Hauraki Gulf shearwaters: Globe-trotting on underwater wings
Those Aucklanders who have spent a summer day fishing in the Motuihe channel or have looked seaward from Takapuna Beach on a winter's morning, will be vaguely familiar with flocks of small black and white seabirds, fluttering shearwaters (Puffinus gavia), flying low and fast on whirring wings. These gulf residents are a spectacular sight. However what may not be apparent is that they are also some of the some of the most amazingly engineered creatures on the face of the planet, capable of movement above and below the seas surface that we can only dream of.
Crafted for movement
What do I mean? Well nearly all vertebrates have been crafted by evolution for movement in one of two fluid environments, water or air. For example, as bipedal apes we humans are supremely adapted for walking surrounded by air, with modifications to the bones of our feet, legs and hips that make our movement 75% less costly than our close relative the chimpanzee. We can walk for days and cover hundreds of kilometres in one sitting. Yet force us to swim 500 metres in the open ocean and many of us, myself included, might not return to tell the tale.
Shearwaters on the other hand can cross the water/air boundary with remarkable efficiency. In air their mobility is remarkable. Come winter time many of the gulf's fluttering shearwaters up and jump the ditch, flying to the coast of Australia in just a few days. Other gulf residents such as the sooty and flesh-footed shearwater head north and spend a few weeks migrating to the north Pacific Ocean on a round trip of 50,000 plus kilometres.
But at the same time these remarkable birds have joined penguins in the underwater realm chasing prey to depths of up to 90 metres, staying under water for minutes at a time and then using the same wings to propel them thousands of kilometres across the globe.
It's the size that counts
"So what!" you might say, "What’s the big deal?" Well here's the big deal: at the interface of air and water, generally what we call sea level, the density of the air above the surface is generally about 800 times less than that of the water below. This means, summarising several textbooks of theory here, that an optimal bird's wing adapted for flight underwater is radically smaller than one optimised for flight above the ocean's surface. In some cases the trade-off is so great that certain diving species abandon flight all together; we call them penguins.
Blood and bone research
So how do shearwaters fly above and below the ocean's surface so efficiently? A collaboration between Auckland Museum and physiologists from the University of Auckland (Drs Brendon Dunphy and Rachel Goddard) has been seeking to answer this question.
Working with bones and fixed wing specimens from the Land Vertebrate collection at the Museum and we now know that shearwater wings show adaptations for underwater flight in comparison with those of similar sized seabirds that don't dive. In particular, the wings of shearwaters are shorter and have compressed wing bones to reduce underwater drag and are powered by exceptionally strong muscles on both the up and down stroke.
At the same time blood samples collected during field research have revealed that the blood of shearwaters is power packed with oxygen rich red blood cells, to fuel that fast whirring flight necessary on shorter wings.
And finally lab analysis of the muscle tissue from shearwaters, unfortunately killed by commercial fishing boats, has shown that, like penguins, shearwaters have an enhanced ability to store oxygen in their muscle tissues in comparison with non-diving birds. These oxygen stores in both blood and muscle enable the birds to reach unassisted depths that us bipedal apes can only ever dream.
Nature truly is remarkable!
Cite this article
Hauraki Gulf shearwaters: Globe-trotting on underwater wings. Auckland War Memorial Museum - Tāmaki Paenga Hira. First published: 9 November 2015. Updated: 11 March 2016.