Breeding success of common cuckoos Cuculus canorus parasitising four sympatric species of Acrocephalus warblers Oddmund Kleven, Arne Moksnes, Eivin Røskaft, Geir Rudolfsen, Ba ˚rd G. Stokke and Marcel Honza Kleven, O., Moksnes, A., Røskaft, E., Rudolfsen, G., Stokke, B. G. and Honza, M. 2004. Breeding success of common cuckoos Cuculus canorus parasitising four sympatric species of Acrocephalus warblers.  / J. Avian Biol. 35: 394  /398. We investigated the level of parasitism, egg mimicry and breeding success of cuckoos parasitising four sympatric species of Acrocephalus warblers in southern Moravia, Czech Republic. The parasitism rate was highest in the marsh warbler Acrocephalus palustris (44.8%) followed by great reed warbler A. arundinaceus (33.8%), sedge warbler A. schoenobaenus (26.5%) and reed warbler A. scirpaceus (11.6%). Although the cuckoo eggs showed a high level of mimicry the eggs of the marsh warbler this host species rejected 72% of the cuckoo eggs, resulting in a cuckoo breeding success of only 4.3%. Cuckoo eggs laid in great reed warbler and reed warbler nests showed a similar hatching success, but the cuckoo chicks survived better in great reed warbler nests, resulting in a breeding success of 30.4%, as compared to 16.4% in nests of the reed warbler. The relationship between the level of parasitism, host rejection of cuckoo eggs, cuckoo chick survival and breeding success is discussed for the four host species. O. Kleven (correspondence), Zoological Museum, Natural History Museums and Botanical Garden, University of Oslo, P.O. Box 1172 Blindern, N-0318 Oslo, Norway. E-mail: oddmund.kleven@nhm.uio.no. A. Moksnes, E. Røskaft and B. G. Stokke, Department of Biology, Norwegian University of Science and Technology, NTNU, N-7491 Trondheim, Norway. G. Rudolfsen, Institute for Biology, University of Tromsø, N-9037 Tromsø, Norway. M. Honza, Institute of Vertebrate Biology, ASCR, Kve ´tna ´ 8, CZ-603 65 Brno, Czech Republic. The common cuckoo Cuculus canorus is an obligate interspecific brood parasite where each female normally specialises on one host species (Wyllie 1981, Dro ¨ scher 1988, Nakamura and Miyazawa 1997), resulting in the evolution of different cuckoo strains or races called gentes that mimic the eggs of their corresponding hosts (Chance 1940, Baker 1942, Moksnes and Røskaft 1995, Gibbs et al. 2000). Different gentes may occur in sympatry (Brooke and Davies 1987, 1988, Edvardsen et al. 2001, Honza et al. 2002) and their population sizes may show considerable fluctuations resulting in varying parasitism rates between years and between different host species (Brooke and Davies 1987, Nakamura 1990, Brooke et al. 1998). There is not much quantitative data available for describing such interactions between host and parasite. Therefore, we investigated the relationship between cuckoo breeding success and parasitism rate in four sympatric and closely related host species of Acrocepha- lus warblers in southern Moravia, Czech Republic. The host species were the great reed warbler Acrocephalus arundinaceus , marsh warbler, A. palustris , reed warbler A. scirpaceus and sedge warbler A. schoenobaenus . Earlier results from radiotelemetry (Honza et al. 2002) # JOURNAL OF AVIAN BIOLOGY RESEARCH LETTERS Research letters are short papers ( 55 printed pages, about 4000 words), ideally presenting new and exciting results. Letters will be given priority, whenever possible, in the publication queue. Contribu- tions should be as concise as possible. An abstract is required. JOURNAL OF AVIAN BIOLOGY 35: 394  /398, 2004 394 JOURNAL OF AVIAN BIOLOGY 35:5 (2004)