Extant Birds - Chapter 22

Extant Birds - Chapter 22

• Chapter focuses on the class Aves (birds)

Class Aves

• There are roughly 10,000 extant species of birds
• Size varies from the Bee Hummingbird of Cuba (~6.1 cm) to the African Ostrich (males up to 9.2 ft tall, 320 lbs, run up to 42 mph)
• Birds live almost everywhere on earth except for the Arctic, Antarctic (except penguins), open ocean, etc.
• Overall morphological diversity is relatively conservative due to constraints on flight-related systems

Avian Characteristics

• Toothless beak covered by a thick keratinized sheath
• Very thin integument with few glands
• Feathers are modified epidermal scales for thermoregulation and flight (shared with advanced theropod dinosaurs)
• Forelimbs are modified as wings, but not always for flight
• No urinary bladder, a derived trait shared with Crocodylia
• Furcula has fused clavicles to form a "wishbone"
• Sternum with a large keel
• Lunate wrist bones allow swivel motion during flight
• Uncinate processes interlock the entire rib cage
• Fused caudal vertebrae form the pygostyle
• Synsacrum involves the pelvic girdle fused to lumbar vertebrae; lumbar & thoracic vertebrae are fused & immobile
• Manus is reduced to 3 digits
• Long flexible neck aids balance
• Tri-dactyl foot has a hallux projecting backward, allowing digitigrade walk
• Pneumatic bones are hollow and light
• Red muscle fibers provide extraordinary capacity for sustained activity with high [Hb] and rapid O2 uptake

Phylogenetic Relationships of Extant Birds (Neornithes)

• Neornithes are divided into Neognathae and Palaeognathae from the Mesozoic, through the Cenozoic
• The image shows a phylogenetic tree showing that evolutionary relationship between different forms of extant birds

Paleognathae – The Ratites

• Primitive, usually flightless birds characterized by an ancestral solid fusion of many sutures in the roof of the mouth
• This limits the size of food items and restricts them tothe southern hemisphere

Neognathae - Modern Birds

• Distinguished by an extensively modified primary palate.

Other Bird Characteristics

• Flight is a defining bird characteristic
• Penguins "fly" through water
• Some bird species are flightless
• Many birds stay on the ground
• Diurnality is a common trait
• Most birds are only active during the day
• Birds have excellent vision

Structural & Functional Adaptations for Flight

• Contour feathers are the majority type, primarily on the body, each comprising

  • A quill that grows out of the skin follicle
  • A shaft that extends length and supports finer structures
  • Barbs, lateral extensions from the shaft arranged in parallel to form the vane
  • Barbules, extensions from each barb that interlink in a herringbone pattern

• Plumulaceous (downy) region is near the base with flexible barbs and barbules lacking hooks, providing insulation

• The vane is sheetlike and exposed, acting as an airfoil, protecting the downy undercoat, shedding water, and absorbing solar radiation

Plumage

• Adult feathers have different colour, texture
• They are dead at maurity, and are subjected to loss, or molting
• Flight and tail feathers are lost in exact pairs that emerges as replacements before the next pair is is lost, so balance is maintained
• Most birds molt at least once annually, usually in late summer after nesting season
• Preening with oil from a gland at the base of the tail provides water-proofing, keeps the plumage clean, and resists ectoparasites

Five Types of Feathers

• Contour Feathers are outermost feathers for the body, wings, and tail
  • It contains remiges (wings feathers) and rectrices (tail feathers)
  • Its outer remiges are tapered to have slots that makes their tip twist
• Semiplumes are contour feathers beneath the covering countour feathers
  • They provide insulation to streamline the body
• Down Feathers are entirely plumulaceous
  • They provide insulation
• Bristles have no barbs or barbs only on the portions closest to the skin
  • They are found around the base, the eyes, and on the head and toes
  • Keeps foriegn particles to stop them from getting into the eyes and nose
• Filoplumes are fine, hairlike feathers
  • Have numerous nerve endings in the follicle walls
  • Transmits info about movement of feathers so flight is better adjusted

