Previous studies have shown that Eurasian Reed Warblers (Acrocephalus scirpaceus) are able to distinguish the Common Cuckoo (Cuculus canorus) from other intruders at the nest and that mobbing is an effective defense measure against brood parasitism. Here we studied the nest-defense behavior of Eurasian Reed Warblers facing four successive exposures to a Common Cuckoo to investigate whether a previous experience of interacting with a brood parasite may play a role in shaping the host's defenses in further encounters. Nesting warblers significantly increased their aggressive behavior from the first to the second presentation of a dummy Common Cuckoo and then sustained their response at the same intensity. The intensity with which the birds tested mobbed the dummy decreased both as the season progressed and with the time of the day. Multiple encounters with the dummy, however, did not increase the warblers' propensity to make recognition errors, i.e., to reject their own eggs in the absence of a Common Cuckoo egg in the nest. We discuss possible explanations of the increased intensity of nest defense with respect to the positive-reinforcement hypothesis and known patterns of nest attendance in the Eurasian Reed Warbler.
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Vol. 112 • No. 4