Absence of egg discrimination in a suitable cuckoo Cuculus canorus host breeding away from trees

There is at present considerable variation in the level of antiparasite defences among different host species of avian brood parasites, but in many potential hosts some individuals reject poorly matching parasite eggs. Here we present unique absence of egg discrimination behaviour backed up by a lack of egg recognition abilities in a suitable common cuckoo Cuculus canorus host, the skylark Alauda arvensis. Skylarks did not show any clear rejection response to experimentally added highly non-mimetic foreign eggs in any behavioural context, even before they had started laying or when the whole clutch was exchanged with foreign eggs. This absence of antiparasite defence can be explained by the breeding habitat of larks consisting of largely treeless open landscapes where cuckoos have little access to the nests, thereby eroding the possibility of coevolutionary interactions. Our results are strikingly consistent with the spatial habitat structure hypothesis proposed to explain the occurrence and extent of avian host-parasite co-adaptation.