EVOLUTION OF VARIATION IN EGG COLOR AND MARKING PATTERN IN EUROPEAN PASSERINES - ADAPTATIONS IN A COEVOLUTIONARY ARMS-RACE WITH THE CUCKOO, CUCULUS-CANORUS

In a coevolutionary arms race between the cuckoo (Cuculus canorus) and its host species, both sides should evolve adaptations that will ensure the survival of their own offspring. The appearance of the eggs is central in this race. We investigated the occurrence of a defense mechanism that has not previously been demonstrated: the evolution of an increase in the variation in egg appearance (color and markings) between clutches, and an increase in the uniformity of eggs within clutches. We quantified the degree of homogeneity within and between clutches in the color and marking pattern of eggs of two groups of species, those regarded as suitable and as unsuitable hosts. The results show that statistically significant differences in the predicted direction existed at the species level for the interclutch variation between the two groups, but not for the intraclutch variation. For 34 species for which the rejection rates of artificial cuckoo eggs were known, the degree of variation was compared with the rejection rates. We found a statistically significant positive relationship between the rejection rate and the degree of interclutch variation in egg appearance, but not between the rejection rate and intraclutch variation. These results support the idea that there has been a coevolutionary arms race between the cuckoo and its hosts in Europe, leading to a high degree of interclutch variation in egg appearance in passerines. The lack of a trend regarding intraclutch variation is discussed.