Energetically demanding but highly efficient. The physics of flight: Four forces acting on a flying bird: Gravity pulls toward the ground

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1 Avian Flight and Energetics Left the global adaptive radiation of birds included multiple unrelated lines of flightless forms. But why? If flying is so great, how would some birds be better off without it? Advantages 1. Speed for catching prey 2. Speed for evading predators 3. Agility for access to food/neing sites 4. Ease of travel through large home range 5. Ease of dispersal 6. Ease of migration to take advantage of favorable environments Disadvantages 1. Modification of forelimbs to be not much use for things other than flying 2. Extreme skeletal modification to reduce weight 3. Energetically coly to maintain ability to fly 4. Selection toward high-energy foods Energetically demanding but highly efficient. The physics of flight: Four forces acting on a flying bird: Gravity pulls toward the ground Drag slowing force from turbulence and friction Lift upward air pressure that counteracts gravity Thru forward force that counteracts drag Airplanes generate lift with wings and thru with engines; birds mu generate both forces with wings. Daniel Bernoulli ( ): the pressure of a fluid (including that of air) decreases as its speed increases. The air passing over a wing s curved surface mu travel faer than the air moving under it. The increased speed of the air above the wing decreases its pressure, resulting in lift. There is vigorous debate on relative influence of air pressure differences and Newton s laws of conservation of momentum: The downwash behind and beneath the wing mu be balanced by an equal and opposite force above and in front. Many bird flap and propel themselves through the air like torpedoes. They obviously aren t relying on the Bernoulli effect when their wings are closed! Wing morphology contributes greatly to various aspects of flight: Angle of attack deviation of the leading edge of an airfoil from the direction of airflow. Chord raight line width of an airfoil from leading to trailing edge. Camber convex shape to an airfoil in cross section. Camber of a bird s wing is greater close to the body (proximally) than at the wingtip (dially). 1

2 NREM/BIOL 4464 Ornithology Dr. Tim O Connell Camber is lowe at the narrow (in cross section) wingtips of birds, so can be a problem there. How is drag reduced? With mini-airfoils within the outretched wing. The alula often extended at slower speeds to reduce alling, especially during landing. Slotting refers to the narrow emarginations on the outer primaries of many soaring birds. At these primary tips, each feather acts like its own airfoil to reduce drag. Lectures Feb Mar 2019 drag Wing Loading the total mass supported by the wing surface area. Heavy birds have high wing loading, but so do lighter birds that have proportionately small wings. Highe wing loadings occur in the heavie of flying birds, e.g., buards, as well as smaller birds with proportionately smaller wings, e.g., alcids. > Aspect Ratio describes the shape of a bird s wing as a ratio of wing length to wing width. The long, thin wings of albatrosses provide and excellent example of high aspect ratio; the broad, ubby wings of grouse illurate low aspect ratio. For maneuverability, low aspect ratio performs better. For soaring and gliding, high aspect ratio is better. The long, thin wings of albatrosses provide and excellent example of high aspect ratio; the broad, ubby wings of quail illurate low aspect ratio. > With their heavy bodies and small, iff wings used for flapping propulsion under water, Alcidae (auks, puffins, etc.) have the highe wing loading among living birds, and they are very close to the theoretical maximum at which they mu become flightless (illurated well by Great Auk!). At the other end of the spectrum are birds with very large flight surface areas supporting relatively miniscule weights: Frigatebirds and Barn Owls! Additional energy savings in flight Riding thermals (columns of rising air that form over sun-warmed ground): raptors, pelicans, orks, cranes, gulls, Riding updrafts along mountain ridges, bridges, etc.: raptors, gulls Riding updrafts along waves! Albatrosses. This takes great skill and a very high aspect ratio wing to be successful. It s a main reason why albatrosses are rericted to some of the windie places on earth. Flying in V-formation: Flying a bit behind and to the side of the bird in front of you can reduce turbulence at your wingtips. Less turbulence means a more smooth flow of air over the wings, thereby generating maximum lift with minimum drag. Energy savings can be 10 50%. Feather coloring: Pigments chemical compounds that efficiently absorb some wavelengths of light and reflect others. The pigments are located as granules within the cells. Structural colors produced through the scattering of various wavelengths of light by microscopic features on the surface of feathers and skin. Almo all blue coloration is ructural. 2

