ABS 2022
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A Review of Sensory Cues for Rejecting  Foreign Eggs from the Nest by the Blackbird (Turdus merula)
Andrew G. Fulmer1, Mark E. Hauber2. 1Fort Lewis College, Durango, Colorado, United States; 2University of Illinois Urbana-Champaign, Urbana, Illinois, United States

Costs of avian brood parasitism select for cognitive processes enabling anti-parasitic host strategies. Eurasian blackbirds (Turdus merula) build open-cup nests and are sympatric with common cuckoos (Cuculus canorus). While blackbirds are parasitized rarely by cuckoos, they are robust rejectors of non-mimetic foreign eggs in the nest. We provide a comprehensive, quantitative review of blackbird studies on foreign egg rejection. Vision is the primary direct sensory modality that blackbirds use to assess eggs, while less commonly studied rejection indirect cues include predator exposure, individual experience, clutch completion stage, and maternal hormonal state. Blackbirds eject even highly mimetic (conspecific) eggs at moderate rates, apparently relying on similar sensory cues. Although the cues involved in foreign egg recognition by Eurasian blackbirds do not appear specialized to non-mimetic cuckoo parasitism, we cannot differentiate between hypotheses that blackbird rejection evolved in response to parasitism by a) conspecifics and/or b) a now-extinct cuckoo gens.