Food shortage for breeding Black-billed Magpies (Pica pica): an experiment using supplemental food

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Abstract
Supplemental food was provided to a breeding population of Black-billed Magpies (Pica pica), starting before and after eggs were laid, to determine if and when natural food was limited. The production of fledgling magpies was increased only when supplemental feeding was begun before laying. Laying date was also advanced when supplemental feeding began before laying, but clutch and egg size were not significantly changed. Because the two feeding treatments were conducted in different years, we cannot conclude whether natural food was limiting before or after egg laying. However, out data suggest that the former may have been the case. An improved design for food supplementation experiments, which can be used to determine the time of food shortage, is suggested.