Antagonistic antiparasite defenses: nest defense and egg rejection in the magpie host of the great spotted cuckoo

Brood parasites dramatically reduce the reproductive success of their hosts , which therefore have developed defenses against brood parasites. The firs t line of defense is protecting the nest against adult parasites. When the parasite has successfully parasitized a host nest, some hosts are able to r ecognize and reject the eggs of the brood parasite, which constitutes the s econd line of defense. Both defense tactics are costly and would be counter acted by brood parasites. While a failure in nest defense implies successfu l parasitism and therefore great reduction of reproductive success of hosts , a host that recognizes parasitic eggs has the opportunity to reduce the e ffect of parasitism by removing the parasitic egg. We hypothesized that, wh en nest defense is counteracted by the brood parasite, hosts that recognize cuckoo eggs should defend their nests at a lower level than nonrecognizers because the former also recognize adult cuckoos. Magpie (Pica pica) hosts that rejected model eggs of the brood parasitic great spotted cuckoo (Clama tor glandarius) showed lower levels of nest defense when exposed to a great spotted cuckoo than when exposed to a nest predator (a carrion crow Corvus corone). Moreover, magpies rejecting cuckoo eggs showed lower levels of ne st defense against great spotted cuckoos than nonrecognizer magpies, wherea s differences in levels of defense disappeared when exposed to a carrion cr ow. These results suggest that hosts specialize in antiparasite defense and that different kinds of defense are antagonistically expressed. We suggest that nest-defense mechanisms are ancestral, whereas egg recognition and re jection is a subsequent stage in the coevolutionary process. However, host recognition ability will not be expressed when brood parasites break this s econd line of defense.