Factors Affecting Prey Preparation by Adult Magpies Feeding Nestlings

We collected 1,138 prey items from the guts of nestling Magpies (Pica pica) using neck collars. We noted the alterations (appendages missing, breakage) shown by the prey, and gave an index of preparation to the most abundant prey types (adult beetles and grasshopper nymphs). We then related these indices to prey size, age of the nestlings, brood size, and sampling date. Fifty-seven percent of the beetles were prepared, most of them being broken. The probability of breaking a beetle increased as the difference between clutch size and brood size increased, and as the residuals of prey size on sampling date increased. The probability of having at least one elytra removed increased as the beetle size increased and as the difference between clutch size and brood size increased. The degree of preparation of beetles decreased as their size decreased, as the nestlings grew older, and as the difference between clutch size and brood size decreased. Seventy-one percent of the grasshopper nymphs were prepared, the degree of preparation decreasing as their size decreased and as the season progressed. We conclude that the degree of prey preparation by Magpies feeding nestlings is the result of a trade-off between the benefits obtained by the nestlings and the costs to the parents.