Northern Gannets Morus bassanus typically forage by diving from high above the water surface. Their subcutaneous (s-c) tissues are invested by an elaborate system of air diverticula that presumably function in cushioning the impact of their entry into the water. The anatomical details of this system were studied by dissection and latex injection in 15 carcasses of these birds. The s-c air diverticula consist mainly of two independent systems of intercommunicating compartments that are bilaterally symmetrical, cover the ventral and lateral regions of the trunk and the proximal portions of the wings and legs, and communicate with the ipsilateral region of the clavicular respiratory air sac. This communication, which opens into the axillary region, is through a narrow gap between the subcoracoideus and coracobrachialis caudalis muscles. Two other, smaller, independent systems of s-c air diverticula, also bilaterally symmetrical, may contribute to cushioning the Northern Gannet’s body during its dives: one at the thoracic inlet, which communicates with the corresponding side of the clavicular air sac, and the other along the neck, which communicates with the nasal cavities and the choanal opening. Further work is required to define more precisely the function of these extensive air diverticula and air circulation within them.

The avian respiratory system is the most efficient among those of all air-breathing vertebrates and is unique in its basic structure (King & McLelland 1984). Its extensive system of air sacs allows a near-continuous flow of fresh air through the pulmonary air capillaries at countercurrent to the blood circulation and throughout the respiratory cycle. Most avian species have four paired air sacs (cervical, cranial thoracic, caudal thoracic, abdominal) and one unpaired air sac (clavicular). Depending on the species, some of these air sacs can project complex systems of diverticula between muscles and into the subcutis and pneumatic bones of the trunk, pectoral and pelvic girdles, and limbs (McLelland 1989; O’Connor 2004). Some members of the order Pelecaniformes have an elaborate and extensive system of subcutaneous (s-c) air diverticula. Northern Gannets Morus bassanus, which typically forage by diving from heights of up to 30 m above water and reaching speeds of up to 100 km/h on impact with water, are thought to use these s-c air diverticula as a means of cushioning this impact (Montagu 1813; Gurney 1913; Nelson 1978). It is not known, however, whether these diverticula are inflated voluntarily prior to diving or whether air is simply prevented from exiting them as the bird hits the water. Regardless, an efficient communication is likely needed between the respiratory tract and the system of s-c air diverticula and among the various compartments of this system. Subcutaneous air diverticula were described, albeit only partially, many years ago in the Northern Gannet (Montagu 1813; Gurney 1913) and in the Brown Pelican Pelecanus occidentalis (Richardson 1939). According to the study of the Brown Pelican by Richardson (1939), the communication between the respiratory system, specifically the clavicular air sac, and the s-c air diverticula is located caudolaterally to the head of the coracoid bone and below the head of the humerus, ‘primarily between the M [muscle] coracobrachialis posterior and the M subcorachoideus’. Similarly, in his study of the Northern Gannet, Gurney (1913), quoting C. B. Ticehurst, states that the s-c air diverticula communicate with the respiratory system by way of a passage just outside the coracoid bone and close to the tendon of the ‘pectoralis minor muscle’ (‘M coraco- brachialis posterior’, according to Richardson (1939)). These authors also briefly describe the distribution of the s-c air diverticula along the ventral region of the trunk and down the thighs and wings and the separation of these diverticula between left and right sides of the body. The description of s-c air diverticula that they offer is, however, insufficient to fully understand the exact pattern of air flow among their various compartments.

The objective of this study was to provide a more detailed description, complemented by photographs, of the anatomy of the s-c air diverticula in the Northern Gannet than is currently available in the literature. More specifically, we describe the distribution of s-c air diverticula along the body and the communication between the system of s-c air diverticula and the respiratory system in this species. We also hypothesise that the wings’ position may alter this communication as it changes from extended away from the body while flying and soaring to flexed against the body when diving. More specif- ically, we predict that wing flexion against the body closes the communication between the two systems, thus preventing air from escaping the s-c air diverticula, thus ensuring a firm cushion on impact.

We thank Shelley Ebbett, Mike Needham, and Fiep de Bie for photography of the specimens and preparation of the figures. We also thank three anonymous reviewers for their excellent comments.

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