Ardea
Official journal of the Netherlands Ornithologists' Union

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Curio E. (1987) Brood defence in the Great Tit: the influence of age, number and quality of young. ARDEA 75 (1): 35-42
The multidimensional defence behaviour of Great Tit parents when confronting a live Pygmy Owl at their nests was subjected to uni- and multivariate analyses; the owl is a predator of both adults and nestlings of hole-nesting birds. A life history model of parental anti-predator defence of a brood by an altricial parent bird balances the value of the brood (number and reproductive value vx of offspring) against the risk of the parent to die in the event. The model predicted successfully an increase of parental defence with 1) age of young in 1st broods, 2) clutch size in 1st broods (2nd brood not tested), and 3) brood size in second broods, i.e. when the residual reproductive value vr (a measure of the future sum of offspring of the parent's sex) is low. The model did neither explain the failure of parents to gear their defence to age of young in 2nd broods as well, nor to brood size or weight of young in 1st broods. These discrepancies are discussed in terms of functional and causal aspects not contained in the model. It is stressed that research should tackle the neglected problem of a facultative assessment of reproductive prospects apart from the reasoning based on the average demography of the species.


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