The mere presence of cuckoos in breeding area alters egg-ejection decisions in Daurian redstarts

Brood parasitic cuckoos and their hosts serve as model systems for
studying host-parasite coevolution. Egg-rejection behavior constitutes an
effective defense against brood parasitism, but some host species show
phenotypic plasticity in egg-rejection behavior. Direct exposure
to a cuckoo near the nest can increase egg-rejection likelihood, and
long-term studies have shown that increased the egg-rejection rates
generally correlate with higher parasite prevalence. However, it remains
unclear whether such increases result from interactions between parasites
and hosts, as these can be surprisingly common, or whether the mere
presence of cuckoos in the breeding area is sufficient. Daurian
redstarts Phoenicurus auroreus are a common
host of the common cuckoo Cuculus canorus that defend
against cuckoo parasitism mainly by ejecting the parasitic egg from the
nest. This species is unique, as its first breeding attempt of the year
takes place before the arrival of cuckoos, excluding the possibility for
direct interactions. We simulated the ambient presence of cuckoos or
hoopoes Upupa epops (control) in sub-populations of
redstarts during their first egg-laying period by presenting
taxidermic models and playing back vocalizations. Redstarts in
cuckoo-treated plots showed significantly higher egg-ejection rates than
individuals in control plots, even though females in both groups were
equally likely to recognize the parasitic egg. Among females that did
recognize the parasitic egg, those exposed to the cuckoo treatment were
more likely to eject it than those exposed to the control treatment. Our
results demonstrate unequivocally that the mere presence of cuckoos in the
environment is sufficient to provoke egg-ejection behavior.