The Economics of Flight, 3 - Boosted over the Sea

 

Great Blue Herons over Halibut Point

Last week we saw that fairly heavy, broad-winged birds can take advantage of updrafts and thermals to buoy their flight over land and along the coast, as the soaring herons pictured above are doing above Halibut Point. On the open ocean they would not find these continentally-generated types of wind currents. Maritime birds with specialized anatomy and aerial techniques adapt to their own environment where winds blow primarily across the water.

Iceland Gull

Their narrow, tapering wings and light bodies give gulls the ability to soar with or against prevailing air currents while retaining maneuverability. Gulls can take off with ease from the water surface flapping scarcely at all when facing into sufficient wind. By adjusting their bearing a bit off the directness of an oncoming wind, and managing the opposing forces of gravity and lift, they are able to maintain altitude, move forward or sideways, and maneuver by "slope soaring"‒all while pointed straight ahead with little or no exertion of their wings. They charm passengers into tossing tidbits their way by slipstreaming in a power boat's air wake and deftly catching the morsels in flight.

Ring-billed Gull hovering

Ring-billed Gulls are the most aerially buoyant of the larger local gull types. This one floats above the shoreline on turbulent air thrust up by breaking waves. By spreading and cupping its wings and tail it utilizes an updraft rather than aerodynamic lift to hover until plunging on a school of fish. Sturdier Herring Gulls and Greater Black-backed Gulls have evolved their own niche advantages cannot work the water surface with the light dexterity of the Ring-bill.

Black Scoters and Long-tailed Ducks

Diving birds are built with heavier musculature to pursue food under water. Their takeoffs require vigorous flapping effort, aided by foot propulsion. Many types achieve lift by literally running on the surface of the water. Once airborne they may join in symmetrical groupings to borrow energy from the aerial backwash of those ahead.

Surf Scoters and Red-breasted Mergansers

Not uncommonly birds of different species align in flocks for conservation of their flight effort. Individuals take turns at the more demanding lead position. Low-flying formations also take advantage of compression lift as the force of their movements deflects back up from the water surface, the "ground" effect described in a previous essay.

Cormorants in low-altitude mutual flight support

Cormorants with wingtips aligned
for aerodynamic advantage in higher-altitude travel

Northern Gannet

Gannets aloft make a compelling sight, trim, tapered, and fast. Their high aspect ratio‒wingspan length divided by width‒approaches that of champion soaring birds from more southerly latitudes, such as pelicans, albatrosses, skimmers and shearwaters. At times those species disappear from sight within the crests of waves where they fly seemingly perilously on the turbulence of wind tumbling into the troughs of moving water. They also are capable of dynamic soaring, a rhythmic pattern of gaining altitude from headwinds and gliding down considerable distances before repeating the ascent.*

A Gannet "surfing" along the crest of a breaking wave

Gannets flying in formation

Gannets excel at all these techniques in the inventory of oceanic flight. Following atmospheric patterns and the migrations of fish they dive from great height in pursuit of underwater prey, and clamber back up to mastery of the sky.

 

* Dynamic Soaring exploits the fact that winds blowing over the sea are slowed by the waves at the surface and gradually increase in velocity with altitude. Relatively heavy birds with long, narrow wings, such as albatrosses, can gain speed high in the “fastest” air and then plunge downwind; when they reach the slower air near the sea surface, they use their momentum to head up again, simultaneously turning into the wind which blows them back aloft – all without a single flap...Chris Leahy, The Birdwatcher's Companion.