Interesting you would ask.
The rate of Serbian Ninja population decline has been tied directly to the shrinking numbers of their ecosystem counterparts, the Samoan Pirates. Pirates versus Ninjas used to be one of the driving dynamics of human evolution. Now that the Pirates have all gone to the hotel trade and offshore casino servers and whatnot, the Ninjas have no natural prey.
Most pretending to be Serbian Ninjas are, in fact, help desk technicians from Des Plaines, IL, and of generally questionable Slavic descent.