I was in Tasmania over the weekend and fortunate to go out on the acclaimed Bruny Island wilderness cruise. We visited Australian fur seal and seabird colonies, learnt a bit of history and were astounded by the towering dolerite cliffs (the tallest sea cliffs in the southern hemisphere). Happily, we had a keen birdwatcher as our captain, and were blessed with the opportunity to observe a colossal aggregation of short-tailed shearwaters, as well as the occasional shy albatross and common diving petrel.
The flock filled the sky, in a stream that stretched for kilometres. I marvelled at the effortless flight of these birds, noticing that, except for when taking off from the water, the birds did not flap their wings once!
In fact, Procellariiformes, which includes the shearwaters, petrels and albatrosses, can fly for several hours (some for days) without needing to flap their wings. In a flying strategy known as dynamic soaring, they use the layers of wind above the sea to gain lift and momentum, effectively harvesting energy from the wind.
As air blows over a surface, the wind layers closer to that surface are slowed down by friction, creating a gradient of differing wind speeds. Ocean going seabirds, such as those in the Procellariiformes group, are able to take advantage of these layers (see figure below). They fly up into the stronger wind layers, which give them lift, then wheel down in a dive towards the water, gaining speed. At the last moment before they hit the surface, the birds use their momentum to again turn into the wind. Because the wind is slower closer to the surface, they lose little speed and any speed that is lost in the climb is gained in the next downward dive. By repeating this cycle continuously, they are able to travel great distances, while expending very little energy.
Dynamic Soaring: Seabirds fly up into strong winds to gain height, then dive towards the ocean to gain momentum, in a pattern that allows them to soar continuously without expending any more energy than it takes to steer their bodies. Source:http://www.ctie.monash.edu.au/hargrave/fogel.html
As I watched the wheeling birds, it was clear they were searching for something. They flocked, spread out and regrouped, all the while travelling at great speed. They were searching for a meal, of small fish or krill.
Not being sight hunters (i.e. gannets), Procellariiformes instead rely on an unusually well developed and highly sensitive sense of smell. This can be attributed to their strange, tubular-shaped nostrils. These enlarged nostrils, which open at the tip rather than the base of the beak as in most birds, provide a very large surface area over which scents can diffuse. Following windborne scents, Procellariiformes hone in on food that is literally miles away.
As soon as one bird landed, the behaviour of the entire flock would change. Birds came from everywhere to land at the site. Food sources can be few and far between in the open ocean, and this behaviour guides whole flocks to food sources. This is an altruistic behaviour that ensures the health of the breeding population.
Unfortunately, these same behaviours make Procellariiformes vulnerable to bycatch on fishing equipment, as thousands of birds flock to the trail of fish guts from fishing vessels.
Alas, eventually we had to turn for home, but the flock was so extensive that the birds accompanied us almost the entire trip back. It was exhilarating to race them against the wind, and I was sad to leave them. A true bucket-list event!