3 Pigments: Melanins produce blacks, browns, grays Carotenoids yellows, oranges, reds, (some greens when blue ructural color overlays yellow pigmentation) Porphyrins rufous brown or greens; brilliant reds and blues. In birds, porphyrins occur only in turacos, buards, and owls. Melanins occur as granules in the deep layer (dermis) of the skin. The density, size, and specific type of melanin determines the depth of color in the resulting skin or feather. The granules that contain the melanin are themselves comprised of keratin the same material from which the feather is comprised. Thus, a feather with lots of melanin granules has extra keratin compared to a feather with little or no melanin. This makes black feathers more resiant to wear than white feathers. Check out this photo of Willets from Charlie Moores. The primary coverts are all black. The primaries are white with black tips. Once you art looking, you ll see examples all over the place of birds with dark feather tips. Melanin also makes feathers more resiant to colonization from bacteria that degrade feathers. So we often see darker birds living in wetter environments. This has traditionally been explained as a camouflage phenomenon, but it could ju as easily have emmed from selective pressure for feathers to withand the feather bacteria that flourish in humid and wet environments. Feather pigmentation, ructure, and age It cannot be underated how simpliic is our coverage of feather ructure and coloration. We already learned a bit about the importance of melanin (comprised of the protein keratin) in birds, both for coloration and reinforcement of feather ructure. Birds are so diverse, however, that there are exceptions to almo everything that might seem like a rule. An example: Velvet Asity (male) photo by Dubi Shapiro. Asities are a family of Passerines endemic to Madagascar. In each of the four species, brilliantly-colored patches of bare skin occur on the face. These colors are produced by light scattering from collagen fibers in the skin that reflect wavelengths of blues and greens. The use of collagen for this purpose is unique among birds and indeed among all vertebrates! Structural coloration is widespread in birds. In mo cases, it is scattered wavelengths reflected from a particular arrangement of melanin granules and the open space between them in the cells that comprise the feather barbs. If all wavelengths of light are scattered back, then the feather will appear white. Thus, white plumage can result from an absence or reduction of melanin, or it can be purely a ructural artifact of the arrangement of melanin. Blue feathers lack blue pigment. Bluebirds, jays, Indigo Buntings none of these species have any blue pigment. It is the particular arrangement of melanin granules at the subcellular level that reflects blue wavelengths to make them appear blue. Right this Indigo Bunting has ju crossed the Gulf of Mexico and is reing on Dauphin Is., AL. He looks blue, but he ain t blue. Among vertebrates, it appears to be ju a few species of fish that actually have blue pigmentation. (Although the blue pigment biliverdin is found in birds it s what makes many birds eggs blue!) 3

4 NREM/BIOL 4464 Ornithology Dr. Tim O Connell Lectures Feb Mar 2019 Iridescent feathers, like those on the gorgets of hummingbirds or the gloss on a grackle, allow the passage of light through the feather, so the same feather can appear to be multicolored depending on the angle of light. From some angles these iridescent feathers will also look black. Carotenoids produce yellows, oranges, and reds in (mo) birds. Unlike the genetic determination of melanin, carotenoids are obtained from birds diets. A great example of this is furnished by Cedar Waxwings: A few decades ago, some birds were showing up with orange tail tips rather than the normal yellow: It was determined by Bob Mulvihill and colleagues at the Powdermill Nature Reserve that the waxwings were picking up the red pigment rhodoxanthin from eating the orange and red berries of the invasive Tatarian honeysuckle. ( Psittacofulvins. Parrots are weird. Research has demonrated that the world s 250 or so species of parrots take in plenty of carotenoids from their diets, but they don t actually use them to color their feathers. Inead, parrots form their own clade of birds that uniquely use their own class of yellow, orange, and red pigments the psittacofulvins. Like melanin, red and orange psittacofulvins have been shown to be rongly resiant to featherdegrading bacteria, which might in part explain the bright colors so typical of parrots. (Yellow psittacofulvins, however, are not anti-bacterial.) These Green Parakeets in their evening roo at the Haings parking lot in McAllen, TX illurate something else: the interaction of yellow pigmentation and ructural blue reflectance to produce... green! In almo all birds, green results from yellow pigment and blue reflectance, i.e., there are no green pigments other than those described below. In parrots, the yellow pigmentation comes from psittacofulvins; in other green birds (e.g., a female Painted Bunting) the yellow pigmentation comes from carotenoids. Porphyrins are modified amino acids that can produce browns, reds, and greens. The iron-rich heme group at the center of a hemoglobin molecule is a porphyrin. Owls, buards, and turacos have porphyrins in their plumage. Right porphyrins will glow under UV light! Check out this glow on a Great Horned Owl (photo by Colt Holley): Turacin and turacoverdin are porphyrin pigments that have a copper base inead of iron, and they are unique to the turacos: brightly colored birds from Sub-Saharan Africa. The green turacos are the only green birds with actual green pigment (although some speculate that a few galliformes and the jacanas might also use these pigments). Left: Guinea Turaco photo by Fran Trabalon. The green comes from turacoverdin; the red in the flight feathers is from turacin. Of course, lots of birds look a bit different in the UV portion of the electromagnetic spectrum. We humans see red, green, and blue in the 3 types of cones in our retinas. Birds, however, have four types of cones: red, green, blue, and ultraviolet. Thus the cardinal in the bushes that looks red to us does not look quite they same to birds. They see it reflecting red, but also reflecting in the ultraviolet. To birds, cardinals are actually kind of purple-red! Right: Nathan Chronier photo from 4

