Rapid decline of Common Cuckoo Cuculus canorus parasitism in Red-backed Shrikes Lanius collurio

Several 19th and 20th century studies on Common Cuckoo Cuculus canorus brood parasitism suggest that Red-backed Shrikes Lanius collurio used to be one of its common hosts in central Europe. However, since the late 1960s parasitism ceased to occur in Red-backed Shrikes in Hungary. Using data from bird ringing records in the Czech Republic and Slovakia we evaluate whether this finding holds on a broader scale. We found a rapid decline in the parasitism rate, expressed as the frequency of Common Cuckoo chicks encountered in Red-backed Shrike nests, during 1964-2006. During the first decade of the study, on average 2.19% of Shrike nests contained a Cuckoo chick, while in the last decade Cuckoos occurred in 0.37% of the nests only. This suggests a sixfold decline over the study period. Parasitism rates showed strong regional variations which were positively related to the size of regional host populations. In addition to the high ability of Shrikes in discriminating Cuckoo eggs, as found in previous studies, we suggest that decreases in regional host population numbers might lead to host abandonment within a few decades. Whether these two factors work in tandem, or independently, remains to be answered.