Experimental evidence that cuckoos preferentially parasitize host nests early in their laying cycle

Brood parasites, such as the Common Cuckoo (Cuculus canorus), rely on manipulating hosts from other species to raise their offspring. Selection should favor individuals that are able to identify and choose host nests that closely match the incubation stage of their own eggs. While intuitive, and supported by the findings of numerous long-term monitoring studies, experimental evidence of this remains sparse. By using video monitoring, we used three experimental groups to explore whether Common Cuckoos preferentially choose host Oriental Reed Warbler (Acrocephalus orientalis) nests that are early in their laying cycle. We found that Common Cuckoos preferentially parasitize nests of the host Oriental Reed Warbler that are earlier in their laying (less vs. more eggs) or nesting cycle (eggs vs. chicks), and videos of our experiments provide direct insights into their decision making process. Our results provide strong experimental evidence that Common Cuckoos are able to assess nesting stage and strategically choose nests that increase the likelihood of successful parasitism.