Exploring mate choice signals in the common cuckoo: the role of polymorphism and territory quality

In brood parasites both sexes are released from parental care creating a unique context in which sexual selection may operate differently than in species with parental care. Reproductive aspects of these species remain poorly studied as research has traditionally focused on host–parasite interactions, nevertheless, some general patterns have emerged including social monogamy and reciprocal mate choice based on cues of individual genetic quality. We investigated these dynamics in the common cuckoo, Cuculus canorus, an obligate brood parasite whose mating behaviour remains largely unexplored. Cuckoo females occur in two colour morphs (grey and rufous), and recent studies suggest that males may prefer a specific morph, whereas female choice remains unknown. To test this, we presented pairs of female models (grey vs rufous) coupled with playback to attract individuals. We also hypothesised that males may provide females with immaterial nuptial gifts by signalling territories containing more parasitable host nests, a key resource for female reproductive success. Territory quality was assessed by creating buffers around experimental sites and counting host nests, classifying sites as potentially parasitable when no nests were in the incubation phase. Behavioural responses were coded and analysed using GLMMs. Although interactions with the models were insufficient to assess morph preference, three behaviours were significantly more frequent in parasitable sites: cu-cu-coo and bark calls, and flights around the models, which were also positively correlated. Our results suggest that male cuckoos may combine vocal and flight display to signal high-quality territories, rich in host nests potentially parasitable. Further studies are needed to test the mechanisms allowing males to assess territory quality, such as by eavesdropping on host vocalisations.