Storm-petrels - Morphology and Flight

Morphology and Flight

Storm petrels are the smallest of all the seabirds, ranging in size from 13–26 cm in length. There are two body shapes in the family; the Oceanitinae have short wings, square tails, elongated skulls, and long legs; the Hydrobatinae have longer wings, forked or wedge-shaped tails and shorter legs. The legs of all storm petrels are proportionally longer than those of other Procellariiformes, but they are very weak and unable to support the bird's weight for more than a few steps.

The plumage of the Oceanitinae is dark with white underparts (with the exception of the Wilson's Storm Petrel) All but two of the Hydrobatinae are mostly dark in colour with varying amounts of white on the rump. Two species have different plumage entirely, the Hornby's Storm Petrel which has white undersides and facial markings, and the Fork-tailed Storm Petrel which has pale grey plumage. This is a notoriously difficult group to identify at sea. Onley and Scofield (2007) state that much published information is incorrect, and that photographs in the major seabird books and websites are frequently incorrectly ascribed as to species. They also consider that several national bird lists include species which have been incorrectly identified or have been accepted on inadequate evidence.

Storm petrels use a variety of techniques to aid flight. Most species will occasionally feed by surface pattering, holding and moving their feet on the water's surface while holding steady above the water. They remain stationary by hovering with rapid fluttering or by using the wind to anchor themselves in place. This method of feeding flight is most commonly used by Oceanitinae storm-petrels. The White-faced Storm Petrel possesses a unique variation on pattering, holding its wings motionless and at an angle into the wind it pushes itself off the water's surface in a succession of bounding jumps. Storm petrels also use dynamic soaring and slope soaring to travel over the ocean surface. Dynamic soaring is used mostly by the Hydrobatinae, gliding across wave fronts gaining energy from the vertical wind gradient. Slope soaring is more straightforward and favoured by the Oceanitinae, the storm petrel turns to the wind, gaining height, from where it can then glide back down to the sea.