September 13, 2000.
White Faced Storm Petrel: Part I
Most Storm petrels are dark brown and of quite similar appearance, but the White-faced Storm-Petrel is easily identified at sea. Its forehead and belly are covered with bright white feathers-in strong contrast to the dark upper parts: glossy jet?black bill, long black legs and a dark streak which trails backward from each eye. White?faced storm-petrels only grow to be about 20 cm (8 inches) long and have a wingspan only twice that. They are close relatives of other storm petrels, and are usually encountered as solitary individuals with a fluttering flight that keeps them close to the sea's surface. The species I am most familiar with is Wilson's storm petrel. It "patters" with its feet on the water while flying. In this odd performance the bird periodically pushes off from the water with one or both webbed feet (it is this behavior which gave Petrels their name: it comes form Peter, a reference to Saint Peter who attempted to walk on the water of Lake Gennesareth). Some theories have it that these birds use their feet to sense their prey or to cause it to give away its position by fleeing.
I believe that what is really going on is that the bird is giving its wings a momentary assist, a tiny rest by pushing off with its feet from the water's surface. Such slight advantages are important to species, and in this case may significantly affect the time the bird can remain on the wing. For example, when geese are migrating for long distances they fly in V formations, each bird positioning behind the one in front so as to get some lift from the updrafts (or tip vortices) coming off the ends of its wings. Thus, eleven geese in formation use about the same amount of energy as ten geese flying alone, every one but the leader getting a bit of help from the air pushed upwards by the others. Much nonsense has been written about heroic ganders leading their flocks through long migrations. But even a superficial look at any V formation of geese reveals that the leader changes constantly and that any bird finding itself in the lead (i.e. at the point of the V) immediately slides over or drops back so as to get behind some other bird's wings (thus receiving the benefit of its updrafts).
However, in the case of lone animals like storm petrels there is no flock with which to share flying effort so some other strategy has had to be invented. I believe it is their 'pattering' that gives the bird a periodical 'lift assist' as it pushes itself off from the surface of the water with its feet.
© 2000 - Roger Payne