The flight of seabirds

Different ways of flying
The flight of a bird and seabirds are no exception is a very complicated and dynamic activity. Due to constantly changing circumstances and windconditions seabirds have to adjust their wings and flight styel continiously. Therefore seabirds show several types of flight behaviour, though the boundaries are not very clear cut. It varies from gliding, soaring, flap-gliding to flapping and hovering. Seabird wings are not only used for flight in the air; a good number of diving seabirds use their wings also for underwater propulsion. Some, such as the penguins are flightless and use their wings exclusively to 'fly' below the surface. Other divers don't use their wings under water but propell themselves with their feet. This means that the 'flight system' of the various seabird species shows a great variation of adaptations to these habits.
The flight of seabirds can be divided in the following categories:

• Gliding
  Gliding is the least active way of moving. With wings held outstreched in the horizontal plane lift and propulsion is generated by environmental or external windforces e.g. lowering of altitude, updrafts and changeing wind gradients. Gliding is the most cost-efficient way of flying but due to the constant changing conditions at sea this is in seabirds mostly seen for only short periods.
• Soaring
  Soaring is an active way of flying in which the bird uses wind, updrafts, air currents that are deflected by rocks, shorelines and waves. There are different types of soaring flights.
  Slope soaring
  In its most elementary form it can be seen done by gulls that follow the dunes or by fulmars that are cruising along rocky cliffs. The deflection of the wind causes a cushion of air on which the bird 'floates' with motionless wings. At the downwind side of a cliff seabirds can take advantage of the updrafts right behind the rocks, caused by turbulence.
  Thermal soaring
  This flight behaviour uses the rising of warm air. It is best known from landbirds such as vultures, migrating storks and cranes that 'climb' the air current by circling upwards during long periods.  This type of lfying has the disatvantage that it produces ony low ground speeds.  Over sea strong rising warm air currents are not very common and only a handful of seabird species are known to use this technique, frigatebirds for instance.
  Dynamic soaring
  This type of flying that is is mostly seen with seabirds that are very much adapted to a pelagic life with strong winds. Kinetic energy is aquired by turning into the wind and climbing up along the wind to a higher wind speed. This kinetic energy is then used to turn again and travel downwind. This way seabirds are capable of covering large distances with high speed at reasonable low costs in terms of energy consumption.
  
  Pelagic seabirds often combine dynamic and slope soaring. Albatrosses are the best examples, but the larger petrels and shearwater are great soarers too. Albatrosses have long and narrow wings that are locked in an outstreched position and kept virtually motionless during long periods of dynamic and slope soaring over the waves, using strong winds and air speed changes caused by the waves. Flapping occurs only when air speed is low or when corrections have to be made.
  This type of flight doesn't require large and strong musculature because they don't have to perform much mechanical work. The muscles produce static forces to keep the long wings outstretched and in a horizontal plane. Soaring seabirds often have strongly pneumatized bones to keep them light.
   

• Flapping
  Many coastal seabirds: gulls, divers, alcids, cormorants and some smaller petrels have flapping as their default setting. The majority of them can glide or soar for a while, but most of the time they move by constantly beating their wings. Some are very strong flyers, such as the skuas which can accellerate to a very high speed to intercept other seabirds and make them throw up their prey. Others are more generalists and master different kinds of flying to a certain extent. Gulls are a good example of generalists. Cormorants, with their high wing loads, can glide only short distances, while pelicans are capable of gliding for longer periods and some pelican species are great thermal soarers.  Flapping requires larger and heavier muscles that use more energy.
• Flap-gliding
  This term covers the flight styles of many small- to medium-sized petrels which progress by glides and soaring interspersed with bursts of flapping. Flap-gliding occurs in various gradations depending on the species and the circumstances. The Procellaria petrels are great soarers but Puffinus shearwaters show considerable differences in flying habits. The Calonectris shearwaters, for instance, glide and soare with a few slow wingbeats every now and then, but the smaller Puffinus shearwaters fly with rapid wingbeats, alternated by gliding and soaring. Gadfly petrels combine flapping with dynamic soaring make high-towering, roller-coaster flights. When wind speeds increase dynamic soaring becomes more dominant.
• Hovering
  A peculiar way of flying used by several seabirds when hunting for food. In still air hovering birds are more or less stationary with constant wingbeating; storm petrels, terns and even frigate birds are good examples. Hovering requires well developed muscles to move the wings rapidly. During hovering the wingtips follow an eight shaped track in the air. Storm petrels also hover with their their feet kept in the water to stay in balance. Other seabirds can hover on updrafts along cliffs, constantly trimming wings an tails to maintain position and balance. Fulmars and Sooty albatrosses for instance are experts on this.
• Subaqueous flying
  This group is formed by flightless birds such as the penguins and flying species that combine the ability of flight with diving for food by propelling themselves under water beating their wings. The latter are partial subaqueous flyers.
  The flightless penguins are the extremes in this group with their flipperlike wings. But the alcids and diving petrels are capable flyers with very quick wing beats that cannot glide at all. They are perfectly suited to pursue a fast swimming fish under water. Most divers are relatively heavy birds with torpedo shaped bodies and small wings. Among the petrels a number of shearwaters, such as the Manx's, Little and Sooty and Short-tailed Shearwater are good divers but not as specialized as the alcids. Their flight muscles have to serve both ways of flying: under water and in the air. Their muscles are also very large and often supported by a large and elongated brestbone. Birds of this type always have a high wing loading and non pneumatized skeletons.

