Chapter 21
Birds: Reptiles by Another
Name
Evolutionary Perspective
• “Class Aves”
– Dozens of shared characteristics
with (other) reptiles
• Single occipital condyle, single ear
ossicle, other skeletal characters,
nucleated red blood cells, nesting
behavior, enothermy (dinosaur lineages)
– Archosaur lineage
• Saurichian branch
– Theropods
• Feathered theropod dinosaurs
Figure 21.2 An artists representation of feathered
theropods and ancient birds.
Evolutionary Perspective
• Ancient birds and the evolution of flight
– Archaeopteryx
• 150 mya (Germany)
• Reptilian tail; clawed fingers; toothed, beaklike
jaws
• Feathered wings, clavicles for flight muscles
– Sinornis
• 135 mya (China)
• Shortened body and tail, sternum for flight
muscle attachment, claws reduced, forelimbs
permitted folding wings at rest
– Eoalulavis
• 115 mya (Spain)
• Alula
Figure 21.3 Archaeopteryx. (a) Fossil. (b) Artist’s
representation.
(a)
(b)
Evolutionary Perspective
• Diversity of modern birds
– Most bird lineages represented by
early fossils became extinct along
with dinosaurs.
– Modern birds diverged from a few
lineages survived into Tertiary
period.
• 10,000 species
• 31 orders (table 21.1)
Figure 21.4 Developmental feather types and anatomy.
(a) Pennaceous feather. (b) Plumulaceous feather.
(a)
(b)
(d)
(c)
Figure 21.5 Formation of bird feathers.
(a) Epidermal cells form feather sheath.
(b) Follicle grows into the dermis.
(c) Barbs form from feather sheath.
(d) Feather emerges from the sheath.
Evolutionary Pressures
• External structure and locomotion
– The skeleton (lightweight and strong)
• Large air spaces and internal strutting
• Reduced number of skull bones
• Bill replaces teeth
– Bill and flexible neck used in nesting and
feeding young.
– Pelvic girdle and vertebral column and ribs
strengthened for flight.
• Synsacrum
• Pygostyle
– Flight muscle attachment
• Keeled sternum
• Furcula
– Appendages
• Fusion of bones
• Perching tendons
Figure 21.6 (a) The
bird skeleton shows
many adaptions for
flight. (b) Perching
tendons. (c) Internal
structure of the
humerus.
Evolutionary Pressures
• External structure and locomotion
– Muscles
• Flight adaptations
– Flight (figure 21.7)
• Form airfoil
• Slotting and alula reduce turbulence.
• Tail
– Balancing, steering, braking
• Kinds of flight
–
–
–
–
Gliding
Flapping
Soaring
Hovering
(e)
Figure 21.7 Mechanics of bird
flight. (a) A bird’s wing acts as
an airfoil. (b-c) Increased angle
of attack increases turbulence.
This turbulence is reduced by
an alula. (d) Wing orientation
during a downstroke. (e) Note
the alula on this bald eagle
(Haliaeetus leucocephalus).
Evolutionary Pressures
• Nutrition and the digestive system
– High rates of food consumption
support rapid metabolic rates.
– Bill modifications (figure 21.9)
– Digestive tract (figure 21.10)
• Crop
– Storage
• Stomach
– Proventriculus
» secretes gastic juices
– Ventriculus (gizzard)
» Muscular and grinding
Figure 21.9 Specializations
of bird bills. (a) Bald eagle
(Haliaeetus leucocephalus).
(b) Cardinal (Cardinalis
cardinalis). (c) Flamingo
(Phoenicopterus ruber)
Figure 21.10
Digestive system of a
pigeon.
Evolutionary Pressures
• Circulation, gas exchange, and
temperature regulation
– Completely separated atria and
ventricles result in separate
pulmonary and systemic circuits.
– Sinus venosus serves as pacemaker.
– Very rapid heart rates and separate
pulmonary and systemic circuits
support flight and endothermy.
Evolutionary Pressures
• Gas exchange
– Syrinx
• Vocalizations
– Bronchi lead to air sacs.
• Extend into bones (figure 21.11a)
– Lungs
• Small air tubes
– Parabronchi function in gas exchange.
– Respiratory cycles
• Two cycles move each volume of air through
the lungs (figure 21.11b,c).
• Uninterrupted airflow through parabronchi
with both inspiration and expiration
promotes efficient gas exchange.
(c)
Figure 21.11 Respiratory
system of a bird. (a) Air
sacs branch from the
respiratory tree. (b) Air
flow during inspiration
and expiration.
(c) Parabronchi.
Evolutionary Pressures
• Thermoregulation
– Body temperatures
• Between 38 and 45oC
– Heat conservation
• Fluffing feathers decreases heat loss.
• Tucking bill into feathers
• Countercurrent heat exchange in legs
– Heat generation
• Shivering
• Muscular activity in flight
Evolutionary Pressures
• Nervous and sensory systems
– Brain
• Enlarged forebrain
– Corpus striatum
» Visual learning, feeding, courtship, and
nesting
– Pineal body
» Ovarian development and responses to light
and dark periods
• Midbrain
– Optic tectum
» Visual processing
• Hindbrain
– Motor coordination
– Regulation of heart and respiratory rates
Evolutionary Pressures
• Nervous and sensory systems
– Vision
• Double-focusing mechanism
– Curvature of lens
– Curvature of cornea
• Retina
– Rods and cones
– Two foveae per eye
» Search fovea
» Pursuit fovea
• Monocular vs. binocular vision (figure 21.13)
– Olfaction
• Minor importance except in vultures
– Hearing
• Well developed
Figure 21.13 Avian vision. Fields of view of (a) a pigeon, (b) a
hawk, and (c) a woodcock. Compare the size of the binocular
and monocular fields of view in regard to position of the eyes.
Evolutionary Pressures
• Excretion and osmoregulation
– Uric acid stored in cloaca
• Promotes water conservation and
development in terrestrial environments
– Supraorbital salt glands
• Marine birds secrete excess salt through
nasal openings.
Evolutionary Pressures
• Reproduction and development
– Oviparous
• Testes paired
• Left ovary develops and releases eggs to
large ostium.
• Albumen secreted by oviduct.
• Shell gland secretes shell around fertilized
egg.
– Territories and courtship common
(figure 21.15)
– Mating
• Monogamous
• Polygynous
• Polyandrous
Figure 21.15 Courtship display. A male greater prairie
chicken (Tympanuchus cupido) displaying in a lek.
Evolutionary Pressures
• Nesting activities
– Clutch size varies.
– Most incubate eggs.
– Altricial
• Entirely dependent on parents at
hatching
– Precocial
• Relatively independent at hatching
Figure 21.16 (a) American robins (Turdus migratorius) have
altricial chicks that are helpless at hatching. (b) Kildeer
(Charadrius vociferus) have precocial chicks.
Evolutionary Pressures
• Migration and navigation
– Migration
• Periodic round trips between breeding and
nonbreeding areas
– Most annual and north/south
– Breeding area resource abundance varies from
season to season.
» Abundant in spring/summer breeding season
• Response to species-specific physiological
conditions
– Innate clocks
– Environmental factors
Figure 21.1 Bird
migration. The arctic
tern (Sterna arctica)
breeds in northern North
America, Greenland, and
the Arctic. Migrating
birds cross the Atlantic
Ocean on their trip to
Antarctica during the
Northern Hemisphere’s
winter season.
Evolutionary Pressures
• Migration and navigation
– Navigation
• Route-based navigation
– Landmarks on outward journey used to guide
return trip.
• Location-based navigation
– Sun compasses, other celestial cues, the
earth’s magnetic field