With 8 figures in the text)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 eggmorphs 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.

Biological responses to climate change have been widely documented across taxa and regions, but it remains unclear whether species are maintaining a good match between phenotype and environment, i.e. whether observed trait changes are adaptive. Here we reviewed 10,090 abstracts and extracted data from 71 studies reported in 58 relevant publications, to assess quantitatively whether phenotypic trait changes associated with climate change are adaptive in animals. A meta-analysis focussing on birds, the taxon best represented in our dataset, suggests that global warming has not systematically affected morphological traits, but has advanced phenological traits. We demonstrate that these advances are adaptive for some species, but imperfect as evidenced by the observed consistent selection for earlier timing. Application of a theoretical model indicates that the evolutionary load imposed by incomplete adaptive responses to ongoing climate change may already be threatening the persistence of species.

Summary1. Why are some common and apparently suitable resources avoided by potential users? This interesting ecological and evolutionary conundrum is vividly illustrated by obligate brood parasites. Parasitic birds lay their eggs into nests of a wide range of host species, including many rare ones, but do not parasitize some commonly co-occurring potential hosts. 2. Attempts to explain the absence of parasitism in common potential hosts are limited and typically focused on single-factor explanations while ignoring other potential factors. We tested why thrushes Turdus spp. are extremely rarely parasitized by common cuckoos Cuculus canorus despite breeding commonly in sympatry and building the most conspicuous nests among forest-breeding passerines. 3. No single examined factor explained cuckoo avoidance of thrushes. Life-history traits of all six European thrush species and the 10 most frequently used cuckoo hosts in Europe were similar except body ⁄ egg size, nest design and nestling diet. 4. Experiments (n = 1211) in several populations across Europe showed that host defences at egg-laying and incubation stages did not account for the lack of cuckoo parasitism in thrushes. However, cross-fostering experiments disclosed that various factors during the nestling period prevent cuckoos from successfully parasitizing thrushes. Specifically, in some thrush species, the nest cup design forced cuckoo chicks to compete with host chicks with fatal consequences for the parasite. Other species were reluctant to care even for lone cuckoo chicks. 5. Importantly, in an apparently phylogenetically homogenous group of hosts, there were interspecific differences in factors responsible for the absence of cuckoo parasitism. 6. This study highlights the importance of considering multiple potential factors and their interactions for understanding absence of parasitism in potential hosts of parasitic birds. In the present study, comparative and experimental procedures are integrated, which represent a novel approach that should prove useful for the understanding of interspecific ecological relationships in general.

BackgroundTrait polymorphism can evolve as a consequence of frequency-dependent selection. Coevolutionary interactions between hosts and parasites may lead to selection on both to evolve extreme phenotypes deviating from the norm, through disruptive selection.Methodology/Principal findingHere, we show through detailed field studies and experimental procedures that the ashy-throated parrotbill (Paradoxornis alphonsianus) and its avian brood parasite, the common cuckoo (Cuculus canorus), have both evolved egg polymorphism manifested in discrete immaculate white, pale blue, and blue egg phenotypes within a single population. In this host-parasite system the most common egg colours were white and blue, with no significant difference in parasitism rates between hosts laying eggs of either colour. Furthermore, selection on parasites for countering the evolution of host egg types appears to be strong, since ashy-throated parrotbills have evolved rejection abilities for even partially mimetic eggs.Conclusions/SignificanceThe parrotbill-cuckoo system constitutes a clear outcome of disruptive selection on both host and parasite egg phenotypes driven by coevolution, due to the cost of parasitism in the host and by host defences in the parasite. The present study is to our knowledge the first to report the influence of disruptive selection on evolution of discrete phenotypes in both parasite and host traits in an avian brood parasitism system.