Evolutionary hypotheses regarding the function of eggshell phenotypes, from solar protection through mimicry, have implicitly assumed that eggshell appearance remains static throughout the laying and incubation periods. However, recent research demonstrates that egg coloration changes over relatively short, biologically relevant timescales. Here, we provide the first evidence that such changes impact brood parasite-host eggshell color mimicry during the incubation stage. First, we use long-term data to establish how rapidly the Acrocephalus arundinaceus Linnaeus (great reed warbler) responded to natural parasitic eggs laid by the Cuculus canorus Linnaeus (common cuckoo). Most hosts rejected parasitic eggs just prior to clutch completion, but the host response period extended well into incubation (~10 days after clutch completion). Using reflectance spectrometry and visual modeling, we demonstrate that eggshell coloration in the great reed warbler and its brood parasite, the common cuckoo, changes rapidly, and the extent of eggshell color mimicry shifts dynamically over the host response period. Specifically, 4 days after being laid, the host should notice achromatic color changes to both cuckoo and warbler eggs, while chromatic color changes would be noticeable after 8 days. Furthermore, we demonstrate that the perceived match between host and cuckoo eggshell color worsened over the incubation period. These findings have important implications for parasite-host coevolution dynamics, because host egg discrimination may be aided by disparate temporal color changes in host and parasite eggs.

Department of Biological Sciences Mount Holyoke College South Hadley Massachusetts 01074 USA; Organismic and Evolutionary Biology Graduate Program University of Massachusetts Amherst Amherst Massachusetts 01003 USA

Department of Psychology Hunter College and the Graduate Center The City University of New York 695 Park Avenue New York City New York 10065 USA

Department of Zoology and Laboratory of Ornithology Palacký University 17 listopadu 50 Olomouc 771 46 Czech Republic

Institute of Vertebrate Biology Academy of Sciences of the Czech Republic v v i Květná 8 603 65 Brno Czech Republic

Institute of Vertebrate Biology Academy of Sciences of the Czech Republicv v i Květná 8603 65 Brno Czech Republic; Department of Ecology Faculty of Science Charles University Prague Viničná 7128 44 Prague 2 Czech Republic

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