Shearwaters are medium-sized long-winged seabirds. There are more than 30 species of shearwaters, a few larger ones in the genus Calonectris and many smaller species in the genus Puffinus. The Procellaria petrels and Bulweria were believed to belong to this group, but are only distantly related based on more recent studies, while the Pseudobulweria and Lugensa "petrels" are more closely related. The genus Puffinus can be divided into a group of small species close to Calonectris and a few larger ones more distantly related to both. One thing that can be agreed upon about taxonomy of Procellariidae is that it is in a state of flux.