EGG-MORPHS AND HOST PREFERENCE IN THE COMMON CUCKOO (CUCULUS-CANORUS) - AN ANALYSIS OF CUCKOO AND HOST EGGS FROM EUROPEAN MUSEUM COLLECTIONS

An examination of about 12000 clutches of European passerines that contained eggs of the common cuckoo (Cuculus canorus), held in museum egg-collections, revealed statistically significant correlations between the cuckoo and the host eggs within a clutch in volume, ground colour, and size and percentage coverage of the spots. Although most cuckoo eggs were yellowish, the range in coloration and the percentage coverage of spots indicate that in these respects the cuckoo eggs are distributed along a continuum. However, a similar distribution was also found among the pooled host eggs. These results provide some support for the 'host preference hypothesis', which states that each cuckoo female specializes on one particular host species. By using a subjective classification, we found that there are at least 15 different cuckoo egg-morphs in Europe, but only 44% of the clutches contained cuckoo eggs of the egg-morph corresponding to the host eggs. The 'host preference hypothesis' therefore cannot provide a satisfactory explanation for the variation found among cuckoo eggs in Europe. However, 77% of the cuckoo eggs had been laid in nests of hosts with nesting sites similar to those of the main host of the egg-morph. This indicates that cuckoos also parasitize several species whose nest sites are similar to those of their main host. These results therefore provide support for the 'nest site hypothesis' which states that each cuckoo female parasitizes a group of host species with similar eggs or nest sites. The 'natal philopatry hypothesis' which states that female cuckoos may search for nests completely at random in their natal habitat is only weakly supported and can probably be rejected. Most of the hosts, main and secondary, nest among low vegetation or on the ground, whereas tree-nesting species are seldom parasitized. The most frequently used hosts in the egg collections we examined were species of the Acrocephalus and Sylvia genera of warblers, all species that breed in low vegetation.