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