Host nest defense against a color-dimorphic brood parasite: great reed warblers (Acrocephalus arundinaceus) versus common cuckoos (Cuculus canorus)

We tested great reed warbler (Acrocephalus arundinaceus) discrimination against two common cuckoo (Cuculus canorus) female color morphs (gray and rufous) in two areas with different parasitism rates and proportions of the two morphs. Hosts recognized the two cuckoo morphs from a control, the feral pigeon (Columba livia), at Apaj, Hungary (where brood parasitism was heavy), whereas no significant differences among the models were recorded at Lužice, Czech Republic (where the parasitism rate was moderate). At Apaj, the hosts discriminated the rufous morph (which is slightly predominant there) better than the gray morph from the control. Between-site comparison (after controlling for background aggression) revealed that great reed warblers were more aggressive towards the rufous morph at Apaj than at Lužice, whereas their responses to the gray morph did not differ, corresponding with much higher between-site difference in the relative abundance of the rufous morph. Our results suggest that both local parasitism pressure and relative abundance of two female color morphs of a brood parasite may significantly influence host nest defenses.