Cuckoo parasitism in a cavity nesting host: near absent egg-rejection in a northern redstart population under heavy apparent (but low effective) brood parasitism

Brood parasite - host systems continue to offer insights into species coevolution. A notable system is the redstart Phoenicurus phoenicurus parasitized by the “redstart-cuckoo” Cuculus canorus gens. Redstarts are the only regular cuckoo hosts that breed in cavities, which challenges adult cuckoos in egg laying and cuckoo chicks in host eviction. We investigated parasitism in this system and found high overall parasitism rates (31.1% of 360 redstart nests), but also that only 33.1% of parasitism events (49 of 148 eggs) were successful in laying eggs into redstart nest cups. The majority of cuckoo eggs were mislaid and found on the rim of the nest; outside the nest cup. All available evidence suggests these eggs were not ejected by hosts. The effective parasitism rate was therefore only 12.8% of redstart nests. Redstarts responded to natural parasitism by deserting their nests in 13.0% of cases, compared to desertion rates of 2.8% for non-parasitized nests. Our egg parasitism experiments ...