Shearwaters

The shearwaters are a morphologically and behaviourally diverse group of medium sized tube-nosed seabirds (Family Procellariidae). This group has been extremely successful within a variety of marine environments and now has a worldwide distribution. As will be discussed below, the systematics of this group remains very complicated, although recent molecular analysis has shed new light on some of outstanding difficulties. Note that the Calonectris shearwaters (Cory's, Cape Verde and Streaked Shearwaters) are considered separate from other shearwaters and are perhaps more closely related to the Procellaria (e.g. White-chinned, Westland Black, Parkinson's and Grey Petrels). I will group them here with Puffinus shearwaters simply because of the English names.

Birds of northern subtropical seas. The genus was constructed by Mathews and Iredale (1915) on the basis of several morphological features including the distinctive plumage coloration, slight lateral compression of the tarsi and long heavy bill. An important difference from the both the Puffinus shearwaters and Procellaria petrels is the strong sexual dimorphism with respect to body size. Bill length x bill height for example can be used to sex birds in the field. In Cory's Shearwater, males are on average 5-7% larger than females and in Streaked Shearwater the ratio is increased to 13%. Sexual dimorphism extends to the voice, that of the female having a overall lower-pitch. On breeding islands, will sing in flight as well as from the ground. It has been suggested that Cory's Shearwaters use echolocation to navigate nesting caves at night.

The flight style is strong and at times albatross-like. Even at great distance, Cory's Shearwater can be identified by its deep, lazy, wing beats interspersed with long glides close to the water surface. Feed by 'dipping' and by 'surface-seizing'. Often attracted to feeding tuna, mackerel shoals and whales, catching small fish and crustacea that are driven to the surface by predators. Will also take scraps from boats etc. In Japan and Korea, many Streaked Shearwaters become trapped in floating cages for storing live anchovies. 

Traditionally, the Puffinus shearwaters are divided into five subgroups: Thyellodroma (e.g. Wedge-tailed and Buller's Shearwaters), Hemipuffinus (e.g. Flesh and Pink-footed Shearwater), Ardenna (Great Shearwater), Neonectris (e.g. Sooty and Short-tailed Shearwaters) and Puffinus (Manx, Audubon's and Little Shearwater types). In this arrangement, which is based on behaviour, osteology, external morphology, distribution and the fossil record, Thyellodroma are considered the most ancestral and the Puffinus subgroup the most derived (Kuroda, 1954). It is thought that shearwaters first evolved in the North Atlantic and then subsequently colonized the world's oceans reaching the Pacific via the then submerged Isthmus of Panama. Recent molecular analysis supports this basic taxonomy and the North Atlantic origins of the group (Austin, 1996). Analysis of partial cytochrome b gene sequences suggests that the extant Puffinus species are derived from two ancestral groups that separared soon after the divergence of Puffinus from the shearwater ancestral stock (Austin, 1996). Rather than repeated invasions of the southern oceans, it seems that a single ancestral population moved to the southern hemisphere and then gave rise to the Thyellodroma, Hemipuffinus, Ardenna and Neonectris subgroups. This may explain the similarities in their transequatorial migrations.

The genus Puffinus is particularly well represented in the oceans of the world although the greatest abundance and diversity occurs in the cool waters surrounding Australia and New Zealand (Marchant and Higgins, 1990). The larger species (subgroups Thyellodroma, Hemipuffinus, Ardenna and Neonectris), are mostly annual trans-equatorial migrants, while members of the subgroup Puffinus tend to be more sedentary although some species (e.g. Manx Shearwater) are highly migratory also. The systematics of the genus remains complex and controversial. Problems are greatest within the Manx, Audubon's and Little Shearwater clusters (reviewed in Austin, 1996).