| Nest predation is the main factor impeding successful breeding in birds.In order to obtain the maximum survival and reproductive fitness,birds have evolved complex and variable nest defense strategies.Nest defense by parents can increase the survival possibility of offspring and themselves,but it also costs defenders in terms of injury or death caused by predators.Therefore,in the face of predators,birds will trade-off between the adaptive benefits and costs,and choose appropriate nest defense behavior.However,there are still some problems to be solved,such as what factors affect the trade-off between benefits and costs of birds in defending predators,and how the bird nest defense behavior adjusting according to these factors.In this study,five secondary cavity-nesting birds,Japanese tit Parus minor,Cinereous tit Parus cinereous,Oriental magpie robin Copsychus saularis,Crested myna Acridotheres cristatellus and Silky starling Sturnus sericeus,were used as the study species.The nest defense behaviors were induced by placing specimens around the nest or the approaching humans.The effects of external factors including immediate predation risk,nest-site characteristics and colonial breeding and internal factors including parental physical conditions and offspring reproductive value on nest defense behavior were explored.The nest defense behavior of Japanese tits performed less stationary observition behavior(P < 0.01),more aggressivity behavior(P < 0.01)and threaten behavior(P = 0.029)against the nest predator chipmunks Tamias sibiricus than the adult predator sparrowhawks Accipiter nisus),and performed less stationary observition behavior(P < 0.01),more aggressivity behavior(P < 0.01)and vigilance behavior(P = 0.049)against the nocturnal raptor Indian scops owl Otus bakkamoena than the diurnal raptor sparrowhawks,suggesting that they could adopt different defense strategies according to predator types.The nest defense behavior scores of the Dongzhai population against chipmunks(P < 0.01)and red squirrels Sciurus vulgaris(P = 0.02)were lower than that of the Zuojia population,suggesting that the more costly hissing call behavior and attack behavior may gradually disappear under lower predation pressure,which is consistent with the relaxed selection hypothesis.The Japanese tits responded similarly to the simulated snake model and the snake specimen,indicating that they cannot discriminate between them and regarded both as the same threat level.The tail length of female Japanese tits with hissing call behavior in the face of human intruder were significantly shorter than those of no-hissing individuals(P = 0.043),while body condition(BCI),bill length and wing length had no effect on the hissing call behavior.Bold Japanese tits tend to perform hissing call behavior when confronted with human intruder,suggesting that personality may influence nest defense behavior.When confronted with adult predator sparrowhawks,large-sized Crested mynas performed higher nest defense behavior score than medium-sized Oriental magpie robins(P = 0.018)and small-sized Cinereous tits(P< 0.012).While confronted with nest predator chipmunks,the distance with specimens of Cinereous tits were shorter than those of Oriental magpie robins(P = 0.057)and Crested mynas(P = 0.045),suggesting that birds of different body sizes could assess the level of predation threat and adopt the most appropriate nest defense behavior.The intensity of hissing call behavior of Japanese tits against human intruder was similar between the pre-hatching and post-hatching periods.The nest defense behavior against the three predators was similar between the incubation and nestling periods.Japanese tits in Zuojia performed more aggressivity behavior(P = 0.051)and threaten behavior(P < 0.01)against nest predators in first breeding than second breeding,while the nest defense behavior of the Japanese tits in Dongzhai was similar between first and second breeding,suggesting that the number of offspring may be the main reason affecting the nest defense of Japanese tits,and the results are consistent with the parental investment theory.Cinereous tits performed similarly and low intensity of nest defense behavior against all three predators between the incubation and nestling periods,suggesting that they may reduce the investment of current reproduction for future reproduction.When confronted with nest predator chipmunks,nest site characteristics did not affect nest defense behavior.However,when confronted with adult predator sparrowhawks,the nest covered of aggressive individuals were lower than that of non-aggressive individuals(P <0.01),and the height of undergrowth shrubs around the nest of vigilant individuals were shorter than that of non-vigilant individuals(P = 0.050),indicating that Japanese tits may assess the risk level of different predator types according to nest site characteristics and then adopt different nest defense strategies.Japanese tits that bred in large-hole size nest boxes responded similarly against chipmunks and squirrels that are both accessible to the nest boxes.However,individuals that bred in small-hole size nest boxes performed more aggressivity behavior against chipmunks that could enter the nest box than squirrels that are not accessible to the nest boxes(P = 0.011),suggesting that cavity-nesting birds may adjust nest defense strategy according to the entrance hole size of nest boxes.Solitary breeding of Silky starlings performed more aggressive behavior to four nest intruder than colonial breeding individuals,suggesting that solitary breeding individuals may compensate for the lack of help from neighbors by increasing the intensity of nest defense behavior.In conclusion,predation risk,parental physical condition,body size,offspring reproductive value,nest site characteristics and colonial breeding could affect nest defense behavior of birds.This study helped to reveal the adjustment mechanism of nest defense behavior of birds and provide basic research data for the study of anti-predation behavior of birds. |
