ORIGINAL PAPER Post-ejection nest-desertion of common cuckoo hosts: a second defense mechanism or avoiding reduced reproductive success? Csaba Moskát & Erik C. Rosendaal & Myra Boers & Anikó Zölei & Miklós Bán & Jan Komdeur Received: 25 March 2010 / Revised: 13 August 2010 / Accepted: 3 November 2010 / Published online: 24 November 2010 # Springer-Verlag 2010 Abstract Hosts of the common cuckoo (Cuculus canorus), an avian brood parasite, develop antiparasite defense mechanisms to increase their reproductive success. Ejection of the parasite egg and desertion of the parasitized nest are the most typical adaptations in response to brood parasit- ism, but nest desertion may also occur in response to partial clutch reduction, independently from parasitism. Some great reed warblers (Acrocephalus arundinaceus) showed both mechanisms in the same incidence of cuckoo parasitism: in 18% of successful ejections of the parasite eggs, they deserted their nests. We studied if such cases of post-ejection nest-desertion are caused by brood parasitism or reduced clutch value. We experimentally parasitized clutches consisting of five or three host eggs with two painted conspecific eggs to mimic parasitic eggs, as multiple parasitism is frequent in the area. Although hosts ejected these parasitic eggs in both clutch categories (100% and 67% for the larger and smaller inital clutch sizes, respectively), we found that after manipulation, post- ejection nest-desertion frequently occurred at small (3-egg) clutches (40%), but rarely at large (5-egg) clutches (17%). The same phenomenon also occurred when unparasitized 3-egg clutches were reduced by two eggs, but not when 5-egg clutches were reduced in the same way. A logistic regression model revealed that only initial clutch size affected nest desertion of parasitized nests in our experiments. Therefore, we conclude that post-ejection nest-desertion is not a second antiparasite mechanism, which might serve as a redundant antiparasite defense, but a reaction to typically small and further decreased clutch size. Keywords Cuckoo . Great reed warbler . Antiparasite defense . Clutch size . Clutch reduction The common cuckoo (Cuculus canorus, hereafter “cuck- oo”) is a well-known brood parasite, which parasitizes more than a hundred small passerine species breeding in the Palearctic (Wyllie 1981; Davies 2000; Payne 2005). However, the number of regularly used hosts is much smaller, probably just over 20, and at least 17 host-specific races of the cuckoo (the so-called “gentes”) with developed egg mimicry were previously found in Europe (Moksnes and Røskaft 1995; Alvarez 1994; Antonov et al. 2007), with additional species in Asia (Higuchi 1998; Lee and Yoo 2004; Takasu et al. 2009). The coevolutionary processes between cuckoos and their avian hosts are typically explained as a coevolutionary arms race (Dawkins and Communicated by M. Soler C. Moskát (*) Animal Ecology Research Group, Hungarian Academy of Sciences, c/o Hungarian Natural History Museum, Ludovika ter 2., (Postal address: H-1431 Budapest P.O. Box 137, Hungary), Budapest 1088, Hungary e-mail: moskat@nhmus.hu E. C. Rosendaal : M. Boers : J. Komdeur Animal Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, Haren, The Netherlands A. Zölei Duna-Ipoly National Park Directorate, Költő u. 21., Budapest 1121, Hungary M. Bán Behavioural Ecology Research Group, Department of Evolutionary Zoology, University of Debrecen, Debrecen, Hungary Behav Ecol Sociobiol (2011) 65:1045–1053 DOI 10.1007/s00265-010-1109-7