5 NREM/BIOL 4464 Ornithology Dr. Tim O Connell Lectures Feb Mar 2019 Plumage and molt Basic plumage feathers the bird has through mo of the year; non-breeding plumage Alternate plumage plumage worn for a shorter period that aligns with the breeding season. Each plumage is the result of a previous molt. Pre-Basic molt results in the basic plumage Pre-Alternate molt results in the alternate plumage All birds have a pre-basic molt, but not all have a pre-alternate molt... Not all birds have a diinctly different alternate plumage from their basic plumage: chickadees, thrushes, mo wrens, jays, crows, mockingbirds, thrashers, doves, mo sparrows, etc., have no diinct alternate plumage, and no diinct pre-alternate molt. Some birds (e.g., House Sparrow, Ruy Blackbird) look like they have a diinctly different alternate plumage, but it s the result of feather edges wearing away several months after the prebasic molt to reveal a different pattern to the plumage. The black bib on a male House Sparrow is there all winter long, it ju becomes much more obvious when the grayish feather edges are fully worn off by spring. Warbler, buntings, tanagers, and a few others have a pre-alternate molt. Plumage sequences in passerines 1. Neling hatched naked; grows juvenal plumage in ne (~ days) 2. Juvenal plumage worn for several weeks in summer 3. Pre-basic molt late summer 4. Produces basic plumage. In juveniles, this does NOT include the flight feathers. In second year and older birds, it includes all feathers. Neling Carolina Wrens. The feathers they grow in the ne (both contour and flight feathers) they mu grow in about 10 days. Sometime between July and October, these nelings will molt and grow a new set of contour feathers (1 pre-basic molt), but they will be uck with this fir set of flight feathers until the following July. (Photo by Jason Heinen.) Juvenile (bottom left) and adult Dark-eyed Junco of the Oregon race. The juvenile s reaky, sparrowlike plumage is its juvenal plumage, worn for ju a few weeks between fledging and its 1 pre-basic molt. Following its 1 pre-basic molt, the young bird will look much more like the adult on its head, brea, and back, but it will retain these same flight feathers. Ever seen a robin like this? That spotted brea indicates that this young robin is in its juvenal plumage. A pre-basic molt includes all the contour and flight feathers. ( complete molt) The 1 pre-basic molt does not. ( partial molt in some species in which some of the flight feathers are routinely replaced during the 1 pre-basic molt, we call it incomplete. ) The 1 pre-basic includes ju the contour feathers. On the wing, it can sometimes be easy to tell where the molt opped in a particular feather tract. There will be a contra in color, feather wear, etc., between the old feather that wasn t molted and the new one that grew in. This is called the molt limit. The be place to look is the greater coverts above the secondaries. 5

