Finding ephemeral, highly dispersed foraging sites in a vast ocean can be difficult, especially on the Western Australian coast where no stable zones of upwelling are thought to exist. Some tube-nosed birds are thought to locate prey using olfactory
cues are most effective in cool climates (Prince and Morgan 1987, Nevitt 1999, 2000). Sight is also used to locate prey and often the movements of other predators are
monitored, including conspecifics (Prince and Morgan 1987, Warham 1996). The energetics involved in searching large expanses of ocean indicate that an interplay may exist between the small scale variables generated large scale weather systems, that facilitate travel in the short-term. Consequently, the climate experienced may influence the foraging efficiency of different species in accordance to the limitations of their functional morphology.
The two congeneric species of shearwater studied, the Wedge-tailed Shearwater and Little Shearwater, are disparate species, thought to be suited for tropical and
temperate conditions respectively (Section 1.5.2). Their breeding distributions overlap on offshore islands along the Western Australian coast in a subtropical
climate. However, the Little Shearwater breeds during the austral winter when a more temperate climate is experienced and the Leeuwin Current achieves maximal flow rates (Cresswell 1990, Cresswell and Griffin 2000). In contrast, the Wedge-tailed Shearwater breeds in the austral summer, when conditions are warm and dry and the Leeuwin Current weakens, leaving behind cold or warm offshore waters in
accordance with its strength during the previous winter (Cresswell 1990, Pearce and Walker 1991, Pearce 1997, Cresswell and Griffin 2000). The difference in
morphology may reflect difference in their ability to adjust to proximal weather conditions and this will be investigated in this study.
Most Procellariiformes engage in dynamic soaring, which provides an energetically efficient way of travelling long distances (Warham 1990). The Little Shearwater is
the lightest species in the genus Puffinus and has skinny wings with a small area, resulting in high aspect ratio and wing loading. Birds with a high wing loading need to maintain higher flight speeds as they are more likely to stall (Warham 1996). Since wing loading is a function of weight, it can be highly variable for each individual depending on the phase of the life cycle. Payloads carried are costly as they may decrease the energy efficiency of flight. The weight problem is further compounded when winds are too low to allow shearwaters to use dynamic soaring as an efficient means of locomotion.
Generally, flapping flight is more energetically expensive than dynamic soaring (Warham 1990). The Little Shearwater is often encountered in flapping flight, travelling close to the water surface in the friction layer and may only use dynamic soaring in stronger winds, perhaps due to its relatively high wing loading. The Little Shearwater is unusual among shearwaters, in being able to fly close to the velocity for maximum range under flapping flight. A flight speed of about 14 ms –1 would be associated with increased power requirements, while flight at 10 ms –1 would be
economical (Alerstam et al. 1993 cited in Warham, 1996). In contrast, the Wedge- tailed Shearwater, with its lower wing loading, is able to use dynamic soaring under less windy conditions and is rarely seen flapping in flight, although it may revert to flap-gliding in times of low winds. Effectively, this study deals with two species that differ in their wing designs and hence, flight potential geared to survive in the
disparate weather patterns expected to prevail in summer and winter. Consequently, parental provisioning in both species may be affected by weather patterns, and this was investigated during this study.
Weather patterns may not only affect the ability of shearwaters to engage in low energy locomotion, but it may also affect the direction of travel and time taken to cover distances. Procellariiformes rarely fly with tailwinds, their flight path generally being at right angles across the wind or at an angle to the head wind. However, species with low wing loading travel into headwinds more often and are more likely to fly with tailwinds, whilst Manx-type shearwaters were rarely seen to do so (Spear and Ainley 1997a). Little Shearwaters are grouped among glide flappers, together with other Manx-type shearwaters and diving petrels. Whilst, Wedge-tailed Shearwaters have been classified as flap gliders among other surface-feeding shearwaters, including the Fleshy-footed Shearwater (Spear and Ainley 1997b). Furthermore, flight speeds generally increase with increasing headwinds (Spear and Ainley 1997b), indicating that birds may be able to travel more rapidly during certain weather conditions. Generally, wind speeds increase as pressure differences between adjacent weather systems increase. Consequently, shearwaters activity at the colony will be measured and tested in regards to different weather variables generated by the passage of different weather systems.
The slight differences in the morphological features which affect flight potential evident between the different shearwater species may also affect dive potential. Wing loading is thought to be inversely proportional to buoyancy and can be seen as
indicative of dive potential. The Wedge-tailed Shearwater has very broad wings and together with Buller’s Shearwater, has an unusually low wing loading for its mass (Warham 1996). This, combined with a long tail, lends the Wedge-tailed Shearwater great aerial dexterity. Consequently, Wedge-tailed Shearwaters are thought to be very buoyant birds that have difficulty diving to great depth (Brown et al. 1978). Indeed,
Wedge-tailed Shearwaters have been observed surface seizing and contact dipping, but are rarely seen at greater depth (Marchant and Higgins 1990). However, a study using maximum depth gauges suggested that Wedge-tailed shearwaters were able to attain an average maximal dive depth of 14 m and a maximum depth of 66 m (Burger 2001), despite being a more buoyant bird in flight (Brown et al. 1978). The narrow wings of the Little Shearwater are indicative of an even greater dive potential and maximum depth gauges will be utilised to measure dive depths in this study.
Feathers in optimal condition are necessary for both efficient flight and underwater propulsion (Warham 1996). The plumage of Procellariiformes is very oily, which allows total submergence without water logging (Warham 1996). Some species of shearwaters are highly skilled at underwater locomotion and engage in pursuit diving, during which their half-folded wings are used for propulsion to apprehend prey underwater (Warham 1990). Little Shearwaters are known to pursuit dive from both the surface and the air in the Western Atlantic and are morphologically suited for diving (Brown et al. 1978), although surface-seizing and surface diving also occur (Marchant and Higgins 1990). Similar behaviours have been observed for the Wedge-tailed Shearwater (Marchant and Higgins 1990). Dive depths are currently not available for the Little Shearwater, but the similarly sized Audubon’s Shearwater reached an average maximal depth of 15 m and a maximum depth of 35 m (Burger 2001).