Egg spotting pattern in common cuckoos and their great reed warbler hosts: a century perspective

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) was 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.