6 Exactly where the molt limit will be is maddeningly variable, both within and among species. Our little robin? He should replace all of his median coverts in his 1 pre-basic molt, 0 9 of his greater coverts, and maybe 1 or 2 tertials. So the be place to see a molt limit would be in the greater coverts, but he might show a molt limit between adjacent tertials as well. If there is a molt limit, you are almo certainly examining a bird that has undergone its 1 pre-basic molt. In other words, the bird is less than one year old. If there is no molt limit, the bird is almo certainly older than 1 year old. (It mu at lea have lived through TWO autumns to have gotten to a subsequent pre-basic molt. Note: In some birds, it s really easy to see a molt limit if it is there; for others it takes a good deal of practice. Here s an easy one: Indigo Bunting male. Both of these photos are from males in the spring. A young male arts his life brown like a female but undergoes a pre-alternate molt in the spring that arts to turn him blue. That young male will show a mix of brown and blue feathers when he returns north for his fir breeding season: Left: Check out tertial 9 (black with brown edge) and tertial 8 (black with blue edge). That s a molt limit. See also the greater coverts. 1 and 2 were not replaced, but 3 9 were (as were all of the median coverts). Finally, look at the difference between greater coverts 3 9 (black with blue edges), and the dull gray primary coverts. This male is < 1 year old (although by convention we ll consider him second year in his fir spring. Now compare our young male Indigo Bunting to the ud above right. All of this bird s wing coverts, primaries, secondaries, and tertials are glossy black with blue edging. There is no contra among tracts, or among individual feathers within a tract. Therefore, there is no molt limit, and this is an older bird. Plumage sequences in passerines Pre-alternate molt late winter/early spring Usually limited to contour feathers Check out this Scarlet Tanager illuration by Peter Burke: Females and 1 fall males are similar, except that after his 1 pre-basic molt, the young male will show black median coverts and greater coverts (mo of them). But that male will molt again in the spring (his 1 pre-alternate molt), this time molting in red contour feathers and fresh black tertials. His primaries and secondaries will be obviously dull and worn by this time. After his fir breeding season, our young male will molt again (pre-basic molt) to achieve his definitive basic plumage: olive green with black wings and tail. In the spring, he ll undergo a pre-alternate molt of all contour feathers to reveal his spectacular definitive alternate plumage: eye-popping red with glossy black wings and tail. Some birds are weirder ill: the Marsh Wren and Bobolink both undergo TWO complete molts each year: ALL feathers replaced in both fall and spring. 6

7 Ages of birds Ages of birds ex) egg laid June 1 Egg Neling baby bird confined to the ne. Fledgling ju left the ne; ill beg parents for food Local/juvenile ill on parent s territory, but independent Immature general term for birds not yet old enough to breed. For mo songbirds, immature applies for less than 12 months; for raptors or gulls that might take several years to reach sexual maturity the term applies correspondingly longer. These (except for the egg and immatures > 1 year old) are all hatch-year birds, abbreviated HY. All birds celebrate a birthday on January 1. Each HY bird becomes a second-year bird, or SY. A more general term is after hatch-year or AHY. All SY birds are AHY, but not all AHYs are SYs. It is relatively easy to determine the age of an HY or SY bird. If age cannot be determined on a bird between, say July and January, then the age should be considered unknown. If age cannot be determined on a bird between Jan. and July, it is safe to li as the general term AHY. If a bird can be reliably determined not to be SY between January and July, this means the bird is older than SY, and may be safely lied as ASY. (Some birds plumage sequences allow determination of ATY or even AFY.) So, our young bird born on June 1 is HY through the summer and autumn. He becomes SY on Jan. 1. If lucky and successful, he is tending his own brood by his fir literal birthday in June. He undergoes a complete molt of contour and flight feathers July to September, and ceases to be recognizable as SY. He is now AHY, and he should be able to be aged as ASY the following spring. Once a bird attains adult plumage (either basic or alternate), we use the term definitive. This means that the bird will look the same, whether in basic or alternate plumage, for the re of its life. Ageing by criteria other than molt and feather contra Skulling The avian cranium is bi-layered, with supporting ruts in between the layers. The layers take several months to develop seeing a pattern of incomplete ossification (pneumatization) through the transluscent skin on a bird s crown is be evidence of a HY bird (right). Other body parts A fleshy gape (corners of the mouth) often las on a young bird for many months. The iris color is often different for adults and immatures, e.g., pale yellow in immature Cooper s Hawks but deep red for adults. Mouth linings can differ, e.g., deep black for an adult Gray Catbird but grayish yellow for HYs. Feather shape Outer rectrices tapered (pointed and narrow) on HY and SY birds. Outer rectrices truncate (rounded and broad) on AHY birds. 7