The Common Buckthorn (Rhamnus catharticus) is an introduced small tree from Eurasia. It is now common in the northern portions of the United States and is considered invasive in some areas.
The seed containing berries are mildly toxic to mammals. However, they are a favorite food of birds.
The berries contain a "cathartic" (thus the name) that allows the seeds to pass quickly through the digestive tract. (In a sense, they are a natural (yet toxic) laxative.) This may allow for more effective seed dispersal.
Thus, one of the other names of the Common Buckthorn is Purging Buckthorn.
