Since the first 2 are quite common "open trackers" (where open refers to the fact that they track any hash you submit), it is often quite likely to find sources

Since the first 2 are quite common "open trackers" (where open refers to the fact that they track any hash you submit), it is often quite likely to find sources there, because other people add them as well when they want more sources. Furthermore my comment will lead to more people, who might not have a DHT-supporting client, adding those open trackers.