How Do Oriental Reed Warblers Recognize Cuckoo Eggs?

Oriental reed warblers (Acrocephalus orientalis) are favorite hosts of common cuckoos (Cuculus canorus). However, the cognitive basis and underlying mechanisms of the host egg discrimination behavior remain not well understood. In this study, we conducted egg recognition experiments using three egg mimicry levels (non, poorly, and highly mimetic eggs) to observe Oriental reed warbler behavior (rejection or acceptance) at three breeding stages (pre-egg-laying, single host egg, and multiple host eggs). Results showed that oriental reed warblers accepted all highly-mimetic (conspecific) eggs across all stages, while rejecting most non- and poorly mimetic eggs. Rejection rates for nonmimetic eggs were 100% in both pre-egg-laying and single-egg stages, and 86.7% in the multiple-egg stage; for poorly mimetic eggs, rejection rates were 83.3%, 70%, and 66.7%, respectively. The ability to reject foreign eggs even at the pre-egg-laying stage suggests that oriental reed warblers may use a memory-based template for egg recognition, relying on innate or experientially acquired cues. However, the consistent acceptance of conspecific eggs also aligns with uncertainty-based acceptance strategies. These findings indicate that oriental reed warblers employ a complex and potentially multi-mechanistic cognitive system for egg recognition.