. We compared host eggs from non-parasitized clutches, as well as host and cuckoo eggs from parasitized clutches. As incubation may change bacterial assemblages on eggshells, we compared these egg types in two stages: the egg-laying stage, when incubation has not been started, and the mid-incubation stage (ca. on days 5–7 in incubation), where heat from the incubating female dries eggshells. Our results obtained by the 16S rRNA gene sequencing technique showed that fresh host and cuckoo eggs had partially different bacterial communities, but they became more similar during incubation in parasitized nests. Cluster analysis revealed that fresh cuckoo eggs and incubated host eggs in unparasitized nests (where no cuckoo effect could have happened) were the most dissimilar from the other groups of eggs. Cuckoo eggs did not reduce the hatchability of great reed warbler eggs. Our results on the cuckoo-great reed warbler relationship supported the idea that brood parasites may change bacterial microbiota in the host nest. Further studies should reveal how bacterial communities of cuckoo eggshells may vary by host-specific races (gentes) of cuckoos.

Keeping eggs warm: thermal and developmental advantages for parasitic cuckoos of laying unusually thick-shelled eggs

Obligate brood parasites have evolved unusually thick-shelled eggs, which are hypothesized to possess a variety of functions such as resistance to puncture ejection by their hosts. In this study, we tested the hypothesis that obligate brood parasites lay unusually thick-shelled eggs to retain more heat for the developing embryo and thus contribute to early hatching of parasite eggs. By doing so, we used an infrared thermal imaging system as a non-invasive method to quantify the temperature of eggshells of common cuckoos (Cuculus canorus) and their Oriental reed warbler (Acrocephalus orientalis) hosts in an experiment that artificially altered the duration of incubation. Our results showed that cuckoo eggshells had higher temperature than host eggs during incubation, but also less fluctuations in temperature during incubation disturbance. Therefore, there was a thermal and hence a developmental advantage for brood parasitic cuckoos of laying thick-shelled eggs, providing another possible explanation for the unusually thick-shelled eggs of obligate brood parasites and earlier hatching of cuckoo eggs compared to those of the host.

Laying date, incubation and egg breakage as determinants of bacterial load on bird eggshells: experimental evidence

Exploring factors guiding interactions of bacterial communities with animals has become of primary importance for ecologists and evolutionary biologists during the last years because of their likely central role in the evolution of animal life history traits. We explored the association between laying date and eggshell bacterial load (mesophilic bacteria, Enterobacteriaceae, Staphylococci, and Enterococci) in natural and artificial magpie (Pica pica) nests containing fresh commercial quail (Coturnix coturnix) eggs. We manipulated hygiene conditions by spilling egg contents on magpie and artificial nests and explored experimental effects during the breeding season. Egg breakage is a common outcome of brood parasitism by great spotted cuckoos (Clamator glandarius) on the nests of magpie, one of its main hosts. We found that the treatment increased eggshell bacterial load in artificial nests, but not in magpie nests with incubating females, which suggests that parental activity prevents the proliferation of bacteria on the eggshells in relation to egg breakage. Moreover, laying date was positively related to eggshell bacterial load in active magpie nests, but negatively in artificial nests. The results suggest that variation in parental characteristics of magpies rather than climatic variation during the breeding season explained the detected positive association. Because the eggshell bacterial load is a proxy of hatching success, the detected positive association between eggshell bacterial loads and laying date in natural, but not in artificial nests, suggests that the generalized negative association between laying date and avian breeding success can be, at least partially, explained by differential bacterial effects.