Streamlining and Weight Reduction

• Passerines (songbirds) fly ≥ 50 km/hr
• Ducks and geese fly 80-90 km/hr
• Peregrine falcons reach 200 km/hr diving
• Contour feathers create smooth transitions between the body, wings, and head
• Feet stay tucked under
• Some birds are slow
• Cranes, flamingoes, and other long-legged birds have extended legs and necks when flying
• Birds lack urinary bladders; females have 1 ovary; males and females have small gonads that only enlarge during the breeding season

Skeleton

• Strong but light
  • For example a Frigate Bird with a seven foot wingspan has a skeleton that weights only four ounces.
• hollow bones that are reinforced by struts to brace the interior
• Strong fliers have a well-developed keel
• Hindfoot is greatly elongated
• Ankle joint is within the tarsals (mesotarsal joint) and fifth toe is lost
• Knee is concealed under the contour feathers
• The tibia fuses with the tarsal bones to form the tibiotarsus

Muscle System

• Pectoralis m.: depresses wing, main power stroke
• Supracoracoideus m.: raises wing (originates on sternum); these 2 muscles comprise 25% of body mass in some birds

Flight and Wing Structure

• The wing is an airfoil - asymmetric flight surface that tapers posteriorly that is capable of generating "lift"
• Lift is generated by differential pressures; a net upward force results from differential pressure over surfaces
• Lower surface is slightly concave, creating lift via positive pressure on the ventral surface
• Dorsal surface is slightly convex, faster laminar airflow negative pressure across the dorsal surface accounts for 2/3 lift
• Angle of attack: tilt of trailing edge of airfoil increases lift; excessive tilt causes a partial vacuum develops, causing blocked laminar flow and stall.
• Counter force to lift and thrust is drag with two types: profile and induced
  • Profile drag - friction between airfoil surface and airflow
  • Induced drag - disruption of airflow by eddies, turbulence, etc; strongest on ventral side of wing tips, & dorsal side of wing base
• When birds land there are two ways to avoid a stall:
  • A wing slot created on the leading edge from the alula, which can prevent stalling while also creating air flow that reduces drag near wing base
  • Slots between primary feathers, which are wing tip slots, that can decrease induced drag on the outer section of the wing.

Structural and Functional Types of Wings Explain Flight Characteristics

• Flight efficiency and wing shape are quantified by aspect ratio that impacts flight: -Aspect ratio = wing length ÷ wing area -Wing loading = weight of bird ÷ wing area -Average aspect ratio would be 7 – 8
• High aspect ratio, long and narrow wings that maximize life and allows soaring minimize drag. Observed in birds that soar on strong winds - albatrosses and shearwaters
• High speed wings are seen in birds that feed in flight with high aspect ratios. Observed in swallows
• Short elliptical wings are seen in birds that live in complicated environments where they have to maneuver around vegetation that a low aspect ratio. Observed in quail and grouse
• High lift wings allow birds to soar in rising air currents with low wing loading and intermediate aspects. Observed in hawks and vultures

High Aspect Wings

• Ideal for dynamic soaring, often seen in seagulls, gannets, and albatrosses
• Birds with high-aspect speed wings have tapered or 'swept back' wingtips with limited slotting and maximize speed
  • Frigate birds, swallows, swifts, and falcons

Flight Wing Properties

• Broad and slotted "high lift" wings are ideal for slow soaring on unpredictable wind currents.
  • Common in hawks, vultures, eagles, & condors

Wing Shape

• Elliptical shaped wings are short, rounded shape for bursts of speed.
  • Allows quick turns, high maneuverability, evident in birds (songbirds), game birds (quail, pheasant, etc.), some water birds (coots, grebes, etc.), and a few others

Other Flight Points

• The tail provides break control and is a rudder in flight

Flight muscle

• Red muscle has a high [Hb] & [mitochondria]
  • Fibers are very narrow and have a high surface:volume ratio, short diffusion distance, contractile power based on aerobic metabolism
  • Dark meat
• White muscle has a low [Hb] & [mitochondria]
  • Anaerobic metabolism, capable of short bursts, but fatigues quickly
  • White meat

