The influence of predation risk on threat display in great tits

In wintering birds, conflicts over food are often resolved bythreat displays. For displays to be effective, there oughtto be a cost associated with displaying. We investigated whetherincreased vulnerability to predators due to reduced vigilancecould be such a cost. Conflicts ought then to be resolved usingfewer or less intense displays in conditions of high risk. We also looked for differences between dominants and subordinatesin their reaction to risk. Because there is considerable evidencethat subordinate wintering birds forage in riskier places thandominants, one might expect dominants to be less successfulin conflicts under high predation risk. In our experiment,nine flocks of four or five wintering male great tits were keptin outdoor aviaries. In the predation risk treatment, a stuffedpygmy owl was briefly shown before birds were allowed accessto a feeder. In the control treatment the owl did not appear.The predator presentation caused a reduction in the amountof aggression shown by subordinates, whereas for dominants there was no statistically significant change. Dominants were at leastas successful in subduing subordinates under high risk as underlow risk. A possible interpretation is that our experimentreflected a natural foraging situation for great tits, whereephemeral 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 wouldgain little by risking a conflict with small chances of winning.