Evidence of long-term structured cuckoo parasitism on individual magpie hosts

Brood parasites usually reduce their host's breeding success, resulting in strong selection for the evolution of host defences. Intriguingly, some host individuals/populations show no defence against parasitism, which has been explained within the frame of three different evolutionary hypotheses. One of these hypotheses posits that intermediate levels of defence at the population level may result from nonrandom distribution of parasitism among host individuals (i.e. structured parasitism). Empirical evidence for structured brood parasitism is, however, lacking for hosts of European cuckoos due to the absence of long-term studies. Here, we seek to identify the patterns of structured parasitism by studying great spotted cuckoo parasitism on individual magpie hosts over five breeding seasons. We also aim to identify whether individual characteristics of female magpies and/or their territories were related to the status of repeated parasitism. We found that 28·3% of the females in our population consistently escaped from cuckoo parasitism. Only 11·3% of females were always parasitized, and the remaining 60·4% changed their parasitism status. The percentage of females that maintained their status of parasitism (i.e. either parasitized or nonparasitized) between consecutive years varied over the study. Females that never suffered cuckoo parasitism built bigger nests than parasitized females at the beginning of the breeding season and smaller nests than those of parasitized females later in the season. Nonparasitized females also moved little from year to year and preferred areas with different characteristics over the course of the breeding season than parasitized females. Overall, females escaping from cuckoo parasitism reared twice as many chicks per year than those that were parasitized. In conclusion, our study reveals for first time the existence of a structured pattern of cuckoo parasitism based on phenotypic characteristics of individual hosts and of their territories.

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

Egg mimicry by obligate avian brood parasites and host rejection of non-mimetic eggs are well-known textbook examples of host-parasite coevolution. By contrast, reciprocal adaptations and counteradaptations beyond the egg stage in brood parasites and their hosts have received less attention. The screaming cowbird (Molothrus rufoaxillaris) is a specialist obligate brood parasite whose fledglings look identical to those of its primary host, the baywing (Agelaioides badius). Such a resemblance has been proposed as an adaptation in response to host discrimination against odd-looking young, but evidence supporting this idea is scarce. Here, we examined this hypothesis by comparing the survival rates of young screaming cowbirds and non-mimetic shiny cowbirds (Molothrus bonariensis) cross-fostered to baywing nests and quantifying the similarity in plumage colour and begging calls between host and cowbird fledglings. Shiny cowbirds suffered higher post-fledging mortality rates (83%) than screaming cowbirds (0%) owing to host rejection. Visual modelling revealed that screaming cowbirds, but not shiny cowbirds, were indistinguishable from host young in plumage colour. Similarly, screaming cowbirds matched baywings' begging calls more closely than shiny cowbirds. Our results strongly support the occurrence of host fledgling mimicry in screaming cowbirds and suggest a role of visual and vocal cues in fledgling discrimination by baywings.

CUCKOO PARASITISM AND PRODUCTIVITY IN DIFFERENT MAGPIE SUBPOPULATIONS PREDICT FREQUENCIES OF THE 457bp ALLELE: A MOSAIC OF COEVOLUTION AT A SMALL GEOGRAPHIC SCALE

The level of defense against great spotted cuckoo (Clamator glandarius) parasitism in different European populations of magpie (Pica pica) depends on selection pressures due to parasitism and gene flow between populations, which suggests the existence of coevolutionary hot spots within a European metapopulation. A mosaic of coevolution is theoretically possible at small geographical scales and with strong gene flow, because, among other reasons, plots may differ in productivity (i.e., reproductive success of hosts in the absence of parasitism) and defensive genotypes theoretically should be more common in plots of high productivity. Here, we tested this prediction by exploring the relationship between parasitism rate, level of defense against parasitism (estimated as both rejection rate and the frequency of the 457bp microsatellite allele associated with foreign egg rejection in magpies), and some variables related to the productivity (average laying date, clutch size, and number of hatchlings per nest) of magpies breeding in different subpopulations. We found that both estimates of defensive ability (egg rejection rate and frequency of the 457bp allele) covaried significantly with between-plot differences in probability of parasitism, laying date, and number of hatchlings per nest. Moreover, the parasitism rate was larger in more productive plots. These results confirm the existence of a mosaic of coevolution at a very local geographical scale, and the association between laying date and number of hatchlings with variables related to defensive ability and the selection pressure arising from parasitism supports the prediction of coevolutionary gradients in relation to host productivity.