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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 ♀ (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.

and Summary We examined the extent to which parental investment, as measured by brood defence, is determined by selection via life history in a short‐lived bird, the great tit (Parus major). Great tit parents tending 1st and 2nd broods of the season were used to test five predictions of a cost/benefit model of brood defence based on the species average demography. The benefit was envisaged as the brood's contribution to a parent's fitness, and the cost as the potential loss if the defender dies in the act of defence; this loss was mirrored by Residual Reproductive Value plus a fraction of the brood dying as a consequence of the defender's death. Univariate and multivariate procedures were applied to six measures of defence response in 221 experimentally naive great tit pairs with nestlings. Stimuli consisted of a live raptor and a multi‐species taped mobbing chorus (both of which triggered strong alarm), the latter alone, and a novel, visual stimulus inhibiting nest visits for some time. As predicted by the model, response strength and associated risk increased with (1) advancing time in the breeding season, (2) the age of young, and (3) the number of young in 2nd broods. The last finding, being non‐trivial, is the strongest and the only unequivocal support for the notion that life‐history has moulded parental investment also in a short‐lived species; findings (1) and (2) could be alternatively accounted for by simply assuming that parents tailor their defence to environmental conditions permitting a new breeding episode. The model failed to predict the change of defence behaviour between 1st and 2nd broods. Instead, proximate factors coupled to the precise breeding area proved to be of prime importance in determining defence level, thereby falsifying the idea of a rigidly preprogrammed change of response level based on population parameter means. Instead, parents might gauge their defence to quality of young, as also allowed for in the model, or they may sometimes be constrained to display the full defence. As a by‐product, the experiments permit us to narrow down the range of selective agents producing the male parent's stronger defence. The sex difference, being prevalent in 1st broods and abating in 2nd broods, indicates that the male, by its defence behaviour, not only invests in its offspring but also in its home range and/or its mate. The latter interpretation is supported most directly by the male's defence behaviour in 1st broods.

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