Effect of Additional Food on Reproductive Success in the Magpie (Pica pica)

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Abstract
Magpie P. pica pairs were given extra food (fish) before and during the breeding season. Reproductive success was compared with that of the same pairs in other seasons, when no food was offered, and with that of a control group, without extra food. Food was also offered in empty territories, occupied in earlier seasons, in the hope of inducing reoccupation. Experimental pairs started egg-laying earlier, had larger clutches, heavier eggs and greater hatching success, and produced more fledglings than control pairs. This was also found when comparing the same females under experimental and control conditions. Food did not attract new settlers to empty territories. Experimental pairs laid earlier and started their laying within a significantly shorter period of time than did controls. The relatively small-scale effects of extra food on timing of breeding suggest that food alone does not determine start of laying, but that it is also stimulated by some other factors. Shortly after the additional factors were perceived by the birds, laying started. As experimental birds were in a better nutritional state than controls, they responded faster, and laid larger clutches and heavier eggs. The difference in reproductive success between the 2 groups was established during the nestling phase. Predation, mainly by hooded crows Corvus cornix L., and starvation, especially during adverse weather, caused the mortality. As invertebrate activity decreases in such periods, food scarcity is suggested to be the ultimate mortality factor. Extra food buffers adverse periods, giving a more stable temporal distribution of food as compared with natural conditions.