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Failure to recognise own eggs (recognition errors) may be an important selective force behind acceptance of parasitic eggs, leading to a balance between rejecters and acceptors in a host population (the equilibrium hypothesis). We predicted that recognition errors should occur frequently among host species with intermediate rejection rates, whose rejection behaviour shows many conditional responses. The reed warbler Acrocephalus scirpaceus and great reed warbler A. arundinaceus fulfil these requirements. These two species were therefore used in an experiment where host birds were exposed to a common cuckoo Cuculuscanorus dummy, either <2 m or 5–10 m from the nest, at fishponds in southern Moravia (Czech Republic). The hosts responded to the cuckoo dummy, great reed warblers being much more aggressive than reed warblers, and both species being more aggressive towards the dummy when it was close to the nest than when it was farther away. We furthermore predicted that there should be more eggs rejected (ejected or nest abandoned) due to recognition errors among hosts exposed to a dummy close to the nest than among both those exposed to a dummy farther away from the nest and towards controls not exposed to cuckoo dummies. When comparing egg loss between groups of birds that were exposed to a cuckoo dummy with those that were not, we found no significant difference. However, partial egg loss was frequent among hosts in the studied population, most probably due to cuckoo depredation. We discuss why there were no detectable recognition errors in the studied population, when other researchers have claimed to have found such errors in host populations elsewhere.

Abstract.A review of all known descriptions of immature stages of the species of the genera Scaeva Fabricius, 1805, Ischiodon Sack, 1913 and Simosyrphus Bigot, 1882 is presented using SEM illustrations. The third instar larval and/or pupal morphology of Scaeva dignota (Rondani, 1857), Scaeva mecogramma (Bigot, 1860) and Simosyrphus grandicornis (Macquart, 1842) are newly described. All species of the genera studied in this paper are very similar for all the studied characters of their immature stages, including the chaetotaxy. Molecular characters of the mitochondrial cox1 gene (1128bp) were used for inferring relationships of the studied taxa. The nuclear internal transcribed spacer 2 (ITS2) was additionally applied for species delimitation of the closely related species Scaeva selenitica and S. dignota. The Palaearctic Scaeva species could be split into two groups based on the analysis of morphology of posterior respiratory process. These groups were previously diagnosed as S. selenitica

Aggression directed by 53 potential host species towards a dummy of the parasitic common cuckoo, Cuculus canorus, was tested in relation to their breeding habitat, their suitability as a host and whether they were breeding in sympatry or not with the cuckoo. Host habitats were divided into three categories: (1) always breeding near trees, (2) some populations breeding near trees, others in open areas, and (3) always breeding in open areas. Each species was also placed in one of five categories according to their suitability as a cuckoo host. Strong support was found for predictions derived from the 'spatial habitat structure hypothesis', which argues that common cuckoos only breed in areas where they have access to vantage points in trees. Thus, species which have some populations breeding near trees and others breeding further from trees have a different cuckoo-host population dynamics than species that always breed near trees, or always breed in open areas. Aggression levels were highest among species regarded as being always suitable as hosts, and species which always breed near trees. However, populations breeding in sympatry with the cuckoo were more aggressive than allopatric populations, indicating the plasticity of aggressive behaviour. Adaptive behaviour in cuckoo hosts can be predicted from the 'spatial habitat structure hypothesis'.