Syntactically aberrant vocalization in cuckoos disrupts communication but triggers host responses

Acoustic signals are primarily used for long-distance communication between conspecifics, but other species may eavesdrop on these signals. Aberrant vocalizations can disrupt information transfer, affecting the responses of both conspecifics and heterospecifics. Individuals with aberrant vocalizations may attempt to compensate, such as by increasing their signal production rate, but compensatory mechanisms remain largely unexplored, especially in species with innate vocalizations. It is also unclear whether such compensation influences the behaviour of eavesdropping heterospecifics. The common cuckoo, Cuculus canorus, an obligate brood parasite, produces a distinct innate ‘cu-coo’ call by males. We conducted field playback experiments during the breeding season using synthesized normal and aberrant (with syntax error, ‘cu-cu-coo’) cuckoo calls, as well as control wood pigeon calls. Our findings showed that cuckoos responded most strongly to high-rate normal cuckoo calls, followed by low-rate normal calls, while showing significantly less interest in syntactically aberrant and control calls, regardless of call rate. Noncuckoo (potential host) bird species also responded most strongly to high-rate normal cuckoo calls, but their response to low-rate normal and high-rate syntactically aberrant calls was similar. These results suggest that while cuckoos and other bird species primarily respond to normal cuckoo calls, noncuckoo species may still respond relatively strongly to high-rate syntactically aberrant calls. This study highlights the complexity of interspecific acoustic interactions and suggests that syntactically aberrant vocalizations in cuckoos may disrupt communication between conspecifics while still influencing the responses of eavesdropping heterospecifics.