Hummingbirds

• There is a very large supracoracoideus (50% size of pectoralis; average = 10% in other birds) observed among them
• Wings work like helicopter blades, are stiff but with high flexibility that requires 55 wing beats/second for flight
• Can fly backwards
• Contain 100% red muscle

Perching

• This activates a toe-lock mechanism, ensuring the bird won't lose perching stance while asleep
• Predatory birds like hawks, eagles, and owls possess strong leg muscles

Bird Feet

• Most birds have 4 toes with different arrangements depending on lifestyle of the bird
• Terrestrial, arboreal, and aquatic birds have ancestral condition of 1 toe pointing backward (anisodactyl)
• Birds with 1 and 4 point backward and 2 and 3 point forward (zygodactyl) are climbers, predators or rapid fliers
• Toes 2 and 3 are fused is syndactyl, found in passerine birds
• Some running birds (ostriches) are didactyl and have only toes 3 and 4
• Emus and rheas have only 3 toes (tridactyl)
• Webbed feet provide swimming adaptations
  • Ducks has webbing between toes 2, 3, and 4
  • Pelicans and boobies has webbing between all 4 toes
  • Grebes have lobes that mix surface are on backstroke

Feeding

• Bird beak shapes depends on how birds get food
  • Generalists possess beaks for grabbing and processing plants and animals
  • Probers and gleaners posses pointed beaks like forceps -food from small spaces
• Some carnivores possess heavy, pointed beaks, used for killing prey
• The tongue and talons assists the beak in securing food

Digestive Systems

• Birds do not have teeth, so there digestive system is capable of enhanced food breakdown
• More food is gathered and stored if there is excess in the crop
• The stomach structure is dependent on specific bird diet
• A 2-chambered stomach has:
  • A Proventriculus to secrete enzymes
  • A Gizzard that stores and processes and grinds for mechanical processing
• The small intestine contains enzymes for chemical digestion to absorb nutrients
• Symbiotic microorganisms in the ceca helps plant material
• The cloaca contains indigestible waste that gets reabsorbed, and gets rid of nitrogenous waste as uric acid, so all contents and fluids exit

Bird Songs

• Most birds can produce some sort of vocal sound
• Some species like pelicans may only have voice
• Birds also possess vocal sounds
• A Call is a brief sound of simple structured acoustic communication found in some birds
• A Song is a slightly longer string of melodic notes
• Songs are usually associated with courtship
• Syrinx is the vocal chords found in birds, composed of membranes and are located at the intersection of bronchi
• Songbirds have the largest number of syrinx muscles at four ot nine pairs, meanwhile most bird groups have one pair in contrast
• Birds evolved from voiceless reptiles, so the adaptation of songs gives thens an evolutionary benefit

Function of Song

• Proclaims the sex/induces other birds of opposite sex
• Usually has a correlation between complexity of song and its dull plumage

Visual Displays

• These are usually associated with bird vocalizations
• Males are generally brighter colors to attract females

Reproduction

• All birds lay eggs
• Females are heterogametic (ZW), males are homogametic (ZZ)
• Birds are closely related to crocodilians, but have unknown reasons for not using TSD
• Incubation helps with egg temperature
• Some females influence sex
• Benefits from large first

Monogamy

• Bird pairing for a part of a season, breeding, or life
• There is parental care for both since birds lay and feed
• 90% are monogamous, and are socially monogamous.

Parental Care

• Birds have young dependent on them after birth
• They are either altricial or precocial.
• Parents typically provide offsping food
• Altricial young are usually naked and dependent on parents
• Precocial are feathers and self-sufficient
• Brood Parastitism have some parasitisms that have birds raising other birds eggs, there is usually at least 7 evolved times
• Benefit includes increased fecundity and resrouces
• The parasitism can result in evolutionary arms race

Bird Migration

• Movement of birds from lower or higher resource locations
• Birds will nest in the Northern Hemisphere
• In Winter, birds move North and South