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

Brood parasites dramatically reduce the reproductive successof theirhosts, which therefore have developed defenses againstbrood parasites. Thefirst line of defense is protecting thenest against adult parasites. When theparasite has successfullyparasitized a host nest, some hosts are able torecognize andreject the eggs of the brood parasite, which constitutes thesecondline of defense. Both defense tactics are costly and would becounteractedby brood parasites. While a failure in nest defense impliessuccessfulparasitism and therefore great reduction of reproductive successofhosts, a host that recognizes parasitic eggs has the opportunityto reduce theeffect of parasitism by removing the parasiticegg. We hypothesized that, whennest defense is counteractedby the brood parasite, hosts that recognizecuckoo eggs shoulddefend their nests at a lower level than nonrecognizersbecausethe former also recognize adult cuckoos. Magpie (Pica pica)hoststhat rejected model eggs of the brood parasitic great spottedcuckoo(Clamator glandarius) showed lower levels of nest defensewhenexposed to a great spotted cuckoo than when exposed toa nest predator (acarrion crow Corvus corone). Moreover, magpiesrejecting cuckoo eggsshowed lower levels of nest defense againstgreat spotted cuckoos thannonrecognizer magpies, whereas differencesin levels of defense disappearedwhen exposed to a carrion crow.These results suggest that hosts specialize inantiparasitedefense and that different kinds of defense are antagonisticallyexpressed.We suggest that nest-defense mechanisms are ancestral, whereaseggrecognition and rejection is a subsequent stage in the coevolutionaryprocess.However, host recognition ability will not be expressedwhen brood parasitesbreak this second line of defense.