Detecting bird’s eggs in the diet of raccoon dog (Nyctereutes procyonoides)

Diet studies of carnivores are often based on analysis of prey remains found in scats or stomachs. In these analyses differences in decomposition rate and the degree of digestion of the prey item must be taken into consideration. In studies of raccoon dog diets, eggshells are rarely found. This may be due to three possible scenarios, which may act in combination: 1) raccoon dogs rarely eat bird´s eggs in the wild, 2) raccoon dogs eat eggs without swallowing shell parts, 3) rapid digestion of swallowed eggshells, so that they are only present briefly in stomach contents. In this study, the feeding behaviour of raccoon dogs on eggs of different sizes was observed under semi-natural conditions in an enclosure and the dissolution time of eggshells was examined. Dissolution time of eggshells of different thicknesses was tested experimentally using different concentrations of hydrochloric acid. The study showed a negative relationship between egg size/shell thickness and the amount of shell ingested by the raccoon dog. No eggshell, independent of bird species, was degraded beyond detectability after 12 hours in hydrochloric acid at pH 3.3. A correction factor estimating the mass of eggshell (g) ingested by the raccoon dog was calculated from the dry mass of eggshell found in the stomach or scat. Also the amount of eggshell estimated to be degraded by gastric acid is given. This correction factor ranged from 14 to 282 depending on eggshell thickness and egg size. From this study, it can be concluded that raccoon dogs do ingest some eggshell and that it is possible to detect eggshells in stomachs and scats of raccoon dogs, suggesting that the lack of eggshell in diet samples probably reflects a low proportion of eggs in the diet and/or preying more heavily on large eggs that are cracked open and have their contents eaten, rather than small eggs that are eaten shell-and-all.