Wintering on Berries

In one of the relationships that undergirds nature as we know it, berries produced by trees, shrubs and vines sustain the birds who help distribute those woody plants far and wide. Typically the birds help themselves to the calories and nutrients in the surrounding pulp package and excrete the hard, undigested seeds to the random fortunes of dispersal. Even birds that are insectivores by preference get through the lean season at Halibut Point  by eating berries.

Robin and cedar berry, Juniperus virginiana

Flocks of Robins that we used to associate with the arrival of spring are now sustaining themselves through the winter on a variety of landscape resources.

Cardinal eating cedar berries

Cardinals have similarly found sufficient food to pioneer a year-round New England presence within recent decades.

Yellow-rumped Warbler

Some Yellow-rumped Warblers have extended their stay locally with help from cedar berries.

Starling with cedar berry

Many species in addition to the Cedar Waxwing depend partially on these trees for winter sustenance.

Robin in Crabapple tree

Succulent fruits that may not attract birds in the cornucopia of fall become important nutrients later in the season.

Cardinal in Honeysuckle shrub

Even in their most desiccated state berries retain some food value for the bird population.

White-throated Sparrow eating Virginia Creeper berries
in preference to abundant Bittersweet

For reasons known to themselves birds may be drawn to a particular fruit or tree in preference to an adjacent one.

Cedar Waxing reaching for Multiflora Rose berries

Cedar Waxwings eating Privet berries

The fact that so many non-native species have been spread by seed dispersal through Halibut Point is proof of the palatability of their berries to birds.

Robin plucking Sumac fruit

The source of some meals might be surprising to us, but the birds know what they need and where to find it.

Bittersweet

Bittersweet berries look like an obvious sustainer but at this point in the season they have drawn relatively little attention. They may have a bigger role to play as winter deepens.

Yellow-breasted Chat amid Bittersweet

Bittersweet did attract my own first sighting at Halibut Point of this warbler relative and its astonishing glint of chromium yellow on a drab chilly day.

Flight and Anatomy

 

Northern Mockingbird

Birds amaze us with their ability to fly. With the aid of a camera we can isolate some of the intricacies of movements that they combine into coordinated flight.

Great Egret taking flight, Folly Cove

Getting into the air typically requires a burst of propulsion from the bird's legs and wings. The necessary musculature is concentrated near the center of its body for aerodynamic reasons, and the power translated by rope-like tendons to working parts in the limbs.

Generating lift

Of course overpowering gravity requires strength and energy, but the cross-sectional shape of the wing contributes inherently to lift, a principle of physics that lowers pressure above a curved surface and raises pressure below it as air rushes past in forward motion. A bird's wing is pulled upward into this 'dynamic vacuum' by the passing air simply as a result of moving forward. It has to generate that forward motion either through its own effort, or by plunging with its wings outstretched, or by borrowing the force of the wind. In takeoff it may be able to blend all three phenomena. The upward force of lift operates mainly on the broader, stiffer, more cambered parts of the inner wing with its relatively fixed feathers. This principle of lift has been carefully applied to aircraft design.

Blue Jay in forward stroke

In their power stroke birds reach down and forward with primary feathers closed to force air past their wings for lift as well as for propulsion. Even in level flight their bodies, when flapping, are oriented on a somewhat up-facing diagonal in the direction of thrust.

Black-capped Chickadee

Feathers are controlled to various degrees by muscles at their point of attachment to the skin and skeleton. The primaries, located past the bend in the wing in the area analogous to our hands, are controlled individually by rotating, spreading, and altering angles. Within a fraction of a second they loop through the figure-eight cycle of a wingbeat.

Great Cormorant

This dexterity extends to the tip of the wing with "fingertip" subtleties of guidance.

Cooper's Hawk

The Accipiter group of hawks have evolved long tails to facilitate their twistings and turnings in pursuit of prey through woodlands, as distinct from the broad-winged Buteos that excel at soaring over open ground and swooping out of the sky.

Eastern Wood-pewee

Most birds lose aerodynamic lift in an upright flight posture. They can maintain verticality only briefly and with great effort.

Ruby-throated Hummingbird

Hummingbirds can "stand still" in the air with propeller-like wings cycling as much as 80 times per second supported by rapid metabolism and a heart rate exceeding1,000 beats per minute. In proportion to their size they use a vast amount of calories while feeding.

