Background For cavity-nesting birds, the nest entrance plays an important role in preventing predators from accessing nests. Several species of nuthatches use mud to narrow the entrance of cavities. In theory, the smaller the entrance hole size, the more effective it is against predators; however, few studies have tested whether narrowing the entrance hole size can affect the estimation of threat levels from nest predators in cavity-nesting birds. Methods Using dummy experiments, we tested whether Eurasian Nuthatches (Sitta europaea, narrow the entrance hole of cavities) and Cinereous Tits (Parus cinereus, do not narrow the entrance hole, as a control) perform different nest defence behaviours against Common Chipmunks (Tamias sibiricus, small nest predator) and Red Squirrels (Sciurus vulgaris, larger nest predator). Results Both nuthatches and tits exhibited stronger response behaviours (high dummy response scores) against chipmunks than against squirrels. Compared with tits, nuthatches exhibited more aggressive behaviours to chipmunks, but their responses to squirrels were similar. Conclusions Nest defence behaviours of nuthatches to chipmunks differed from tits, and the results suggested that nuthatches might estimate threat levels of nest predators according to their narrowed entrance-hole size.

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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), DLS (18° N) and DZ (31° N) populations, respectively. The response scores varied significantly across the four 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 two populations. However, tits among the three 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 three 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.

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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.