Egg spotting pattern in the common cuckoo and its great reed warbler host: a century perspective

This is the author accepted manuscript. The final version is available from OUP via the DOI in this record.The characteristics of common cuckoo (Cuculus canorus) and host eggs are widely thought to have coevolved over time, but few studies have tested this prediction. We compared cuckoo eggs with those of its primary host, the great reed warbler (Acrocephalus arundinaceus) from four time periods spanning >100 years (between 1900 and 2014), and studied if cuckoo eggshell patterns better resembled those of their hosts over time. We used image analysis to compare five eggshell pattern variables, relating to marking size, diversity, contrast, coverage, and distribution on the egg surface. Each feature showed different temporal trends. All but one of these variables ("dispersion" of spots among egg regions) were species-specific and differed between hosts and parasites. The magnitude of change was greater for hosts than cuckoos, which could be a consequence of host eggs’ more intensive and variable spottiness. Specifically, the proportion of the egg surface covered with pattern increased marginally over time, and the dispersion of spotting became more even over the egg surface. Egg marking contrast showed a decreasing trend, with species differences also decreasing, suggesting better mimicry. Our results suggest multidirectional evolution of eggshell components in this system, with potential implications for mimicry and rejection over time.This study was supported by the Hungarian National Research Fund (OTKA, grant no. 83217 to CM) and the National Research, Development and Innovation Office (NKFIH, NN118194 to CM). We are thankful to Douglas Russell for his help during the work in the Bird Collections, Natural History Museum, Tring, and to Gergely Babocsay for his help in the Vertebrate Collections, Mátra Museum, Gyöngyös, a division of the Hungarian Natural History Museum, Budapest. We are grateful to Michael I. Cherry and two anonymous reviewers for their valuable comments on the manuscript and to István Zsoldos for his assistance in field work. The Lower-Danube-Valley Inspectorate for Environmental Protection, Nature Conservation and Water Management provided permission for research. M.S. was funded by a BBSRC David Phillips Research Fellowship (BB/G022887/1). N.G. was supported by the Campus Hungary program, executed with the financial support of the European Union within the framework of the Social Renewal Operational Program (TÁMOP) of Hungary

Post-ejection nest-desertion of common cuckoo hosts: a second defense mechanism or avoiding reduced reproductive success?

Hosts of the common cuckoo (Cuculus canorus), an avian brood parasite, develop antiparasite defense mechanisms to increase their reproductive success. Ejection of the parasite egg and desertion of the parasitized nest are the most typical adaptations in response to brood parasitism, but nest desertion may also occur in response to partial clutch reduction, independently from parasitism. Some great reed warblers (Acrocephalus arundinaceus) showed both mechanisms in the same incidence of cuckoo parasitism: in 18% of successful ejections of the parasite eggs, they deserted their nests. We studied if such cases of post-ejection nest-desertion are caused by brood parasitism or reduced clutch value. We experimentally parasitized clutches consisting of five or three host eggs with two painted conspecific eggs to mimic parasitic eggs, as multiple parasitism is frequent in the area. Although hosts ejected these parasitic eggs in both clutch categories (100% and 67% for the larger and smaller inital clutch sizes, respectively), we found that after manipulation, post-ejection nest-desertion frequently occurred at small (3-egg) clutches (40%), but rarely at large (5-egg) clutches (17%). The same phenomenon also occurred when unparasitized 3-egg clutches were reduced by two eggs, but not when 5-egg clutches were reduced in the same way. A logistic regression model revealed that only initial clutch size affected nest desertion of parasitized nests in our experiments. Therefore, we conclude that post-ejection nest-desertion is not a second antiparasite mechanism, which might serve as a redundant antiparasite defense, but a reaction to typically small and further decreased clutch size

A shared chemical basis of avian host-parasite egg colour mimicry

Avian brood parasites lay their eggs in other birds’ nests and impose considerable fitness costs on their hosts. Historically and scientifically, the best studied example of circumventing host defences is the mimicry of host eggshell colour by the common cuckoo (Cuculus canorus). Yet the chemical basis of eggshell colour similarity, which impacts hosts’ tolerance towards parasitic eggs, remains unknown. We tested the alternative scenarios that (i) cuckoos replicate host egg pigment chemistry, or (ii) cuckoos use alternative mechanisms to produce a similar perceptual effect to mimic host egg appearance. In parallel with patterns of similarity in avian-perceived colour mimicry, the concentrations of the two key eggshell pigments, biliverdin and protoporphyrin, were most similar between the cuckoo host-races and their respective hosts. Thus, the chemical basis of avian host–parasite egg colour mimicry is evolutionarily conserved, but also intraspecifically flexible. These analyses of pigment composition reveal a novel proximate dimension of coevolutionary interactions between avian brood parasites and hosts, and imply that alternative phenotypes may arise by the modifications of already existing biochemical and physiological mechanisms and pathways.Branislav Igic, Phillip Cassey, Tomáš Grim, David R. Greenwood, Csaba Moskát, Jarkko Rutila and Mark E. Haube

Do cuckoos choose nests of great reed warblers on the basis of host egg appearance?

Prevailing theory assumes cuckoos lay at random among host nests within a population, although it has been suggested that cuckoos could choose large nests and relatively active pairs within host populations. We tested the hypothesis that egg matching could be improved by cuckoos choosing nests in which host eggs more closely match their own, by assessing matching and monitoring nest fate in great reed warblers naturally or experimentally parasitized by eggs of European cuckoos. A positive correlation between cuckoo and host egg visual features suggests that cuckoos do not lay at random within a population, but choose nests and this improves egg matching: naturally parasitized cuckoo eggs were more similar to host eggs as perceived by humans and as measured by spectrophotometry. Our results suggest a hitherto overlooked step in cuckoo-host evolutionary arms races, and have nontrivial implications for the common experimental practice of artificially parasitizing clutches

Rapid change in host use of the common cuckoo Cuculus canorus linked to climate change

Parasites require synchrony with their hosts so if host timing changes with climate change, some parasites may decline and eventually go extinct. Residents and short-distance migrant hosts of the brood parasitic common cuckoo, Cuculus canorus, have advanced their phenology in response to climate change more than long-distance migrants, including the cuckoo itself. Because different parts of Europe show different degrees of climate change, we predicted that use of residents or short-distance migrants as hosts should have declined in areas with greater increase in spring temperature. Comparing relative frequency of parasitism of the two host categories in 23 European countries before and after 1990, when spring temperatures in many areas had started to increase, we found that relative parasitism of residents and short-distance migrants decreased. This change in host use was positively related to increase in spring temperature, consistent with the prediction that relative change in phenology for different migrant classes drives host-use patterns. These findings are consistent with the hypothesis that climate change affects the relative abundance of different host races of the common cuckoo