Tristan is primarily known for its wildlife. The island has been identified as an Important Bird Area by BirdLife International because there are 13 known species of breeding seabirds on the island and two species of resident land birds. The seabirds include northern rockhopper penguins, Atlantic yellow-nosed albatrosses, sooty albatrosses, Atlantic petrels, great-winged petrels, soft-plumaged petrels, broad-billed prions, grey petrels, great shearwaters, sooty shearwaters, Tristan skuas, Antarctic terns and brown noddies. Tristan and Gough Islands are the only known breeding sites in the world for the Atlantic petrel (Pterodroma incerta; IUCN status EN). Inaccessible Island is also the only known breeding ground of the Spectacled Petrel (Procellaria conspicillata; IUCN Vulnerable). The Tristan albatross (IUCN status CR) is known to breed only on Gough and Inaccessible Islands: all nest on Gough except for one or two pairs who nest on Inaccessible Island.
There are 13 species of resident land birds on the islands and how many species of breeding seabirds? (If the question is unanswerable, say "unanswerable")
unanswerable