Only tiny but well-muscled creatures hover in this way, supported by extraordinary respiratory and circulatory systems.

Bufflehead landing

All birds need a technique for returning to earth, mobilizing their wings and legs in something of a reversal of takeoff maneuvers. While water landings can be relatively cushioned and random, learning to come in precisely on a perch demands fair practice from juveniles.

Barn Swallow feeding its young
on a Halibut Point quarry cliff

Evolutionary success for birds has balanced opposing factors such as flexibility and durability, lightness and strength, individuality and reproduction. Respiratory sacs dispersed within hollowed bones reduce lung size to conserve core space for denser organs. They get similar advantage by processing food with lightweight beaks and a gizzard rather than heavy teeth away from their center of gravity. 

These features and many more can be explored in a fine online encapsulation How Birds Fly by the University of Wisconsin, or in the encyclopedic essays offered by local resident Chris Leahy in The Birdwatcher's Companion.

 

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.

The Economics of Flight, 2 - Soaring over Land

The flying ability of birds delights and amazes us from childhood up through scientific investigation. Birds take to the air, an invisible medium in itself. They generally work very hard at this with vigorous flapping of their wings, helped or hindered by the wind. Certain ones have achieved the ability to make flight look effortless by appearing to float in the sky. Among these gravity-defying birds are some of the biggest and heaviest we are likely to see.

A Turkey Vulture soaring past the Visitors Center,
Halibut Point State Park

Soaring birds keep themselves in the air with the assistance of wind currents. The ones that achieve this over land generally have broad wings and tails to take advantage of warm rising air. They get themselves up high for two reasons: long-distance travel or searching for food. They're well designed for soaring but their takeoff is more demanding. They often perch in elevated places that offer a better start to flight.

Turkey Vulture perched on the grout pile, Halibut Point

The particular features of each bird species represent a compromise of possibilities for specialization. Gracefulness in soaring may come at the cost of clumsiness and vulnerability on land. Vultures have exceptional senses of sight and smell to locate carrion from up high. They're unlikely to come to the ground unless they see a meal.

The Turkey Vulture taking off

To return to the sky from the top of the grout pile this vulture has oriented and cupped its wings for maximum capture of the air, feathers spread but closed. It found energy assistance from whatever wind currents were available. It developed further lift from its own aerodynamic design, which we will explore in a later essay.

A young Red-tailed Hawk "kiting" over Halibut Point

Air from land masses differentially warmed by the sun spirals upward, an effect more prominent over the continents than over the oceans. These thermals support soaring birds that fly in broad circles around the columns of rising air. Such a column of energy allows this hawk to hover almost motionless while focused on prey below, tilting up its wings to stall forward progress, keeping them and the tail outstretched to maximize its buoyancy. Slots between the feathers add aerodynamic stability while it is suspended before plunging or resuming travel.

Soaring Turkey Vulture

Similarly, this vulture's widespread wingtip feathers (primaries) contribute to fine-tuned flight control and minimized turbulence from air pushed past the wings. It would close these feathers during flapping to help generate forward thrust.

Black Vulture from above

Late this past December a Black Vulture paid a rare visit over Halibut Point, showing its distinctive grey head color, plumage pattern, smaller size, shorter tail, and flatter wingspread.

Black Vulture from below.
In comparison with the Turkey Vulture,
it is silvery only at the wingtips

Black Vultures rely more on sight than Turkey Vultures since their sense of smell is not as keen. At low altitudes both take advantage of updrafts created by wind deflected skyward by trees, hills, and even buildings. Black Vultures maintain a level plane of their wings even while tilted during turns. Turkey Vultures, on the other hand, soar with a distinctive dihedral (shallow V) shape, teetering from side to side as they finesse the winds and flapping less infrequently than Black Vultures. 

While it may seem counterintuitive, soaring birds often fly directly or nearly into the wind. Headwinds increase the lifting force that results from air passing over the cross-sectional shape of their wings, an outline copied in aircraft design. Of course that same headwind slows their forward motion. Birds continuously adjust their body parts to gain or lose altitude and speed and to alter direction as they fly where necessity takes them.