Species facing similar selection pressures should recognize heterospecific alarm signals. However, no study has so far examined heterospecific alarm-call recognition in response to parasitism by cuckoos. In this study, we tested whether two sympatric host species of the common cuckoo Cuculus canorus, Oriental reed warbler Acrocephalus orientalis (ORW, main host), and black-browed reed warbler Acrocephalus bistrigiceps (BRW, rare host), could recognize each other’s alarm calls in response to cuckoos. Dummies of common cuckoo (parasite) and Eurasian sparrowhawk Accipiter nisus (predator) were used to induce and record alarm calls of the two warbler species, respectively. In the conspecific alarm-call playback experiments, ORW responded more strongly to cuckoo alarm calls than to sparrowhawk alarm calls, while BRW responded less strongly to cuckoo alarm calls than to sparrowhawk alarm calls. In the heterospecific alarm-call playback experiments, both ORW and BRW responded less strongly to cuckoo alarm calls than sparrowhawk alarm calls. BRW seemed to learn the association between parasite-related alarm calls of the ORW and the cuckoo by observing the process of ORW attacking cuckoos. In contrast, alarm calls of BRW to cuckoos were rarely recorded in most cases. BRW with low parasite pressure still developed recognition of heterospecific parasite-related alarm call. Unintended receivers in the same community should recognize heterospecific alarm calls precisely to extract valuable information.

Background: Brood parasitic birds such as cuckoos (Cuculusspp.) can reduce their host’s reproductive success. Such selection pressure on the hosts has driven the evolution of defense behaviors such as egg rejection against cuckoo parasitism. Studies have shown that Cinereous Tits (Parus cinereus) in China have a good ability for recognizing foreign eggs. However, it is unclear whether egg spots play a role in egg recognition. The aims of our study were to investigate the egg recognition ability of two Cinereous Tit populations in China and to explore the role of spots in egg recognition.Methods: To test the effect of eggshell spots on egg recognition, pure white eggs of the White-rumped Munia (Lonchura striata) and eggs of White-rumped Munia painted with red brown spots were used to simulate experimental parasitism.Results: Egg experiments showed that Cinereous Tits rejected 51.5% of pure white eggs of the White-rumped Munia, but only 14.3% of spotted eggs of the White-rumped Munia. There was a significant difference in egg recognition and rejection rate between the two egg types.Conclusions: We conclude that eggshell spots on Cinereous Tit eggs had a signaling function and may be essential to tits for recognizing and rejecting parasitic eggs.

Geographic variability in acoustic signals has been documented in many bird species. However, geographic variations in alarm calls have been so far neglected despite their crucial role on reducing risk to group members and relatives. We analyzed the note types and acoustic parameters of Japanese tit (Parus minor) alarm calls to three types of intruders (a nest predator, an adult predator, and a harmless species) from three populations in China. Our results revealed that tits in the same population produce similar note types to different intruders, but the three populations only shared six note types and each population had unique note types. The frequency and duration parameters of three shared common note types were significantly different among populations. The three populations belong to the same species, thus they have shared note types. We suspect that the unique note types occurring in each population may be related to three potential reasons: founder effect, predation pressure, and vocal learning. The differences in acoustic parameters of common notes among populations may be a consequence of adaptations to their environments. We suggest that population differences in the note levels of bird alarm calls do exist.

Behavioral divergence among populations is common across taxonomic groups, still we know very little about anti-predator behaviors. Animal exposure to predation risk is variable in different ecological contexts. In addition, reproduction value of animals in different geographical regions usually varies. In this study, we tested whether cinereous tits Parus cinereus in different populations exhibited nest defense behaviors similar to those of nest or adult predators and whether their nest defense behaviors showed geographical variation. By using field dummy experiments, we observed tits’ nest defense behavior in nest predator common chipmunk Tamias sibiricus and red squirrel Sciurus vulgaris, adult predator Eurasian sparrowhawk Accipiter nisus and nonthreatening species Oriental turtle dove Streptopelia orientalis in the ZJ (44° N), DZ (31° N), and DLS (18° N) populations, respectively. The response scores varied significantly across the 4 dummies in ZJ-tits and DLS-tits but did not in DZ-tits. When facing the chipmunk, ZJ-tits showed the highest response score and DZ-tits showed the lowest response score. When facing the squirrel, ZJ-tits showed a higher response score than tits in the other 2 populations. However, tits among the 3 populations responded similarly to a sparrowhawk or dove with slight response behaviors. In addition, response scores to nest predators were positively correlated with brood size across the 3 populations, but no trend was found for sparrowhawks or doves. Our results indicated that the nest defense behaviors of cinereous tits have evolved in response to large-scale geographical variation in ecological contexts and reproduction value.