A good question without a definite answer, as new seabird species are being discovered every year, while known seabird species are rapidly being driven to extinction.
Seabirds include the following groups of birds:
– Penguins (approximately 16 species)
– “Tube-nosed” seabirds -albatrosses, petrels and shearwaters (approx. 93 species)
– Pelicans, gannets, frigatebirds, tropicbirds (approx. 57 species)
– Gull, terns, skuas, skimmers and auks (approx. 305 species, but not all of these are true seabirds)
One of the amazing things about seabirds is how little is still known about them! New species are being discovered all the time -I was recently involved in the discovery of a new species of storm petrel in the Azores (a group of islands in the Atlantic). What was thought to be a single species breeding there turned out to be 2 separate species, breeding in the same burrows -one species breeding in the summer and the other species breeding in the winter!
Meanwhile, other seabird species are being driven to extinction through human activities, including habitat destruction, climate change and (especially) the introduction of predators such as rats, cats and stoats onto the islands where seabirds breed. Some species are already extinct and others are critically endangered. To learn more about this important conservation issue, have a look at this website: http://www.birdlife.org/seabirds/