Shearwaters: flight apparatus


The shearwater wing consists of 11 bones:

The arm part (4 bones): humerus (1) and ulna/radius (3&4) are nearly of the same length and are rather long to the hand wing. A sesamoid or spreader bone (2) is situated in the pit of the elbow which is connected to the ectepicondylar process of the humerus by a ligament, forming a supporting structure for the patagial fan in the outstreched wing. This is also found in other petrels and albatrosses, with the exception of the Fulmarines, Prions and Blue Petrel.
The hand wing consists of the radiale and ulnare (5): two small bones in the wrist, carpometacarpus (6), alula (7) and the digits (8).
In the more aerial species, such as Calonectris the arm part is proportionally longer than in the more aquatic species such as the Sooty Shearwater Ardenna grisea and the Manx's group. The diving habits of the latter require shorter and stronger bones. All wing bones, but most strikingly the humerus, are stronger built and somewhat flattened to provide streamline and more strength to cope with the forces to which the wing is submitted during underwater propulsion by the wings. In the aerial species the humerus is more rounded and slenderer.


Similarly sized humeri showing the difference in flatness in the more aerial Wedge-tailed Shearwater Ardenna pacifica (upper) and the more aquatic Short-tailed Shearwater Ardenna tenuirostris (lower).

Sternum, coracoid and furcula

The sternum or breastbone of a shearwater is, depending on the species, variable in size and shape. It is somewhat constricted in the middle and has four distal notches [in individual cases two or up to six]. The processes form a frame to a flexible membrane that protects the intestinals. The more aerial species in which a gliding flight dominates have a relatively small, short, ‘square’ and pneumatized sternums: one to a few cavities at the base of the keel in the middle interior side. The 'squarish' shape implicates relatively small and short flight musculature, more suited for static forces. The diving species have proportionally large, elongated and non-pneumatized sterna to provide a basis to the large and longer flight muscles that are needed for underwater propulsion. Sternum, coracoids and furcula are lighter built in the aerial species compared with their diving kin. The pictures below show the differences between te species, all at the same scale.
N.B. The differences are clear, but be careful using them as an identification aid; there is variation in size and a little bit in shape within species. The bends in the keels of some sterna shown here arise after cleaning and drying the specimens. In live birds it is normally straight.


Calonectris shearwaters are gliders and don't dive very deep. As a result they have rather short sterna. Shortest in C. leucomelas wich is the the most aerial of all.
sternum calonectris leucomelas
Streaked Shearwater Calonectris leucomelas
Sternum calonectris leucomelas
Scopoli's Shearwater Calonectris (d) diomedea
sternum calonectris borealis 2006002Sternum calonectris borealis
Cory's Shearwater Calonectris (d) borealis
Sternum calonectris edwardsiiSternum Calonectris edwardsii
Cape Verde Shearwater Calonectris edwardsii


Buller's and Wedge-tailed shearwaters are the most aerial Ardenna species. Sooty and Short-tailed the most aquatic. Sooty is capable of very deep dives and has a very large sternum. Flesh- and Pink-footed and Great are comparable with the Calonectris species in this respect, good gliders but moderate divers and sit in de middle range.
Sternum ardenna puffinua bulleriSternum Ardenna Puffinus bulleri
Buller's Shearwater Ardenna bulleri (Thyellodroma)
Sternum ardenna pacifica puffinus pacificusSternum ardenna pacifica puffinus pacificus
Wedge-tailed Shearwater Ardenna pacifica (Thyellodroma)
sternum ardenna grisea 2003041
Sooty Shearwater Ardenna grisea (Neonectris)
Ardenna tenuirostrisSternum ardenna puffinus tenuirostris
Short-tailed Shearwater Ardenna tenuirostris (Neonectris)
Sternum ardenna puffinus gravissternum ardenna puffinus gravis
Great Shearwater Ardenna gravis (Ardenna)
Sternum ardenna puffinus creatopa
Pink-footed shearwater
Ardenna c. creatopa (Hemipuffinus)
Sternum ardenna puffinus carneipes
Flesh-footed Shearwater Ardenna (c.) carneipes (Hemipuffinus)


Puffinus shearwaters are capable divers and have proportionally large keels and long sterna with long
extensions to support the protective membrane, similar to the Sooty and Short-tailed Shearwater.

Manx's Shearwater Puffinus puffinus
Sternum puffinus yelkouanSternum puffinus yelkouan

Yelkouan or Levantine Shearwater Puffinus yelkouan

Balearic Shearwater Puffinus mauretanicus
Sternum puffinus gavia

Fluttering Shearwater Puffinus gavia

Hutton's Shearwater Puffinus huttoni
Sternum Subantarctic Shearwater Puffinus elegans
Subantarctic Shearwater Puffinus elegans

Baroli's Shearwater Puffinus baroli

Coracoid and Furcula