Avian nest defence behaviour: assessment in relation to predator distance and type, and nest height

We examined nest defence behaviour in relation to predator distance and type, and nest height in three closely related Acrocephalus warblers: moustached warbler, A. melanopogon, reed warbler, A. scirpaceus and great reed warbler, A. arundinaceus. We tested predictions of the dynamic risk assessment hypothesis for nest defence using experimental placement of three predator types (snake, stoat, harrier) 1 and 5 m from the nest. All species responded strongly to changes in predator distance, with less defence towards predators further from the nest. We found no interaction between predator type and distance. Nest defence was stronger at low nests to ground predators and at high nests to the aerial predator. There were combined interaction effects for predator type × nest height in relation to predator distance; for example, nest height was negatively related to nest defence to snakes, but only for snakes close to the nest. These results point to dynamic risk assessment and nest defence behaviour in Acrocephalus warblers.