Locomotion in Birds IGliding & Soaring

JodyLee Estrada Duek, Ph.D.With much material from Dr. Gary Ritchison

http://people.eku.edu/ritchisong/avian_biology.htm

Gliding • Birds fly many ways: gliding, soaring, flapping, hovering • Simplest type is gliding • A gliding bird uses its weight (mass) to overcome air

resistance to its forward motion. • Requires a certain mass: only large birds, such as vultures,

glide on a regular basis• When gliding, a bird loses altitude at some 'sinking speed'

(Vs) while traveling forward at some 'flight speed' (V) • Glide ratio equals V/Vs (the distance traveled forward per

unit of altitude lost) • Some of the best 'bird gliders' (such as Black Vultures) may

travel up to 20 meters for every meter of altitude lost (or, a glide ratio of 20) (Check this short video of a Bald Eagle gliding).

Soaring• A soaring bird (e.g., Turkey Vultures) maintains or

increases its altitude without flapping its wings• short video of a soaring vulture • One way is to take advantage of rising air

– updrafts are generated when a steady wind strikes a hill, cliff, or building, & this is referred to as obstruction lift:

Soaring, cont’d

• Second way is thermals, or updrafts caused by the uneven heating of air near the earth's surface

• Air over fields heats faster than air over a forest or lake • Warmer air over a field is lighter than the surrounding

cooler air: it rises• At high altitudes the warm air begins to cool & sink• As a result, birds using thermals for lift typically fly in

circles to stay in the area of rising air (short pelecan video)

Argentavis magnificens • from upper Miocene (6 my ago) of Argentina, estimated mass of 70kg

wingspan of 7 m: world's largest known flying bird. • fossils of Argentavis found in the foothills of the Andes to the

pampas; likely that it used slope soaring over the windward slopes of the Andes and thermal soaring over the open pampas.

• Cranial morphology indicates that Argentavis, like other teratorns,

was an active predator rather than a scavenger• probably a diurnal predator, dependent on thermals for flight activity

much as large, broad-winged carnivorous birds today• Strong thermals occur by mid-day and disappear in the evening, so

thermal soaring for Argentavis possible only between those times• With a skull >55 cm long and 15 cm wide, Argentavis was capable of

catching sizeable prey with its formidable beak -- From: Chatterjee et al. (2007).

Soaring with Updrafts, then Gliding

Soaring with Thermals, Storm Fronts

Dynamic Soaring• Over the open ocean, large birds like the Wandering

Albatross (check this video!!! and this one!!) shown below take advantage of wind velocity gradients in a type of soaring called dynamic soaring.

Wandering Albatross © Paul Ward and Cool Antarctica

What eats an albatross?

• Every year, at the end of the nesting season, young albatrosses take their first flight to the sea. 

• The young albatrosses do not return for several years   • Some of them don't return at all because they are

eaten by sharks.  • Every year at fledging time, tiger sharks gather in the

lagoon in French Frigate Shoals atoll (Hawaiian Islands)• They wait for the fledglings to hit the water on their first

flights and then they attack • About 10% of fledglings become shark food

Dynamic soaring • Albatrosses perform a fascinating and

complicated flight maneuver called dynamic soaring

• Energy can be extracted from horizontally moving air and transferred to the bird so that an energy gain is achieved which enables it to fly continuously without flapping.

• Dynamic soaring is possible when the wind speed changes with altitude

• This type of wind, which is called shear flow, exists in the boundary layer above the ocean surface in areas in which albatrosses are found

• Dynamic soaring consists of periodically repeated cycles, with one cycle illustrated to the left: 1 - climb (windward flight); 2 - upper curve (change of flight direction to leeward); 3 - descent (leeward flight); & 4 - lower curve (change of flight direction to windward) (Sachs 2005).

Dynamic soaring is energetically efficient. Heart rate of a Wandering Albatross recorded over a two-day period

Heart rate was just above resting rates when soaring, suggesting dynamic soaring requires little more energy than resting on land (Weimerskirch et al. 2000).