Rules of Food Allocation between Nestlings of the Black-billed Magpie Pica Pica, a Species Showing Brood Reduction

—Rules of food allocation between nestlings of the black-billed magpie Pica pica, a species showing brood reduction. Aims: The existence of a size hierarchy of nestlings in a brood facilitates a secondary readjustment of brood size to resource availability, through the death of the smaller chicks when food is scarce. A mechanism to facilitate brood reduction would be for parents preferentially to feed the biggest nestlings in the nest. Therefore, feeding rules employed by adults are important because they predetermine the extent of brood reduction when food is scarce. In this study, feeding rules are studied in a species with habitual brood reduction, the black-billed magpie (Pica pica). Location: Hoya de Guadix, southeastern Spain. Methods: Nests were filmed using a micro-camera. Results: Parents preferentially fed the bigger nestlings, and those with higher begging levels, i.e. those which came closer to parents, responded most quickly, had higher begging intensity and adopted a higher posture when begging. Moreover, the preference by parents for the biggest nestlings was independent of the level of begging and the position in the nest. The process of multiple feeding (when more than one nestling was fed during one feeding event) was also analysed, in relation to nestling age and size: such events were more frequent with younger chicks in the nest, and when the first chick fed was relatively small. Conclusions: Results are congruent with the adaptive brood reduction hypothesis.