The influence of predation risk on threat display in great tits

In wintering birds, conflicts over food are often resolved by threat displays. For displays to be effective, there ought to be a cost associated with displaying. We investigated whether increased vulnerability to predators due to reduced vigilance could be such a cost. Conflicts ought then to be resolved using fewer or less intense displays in conditions of high risk. We also looked for differences between dominants and subordinates in their reaction to risk. Because there is considerable evidence that subordinate wintering birds forage in riskier places than dominants, one might expect dominants to be less successful in conflicts under high predation risk. In our experiment, nine flocks of four or five wintering male great tits were kept in outdoor aviaries. In the predation risk treatment, a stuffed pygmy owl was briefly shown before birds were allowed access to a feeder. In the control treatment the owl did not appear. The predator presentation caused a reduction in the amount of aggression shown by subordinates, whereas for dominants there was no statistically significant change. Dominants were at least as successful in subduing subordinates under high risk as under low risk. A possible interpretation is that our experiment reflected a natural foraging situation for great tits, where ephemeral resources can appear unpredictably. In such situations, dominants may need to be bold to gain priority of access even under increased risk of predation, whereas a subordinate would gain little by risking a conflict with small chances of winning.