The flight apparatus of seabirds

The flight apparatus of (sea-) birds consists of a complex of collaborating bones, muscles and tendons and of course feathers. Although in fact the whole body of a bird plays a rol in flying, the de most important bones involved are the wing bones, the sternum, clavicles (furcula), coracoids and scapulas.

Different types of wings
The different ways of flying, and especially the combination of techniques require wings that are suitable for their variable job. Seabird wings are very complex airfoils that can adapt to the the flight technique the bird needs at any moment. The Sooty Shearwater for instance flies around the Pacific or Atlantic Ocean, from the Roaring Fourties with extremely high wind speeds, through the Tropical seas with totally different wind conditions all the way to the Arctic seas and back to their breeding grounds. It is also one of the best divers of its kind and can 'fly' several tens of meters below the water's surface with astounding agility. To be good in such different environments requires a very adaptable wing.

The qualities of a bird's wing depend on several variables of wich the most important are:

• Wing load
  This is the ratio wing surface to bodymass. A high wing loading means a relative high body mass. This is found among many of the diving seabirds: relative large and heavy bodies and small wings. Albatrosses on the other hand have also high wing loadings: long, but very narrow wings compared to the total body mass. Low wing loadings are found in storm petrels which can maneouvre with great ease and also in frigatibirds which have the lowest wing loads of all birds. Since indviual seabirds can vary a lot in weight, before and after foraging for instance, wing loadings vary als quite a lot.
• Aspect-ratio
  This a measure of the narrowness of a wing. Storm petrels for instance are broad winged and have low aspect ratios, which enables the birds to a high degree manouverability. With the increase of the size of the bird wings tend to become longer and therefore having higher aspect ratios. Albatrosses have high aspect ratios and high wing loads and are consequently clumsy when it comes to maneouvering at low speeds.
• Camber
  Bird wings have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, and always  with camber. Camber concerns the asymmetry e.g. the difference of the curvature between the upper- and underside of an airfoil and is an important feature. The difference in speed over and under the wing determines the lift that can be produced by the air current perpendicular to the wing.

These basic principles define the aerodyamic properties of the wing. Apart from their basic form, birds are able to adjust these properties to a high degree during flight., depending the specific activty that is performed and wind conditions. Nevertheless, the basic form of a wing gives an indication of the life habits of the species. Among tubenoses, small body size is correlated with low aspect-ratio wings, low wing loadings, low flight speeds and high maneouvrability, large body size with high aspect-ratio wings, high wing loadings, high flight speeds and poor maneouvrability. Intermediate body size is associated with intermediate values for these characterisitics. (Warham, 1990) Most other seabird species are adapted to a repertoire of flight habits.

Different parts of the flight apparatus

Birds differ from all other animals by their keeled brestbone (sternum) that forms the basis of a birds flying capacity. The flight muscles are attached to the keel and the plate. The size and shape of the plate and keel depends very much on the life style of its owner. Flightless birds, such as the Galapagos Shag, have virtually no keel. The shoulder blades (scapulas), the fused clavicles (furcula) and the coracoids complete the basis of the flight apparatus.
The wing itself is the adapted fore limb and can roughly be divided in an arm section, internally consisiting of the upper arm (humerus) lower arm (ulna and radius). Externally this part supports the secondary flight feathers, covered by several layers of wing coverts. The tendons, skin and covering feathers between the shoulder, elbow and carpal joint is an important part of the airfoil and forms a rounded trailing edge and defines the camber of the wing. The arm section varies considerable among seabirds. Large tubenoses such as albatrosses have long arm sections, but the under water wing propelling alcids have short arms. The hand section or manus consists internally of the carpometacarpus, and the four phalanges or digits. Externally the manus forms the basis for the primary feathers, mostly around ten. One little digit, the thumb forms the alula that can be moved separately from the primaries. It has an important function in regulating the airflow over the wing.

The relative size of the armbones varies with the dominant flight technique of the species.

Sternum Shoulder girdle: coracoid, clavicle and scapula Wing: humerus, ulna, radius and manus

Sternum and shouldergirdle of Fulmarus glacialis

1. Sternum 2. Coracoid 3. Clavicles / furcula 4. Scapula 5. Joint with the wing 6. Foramen trioceum

- Dijkstra, K., 2003, Gliding or flapping in the Antarctic. Flight morphology as an idicator for ecological differentiation. R.U. Groningen
- Warham, J., 1990, The Petrels, their Ecology and Breeding Systems, Academic Press, London