Long-term changes in eggshell appearance in Lanius collurio (Red-backed Shrike): implications for declining Cuculus canorus (Common Cuckoo) parasitism

Brood parasitism imposes strong selective pressures on host species, driving the evolution of eggshell coloration and egg discrimination mechanisms. We investigated changes in the egg appearance of Lanius collurio (Red-backed Shrike) over 130 yr to understand their role in declining parasitism rates by Cuculus canorus (Common Cuckoo). We hypothesised that a decrease in intra-clutch variability accompanied by an increase in inter-clutch variability enhanced the shrike’s ability to distinguish parasitic eggs. We used calibrated digital photography to quantify eggshell coloration in both contemporary eggs and eggs from museum collections. Then, using visual modelling, we assessed intra- and inter-clutch variability by means of chromatic and achromatic contrasts as well as spotting pattern differences. The results revealed a significant increase in the divergence between inter- and intra-clutch chromatic contrast over time. A similar pattern was observed for pattern spectra differences. In the case of achromatic contrast, however, the opposite trend was noted. These findings suggest that long-term changes in the variability of eggshell coloration and patterning may have improved host defenses by increasing the detection and rejection of cuckoo eggs, potentially contributing to the decline in parasitism rates in L. collurio nests.