ABSTRACT
Two semi-quantitative predictions about the intensity of defence against a predator based on the associated costs and benefits as a function of predator species, were examined in great tits (Parus major) feeding nestlings. One premise was that defence behaviour is adaptive. Defence comprised of vocalizing and homing in on a live raptor near the nest hole. The intensity of defence as judged by two measures of approaching (Minimum Distance, Average Distance) varies with the species of raptor, i.e. sparrowhawk â (Accipiter nisus), pigmy owl (Glaucidium perlatum), and tawny owl (Strix aluco). With the exception of the response to the pigmy owl, defence intensities proved to conform to both predictions; the tits correctly assessed the relative overall risk from each predator ("predator pressure") i.e. its degree of specialization on great tits, and the immediate risk of defence. The failure to verify the predictions regarding the pigmy owl is thought to derive from our incomplete assessment of the cost function and/or from the response being mal-adaptive.The male takes a greater risk, exceeding the female's by an amount independent of the species of raptor. The sexual difference remains functionally unexplained.
