The magpie (Corvidae) does not occur naturally in Japan. However, a small population has been established since the 1990s in Tomakomai City, Hokkaido, where artificial food might support magpies in winter when snow covers their foraging grounds. Our purpose is to estimate how much artificial resources such as pet food contribute to the magpies' diet using stable isotopes.

We collected eggs and feathers as magpie samples, and ground invertebrates as natural food in the field. Eleven kinds of commercial pet foods were used as artificial food. Eggs and feathers reflect the diet in the early spring and in the summer, respectively. Samples were analyzed for carbon (delta¹³C) and nitrogen (delta¹⁵N) stable isotopes by EA/IRMS. The proportions of different foods in the diet were estimated by the SIAR package for R.

Delta¹³C values of pet foods were much higher than those of natural resources. The proportion of pet food was substantial in both eggs, about 50%, and feathers, about 40%. These estimates imply that magpies in Tomakomai depend highly on artificial food, especially in the early spring.